Physics Lab

Experiment 1: The Pendulum & Gravity

Big idea: A swinging pendulum is a clock governed by gravity. Its period (the time for one complete back-and-forth swing) depends on the length of the string — not on how heavy the bob is. You will measure this relationship and see the mathematics of motion in action: T = 2π√(L/g).

Caution: tie the string securely so the weight cannot fly off; swing the pendulum away from people and breakable objects.

Materials: string or dental floss (about 1.2 m), a metal washer, nut, or set of keys (the "bob"), tape, a ruler or measuring tape, a chair or table edge to tie the string to, a phone stopwatch.

Procedure:

1. Tie the bob firmly to one end of the string. Tape the other end to a chair back or table edge so the bob hangs freely.
2. Measure the string length L from the pivot point to the center of the bob. Set it to 1.00 m.
3. Pull the bob to one side (a small angle, about 15°) and release it — do not push it.
4. Start the stopwatch and count 10 complete swings (back-and-forth = one swing). Stop the watch at the end of the 10th swing.
5. Repeat two more times at 1.00 m and record each trial.
6. Shorten the string to 0.75 m, 0.50 m, and 0.25 m, repeating steps 3–5 for each length.
7. Divide each 10-swing time by 10 to get the period T for that trial, then average the three trials per length.

Conclusion questions:

1. How does the period change as the string gets shorter? Is the relationship linear?
2. Plot T² against L (or compare the columns) — what pattern do you see? What does it tell you about the formula T = 2π√(L/g)?
3. Try a heavier bob at the same length. Does the period change? Why is that surprising, and what does it reveal about gravity?

Experiment 2: Paper Airplanes — Variables and Fair Tests

Question: Does wing shape change how far a paper airplane flies? A good experiment changes one thing at a time. The independent variable is what you change (wing shape: dart vs. glider); the dependent variable is what you measure (flight distance); controlled variables are everything kept the same (same paper, same thrower, same launch height, same room, no wind). Changing two things at once makes the result meaningless — you cannot tell which change mattered.

Procedure: (1) Build three identical dart planes and three identical glider planes from the same paper. (2) Mark a launch line. (3) Throw each plane three times with the same effort and measure the distance with a tape measure. (4) Record every flight in a table — do not throw away “bad” flights; a nosedive is data. Safety: throw away from people and faces; never throw toward anyone.

Analysis: average the three flights per design. Suppose the dart averages 8.4 m and the glider 6.1 m. The dart flies farther, but the range of flights matters too: if the glider's flights were 6.0, 6.1, 6.2 m and the dart's were 3.0, 9.5, 12.7 m, the glider is more consistent. Conclusion: state the claim, the evidence, and a possible next test (e.g. does adding a paperclip to the nose change the result?). Real-world link: aircraft designers test wing shapes in wind tunnels the same way — one variable at a time.

Try it yourself: Name the independent, dependent, and two controlled variables if you test whether launch angle changes flight distance. Why throw each plane three times? Answers: Independent: launch angle; dependent: flight distance; controlled: plane design, thrower/paper (any two). Repeats average out random variation and reveal consistency.

Experiment 3: The Friction Ramp

Big idea: Friction is the force that resists motion between two surfaces in contact. Rougher surfaces and heavier objects create more friction. You will test how surface material affects how far a sliding object travels — or the ramp angle needed to start it moving.

Caution: keep the ramp stable so it cannot slip and fall; keep fingers clear of the sliding object.

Materials: a smooth board (cutting board, shelf plank, or large hardcover book), a stack of books to prop one end, a wooden block, small toy car, or bar of soap (the "slider"), a towel, a sheet of wax paper or plastic wrap, tape, a ruler.

Procedure:

1. Prop one end of the board on books to make a gentle ramp. Mark a "start line" near the top with tape.
2. Surface 1 (bare board): place the slider at the start line, release it without pushing, and measure how far it travels past the bottom of the ramp in centimeters. Repeat 3 times.
3. Surface 2 (towel): tape a towel flat over the board so it covers the sliding path. Repeat 3 trials.
4. Surface 3 (wax paper): replace the towel with wax paper taped smooth. Repeat 3 trials.
5. Keep the ramp angle and the release point identical for every trial.
6. Average the three trials for each surface and record your results.

Conclusion questions:

1. Rank the surfaces from most to least friction. How do you know?
2. Where did the slider's energy "go" on the high-friction surface? (Hint: touch the towel after several runs — what do you feel?)
3. Why was it important to keep the ramp angle and release point the same? What is this called in experimental design?

Experiment 4: The Bouncing Ball — Energy Loss

Question: What fraction of a ball's energy survives each bounce? Drop a tennis ball from 100 cm and it bounces to perhaps 55 cm — it never returns to the drop height, because every bounce converts some kinetic energy into heat, sound, and deformation of the ball and floor. The coefficient of restitution e = (bounce height)/(drop height) summarizes the bounciness: e = 1 would be a perfectly elastic bounce; real balls are 0.5–0.9.

Procedure: (1) Tape a meter stick vertically to a wall. (2) Hold the ball so its bottom is at 100 cm, release it without pushing, and watch the top of the bounce (a partner helps: one drops, one watches). (3) Record the first bounce height. Repeat three times and average. (4) Try a different ball (rubber ball, golf ball) with the same drop height. Safety: keep the area clear; the ball will bounce unpredictably.

Analysis: If the first bounce averages 55 cm, e = 0.55, and the energy retained is roughly e² ≈ 0.30 — about 70% of the energy is lost each bounce (energy is proportional to height). Predict the second bounce: 55 × 0.55 ≈ 30 cm. Test it: does the ball actually reach ~30 cm? Conclusion: compare balls and explain the ranking in terms of materials: a superball's rubber returns more energy than a clay ball, which barely bounces at all.

Try it yourself: A ball dropped from 80 cm bounces to 40 cm. Find e and predict the second bounce. Answers: e = 40/80 = 0.5; second bounce ≈ 40 × 0.5 = 20 cm.

Key vocabulary: Coefficient of restitution — bounce height ÷ drop height; kinetic energy — energy of motion; elastic — a bounce with no energy loss (idealized).

Experiment 5: Water Displacement — Density of Irregular Objects

Question: How do you find the volume of a rock? You cannot measure it with a ruler, but you can use displacement: a submerged object pushes aside a volume of water equal to its own volume. This is the trick Archimedes used in his bathtub, and it lets you compute density (mass ÷ volume) for any object, however lumpy.

Procedure: (1) Weigh the object on a kitchen scale (mass in grams). (2) Fill a graduated cylinder or measuring jug partway with water and record the starting level. (3) Lower the object in gently (tilt the container or use a string so no water splashes out). (4) Record the new level; the difference is the object's volume in milliliters (1 mL = 1 cm³). Safety: dry the area; wet floors are slippery. Do not use objects that dissolve or react with water.

Analysis: density = mass ÷ volume. A 60 g rock displacing 25 mL has density 60/25 = 2.4 g/cm³ — typical of stone (2–3 g/cm³). Compare with water (1.0 g/cm³): denser than water, so it sinks; an object less dense than water floats. Use your result to predict: will the object float in oil (density ~0.9 g/cm³)? Conclusion: report the density and identify the likely material from a density table.

Try it yourself: A cork weighs 5 g and displaces 20 mL. What is its density, and will it float in water? Answers: 5/20 = 0.25 g/cm³, less than water's 1.0, so it floats.

Key vocabulary: Displacement — the volume of fluid pushed aside by an object; density — mass ÷ volume; meniscus — the curved water surface to read at eye level.

Experiment 6: Balloon Rocket — Newton's Third Law

Big idea: Newton's third law states that for every action there is an equal and opposite reaction. A balloon rocket makes this visible: air rushes out the back (action), and the balloon shoots forward (reaction) — the same principle that launches real rockets.

Caution: do not inflate balloons near your face; stop if a balloon looks overstretched. Keep the flight path clear of people.

Materials: 3–4 long balloons, about 4 m of string or fishing line, a plastic straw, tape, two chairs, a measuring tape, a phone stopwatch.

Procedure:

1. Thread the string through the straw, then tie the string taut between two chairs — this is your flight track.
2. Inflate a balloon but do not tie it — pinch the neck shut. Tape the balloon to the straw (two pieces of tape, balloon neck pointing backward).
3. Pull the balloon-straw assembly to one end of the string. Release the neck and let it fly!
4. Measure the distance traveled and the time of flight with the stopwatch. Repeat 3 trials with the same balloon size.
5. Repeat with a half-inflated balloon and a fully inflated balloon (3 trials each).
6. Compute average speed for each condition: speed = distance ÷ time.

Conclusion questions:

1. Identify the action and the reaction in your balloon rocket.
2. How did balloon size affect speed and distance? Explain using Newton's third law.
3. Why does the rocket eventually stop? Name at least two forces acting against it.

Experiment 7: Marble Collisions — Conservation of Momentum

Question: In a collision, is momentum really conserved? Roll a marble down a ruler ramp into a stationary marble and watch: the first marble slows (or stops) while the second shoots away. Momentum (mass × velocity) is conserved in the collision: what the first marble loses, the second gains. This is the same physics behind pool breaks and car crashes.

Procedure: (1) Make a ramp from a ruler propped on books, ending at a flat floor with a meter stick alongside. (2) Place marble A at the ramp's bottom and marble B a short distance ahead, both on the meter stick line. (3) Release marble A from a marked height and film the collision with a phone in slow motion. (4) Estimate each marble's speed from the video (distance between floor marks ÷ time) or compare travel distances as a proxy. Try: equal masses; heavy marble into light marble; light marble into heavy marble. Safety: keep marbles off walkways — they are a slipping hazard.

Analysis: With equal masses, the moving marble nearly stops and the target takes nearly all the speed — momentum transfer is almost complete. A heavy marble hitting a light one keeps going while the light one flies; a light marble hitting a heavy one bounces back. In every case, total momentum after is close to total momentum before; the small losses are friction and sound. Conclusion: state whether momentum was conserved within your measurement error, and name the main sources of error (friction, timing, off-center hits).

Try it yourself: A 20 g marble at 1.0 m/s hits a stationary 10 g marble and stops. How fast does the small marble go (ideal)? Answers: 20 × 1.0 = 10 × v, so v = 2.0 m/s.

Key vocabulary: Momentum — mass × velocity; conservation of momentum — total momentum constant in a closed collision; elastic collision — kinetic energy also conserved.

Experiment 8: Sound Waves with a Ruler (and a Phone)

Big idea: Sound is a vibration traveling through air. When you pluck a ruler overhanging a desk, the vibrating part creates a tone — and the shorter the vibrating length, the higher the pitch (frequency). You will map the relationship between length and pitch, just as instrument makers do.

Caution: hold the ruler firmly so it cannot snap back; keep fingers away from the vibrating edge.

Materials: a 30 cm plastic or wooden ruler, a desk or table, a phone with a free frequency-analyzer / tuner app (e.g. a guitar-tuner app), a ruler or measuring tape.

Procedure:

1. Place the ruler on the desk with 25 cm overhanging the edge. Hold the desk end down firmly.
2. Pluck (flick) the overhanging end downward and release. Listen to the pitch and, if your app supports it, record the measured frequency in Hz.
3. Repeat with overhangs of 20 cm, 15 cm, 10 cm, and 5 cm. Pluck with the same strength each time.
4. Do 2 trials per length and record your observations (pitch description and/or Hz).

Conclusion questions:

1. What is the relationship between vibrating length and pitch? Express it as a rule ("As length ___, pitch ___").
2. How does this explain why a violin (short strings) sounds higher than a cello (long strings)?
3. What is vibrating to make the sound — and what carries that vibration to your ear?

Experiment 9: String Telephone — Sound Through Solids

Question: Can sound travel through a solid string better than through air? Build two paper cups joined by a taut string: one person talks into a cup while the other listens. Sound is a vibration; the cup's bottom vibrates with the speaker's voice, the string carries the vibration, and the other cup reproduces it at the listener's ear. If the string goes slack, the telephone dies — the string must be taut to transmit the wave.

Procedure: (1) Poke a small hole in the bottom of each of two paper cups, thread a long string through, and knot it (a paperclip inside stops it pulling through). (2) Walk apart until the string is taut — 10 to 20 meters is possible outdoors. (3) One person speaks normally into a cup (mouth close, not touching); the other holds the cup to their ear. (4) Test variations: slack vs. taut string; string touching a wall or a finger (does it still work?); a wire instead of string; whispering. Safety: watch for tripping on the string; do not run while holding it.

Analysis: The taut string transmits vibrations efficiently because solids are stiffer than air — sound actually travels faster in solids (about 5,000 m/s in steel vs. 343 m/s in air). Touching the string damps the vibration and kills the signal, exactly like touching a guitar string stops its note. Conclusion: sound needs a medium, and the medium's stiffness and tension control how well it travels — the principle behind stethoscopes and submarine sonar.

Try it yourself: Why does the telephone fail when the string touches a fence? Why must you not hold the string in the middle? Answers: Contact damps the vibration (energy leaks into the fence); holding it absorbs the wave, like muting a string.

Key vocabulary: Vibration — rapid back-and-forth motion; medium — the material a wave travels through; damping — loss of vibrational energy to surroundings.

Experiment 10: A Simple Electric Circuit

Big idea: An electric circuit is a closed loop that lets charge flow from a power source, through a load (like an LED), and back. You will build one with batteries and an LED, discover that LEDs only work in one direction (polarity), and test which household materials conduct electricity.

Caution (adult supervision required): use only AA or AAA batteries — never a wall outlet. Do not connect the two battery ends directly with wire (a short circuit makes batteries hot). If a battery feels warm, disconnect everything immediately. Keep water away and work with dry hands.

Materials: 2 AA batteries, battery holder if available (or tape to hold batteries end-to-end), one LED (from an electronics kit, LED tea light, or old string lights — note the longer leg), insulated wire with stripped ends (or strips of aluminum foil folded into "wires"), electrical tape, test objects: paperclip, coin, pencil (wood and graphite tip), rubber band, plastic spoon.

Procedure:

1. Place the two batteries end-to-end (positive to negative) and tape them into a "battery pack" — this is your power source (~3 V).
2. Connect one wire from the battery pack's positive end to the LED's longer leg (the anode). Connect another wire from the LED's shorter leg back to the battery pack's negative end.
3. The LED should light. If not, reverse the LED legs and try again.
4. Record: does the LED light in both directions or only one? This is polarity.
5. Now break the loop on one side and insert a test object (paperclip, coin, pencil tip, rubber band, spoon) to complete the circuit. Record whether the LED lights for each.
6. Classify each test object as a conductor (LED lights) or insulator (LED stays dark).

Conclusion questions:

1. Why must the circuit be a closed loop for the LED to light?
2. What does the one-way behavior of the LED tell you about how it differs from a regular light bulb?
3. Look at your conductor/insulator results — what do the conductors have in common? (Hint: think about the materials.)

Experiment 11: Homemade Compass — Earth's Magnetic Field

Question: Can you build a compass from a needle? A magnetized needle, free to rotate, aligns with Earth's magnetic field — that is all a compass is. Magnetize a steel needle by stroking it in one direction with a strong magnet about 50 times (always the same direction), float it on a small piece of cork in a bowl of water, and watch it settle pointing north-south.

Procedure: (1) Stroke the needle one way with a magnet, 50 strokes. (2) Test magnetization: does it pick up a paperclip? (3) Float it on cork in still water, away from metal objects and electronics. (4) Let it settle; mark the direction with a known compass or the Sun's position. (5) Test interference: bring a magnet or a phone near — the needle swings away, showing that local fields overwhelm Earth's weak field. Safety: needles are sharp — handle by the eye end, store in cork, and keep away from young children. Keep magnets away from phones and cards.

Analysis: Earth's magnetic field is weak (about 50 microteslas), so the needle must be nearly frictionless — hence floating on water. The needle points to magnetic north, which is near (but not exactly at) geographic north; the difference is called magnetic declination and varies by location. Conclusion: explain why the needle aligns (torque from Earth's field), why it must float freely, and why nearby metal distorts readings — the reason real compasses are used away from cars and buildings.

Try it yourself: Why stroke the needle in one direction only? What happens if you stroke back and forth? Answers: One-way stroking aligns the magnetic domains; stroking back and forth cancels the alignment and demagnetizes it.

Key vocabulary: Magnetize — aligning magnetic domains to make a magnet; magnetic declination — the angle between magnetic and geographic north; compass — a free-rotating magnet aligning with Earth's field.

Experiment 12: Series vs. Parallel Circuits — Bulb Brightness

Big idea: The same two bulbs behave very differently wired in series versus in parallel. In series, the same current flows through both and the voltage is shared; in parallel, each bulb gets the full battery voltage and draws its own current. Brightness reveals the power each bulb receives, so your eyes become the measuring instrument.

Hypothesis: Two bulbs in parallel will each be as bright as a single bulb, while two bulbs in series will each be dimmer than a single bulb.

Materials: 2 AA batteries in a holder, 2 identical small bulbs in holders (or LEDs with resistors — ask an adult), 4–6 wires with clips, a switch (optional). Caution: use only low-voltage batteries, never wall power; disconnect the battery when rewiring.

Procedure:

1. Build the single-bulb circuit: battery → bulb → battery. Note the brightness (call it "normal").
2. Rewire the two bulbs in series: battery → bulb 1 → bulb 2 → battery. Record each bulb's brightness.
3. Rewire the two bulbs in parallel: each bulb has its own path to the battery. Record brightness.
4. Unscrew one bulb in each arrangement and observe what happens to the other bulb.
5. Repeat each arrangement twice to check your observations are consistent.

Analysis questions:

1. Why are series bulbs dimmer? (Hint: total resistance rises, so total current falls.)
2. Why does removing one series bulb kill the other, while removing one parallel bulb does not?
3. Why is household wiring parallel, not series? What would happen to your house if one appliance "burned out" in a series house?

Try it yourself: Three identical bulbs in series vs. three in parallel: predict the relative brightness before testing. Answers: Series: each gets 1/3 of the voltage — very dim. Parallel: each gets full voltage — full brightness (but the battery drains three times faster).

Real-world application: Holiday-light strings, car headlights, and home wiring all depend on choosing series vs. parallel correctly.

Key vocabulary: Series circuit — one path, shared current; parallel circuit — separate paths, full voltage each; brightness — a visible proxy for electrical power.

Experiment 13: The Inclined Plane — Measuring Acceleration

Big idea: Galileo used inclined planes to slow down falling motion enough to measure it. A ball rolling down a gentle ramp accelerates steadily, and you can measure that acceleration with nothing more than a ruler, a stopwatch, and the equation d = ½at² (starting from rest).

Hypothesis: A ball rolling down a steeper ramp will have a greater acceleration, and the distance it travels will grow with the square of the time.

Materials: a smooth board or plank (1–2 m), books to raise one end, a marble or ball, a ruler or tape measure, a phone stopwatch, tape for marking distances.

Procedure:

1. Prop one end of the board on books so it forms a gentle slope. Mark start and finish lines 1.00 m apart.
2. Release the ball from rest at the start line (do not push) and time it to the finish line. Repeat 3 times and average.
3. Repeat for distances of 0.25 m, 0.50 m, and 0.75 m (same slope), timing each 3 times.
4. Raise the slope (more books) and repeat the 1.00-m timing.
5. Compute acceleration for each run: a = 2d/t².

Analysis questions:

1. Is your computed acceleration roughly constant across distances on the same slope? If not, what caused the variation (reaction time? friction? rolling vs. sliding)?
2. Does the distance-time pattern fit d ∝ t²? Check: does quadrupling the distance double the time?
3. Why did Galileo use ramps instead of dropping balls? What limits your measurement most?

Try it yourself: If a ball takes 2.0 s to roll 1.0 m from rest, find its acceleration and its speed at the finish. Answers: a = 2(1.0)/4.0 = 0.50 m/s²; v = at = 1.0 m/s.

Real-world application: Highway runaway-truck ramps, wheelchair-access ramps, and crash-test sleds all use inclined-plane physics.

Key vocabulary: Acceleration — rate of change of velocity; reaction time — the delay in starting/stopping a stopwatch; constant acceleration — velocity changing by equal amounts each second.

Experiment 14: Hooke's Law — Stretching Springs

Big idea: Hooke's law, F = kx, says a spring's stretch is proportional to the force pulling it — until it is overstretched. By hanging known weights on a spring and measuring its stretch, you can find its spring constant k and build a working force meter.

Hypothesis: Doubling the hanging weight will double the spring's extension.

Materials: a coil spring or rubber band, a ruler, a clamp or hook to hang the spring from, identical weights (e.g., 100-g masses, or equal stacks of coins), tape.

Procedure:

1. Hang the spring and mark the starting position of its bottom end (zero load). Record the ruler reading.
2. Add one weight. Wait for it to stop bouncing, then record the new bottom position.
3. Add weights one at a time (up to 5), recording the position each time.
4. Remove the weights one at a time and check the spring returns to its original position.
5. Compute extension = position − starting position for each load, and force = mass × 9.8 (or just use "number of weights" as force units).

Analysis questions:

1. Is F/x roughly constant? Plot F vs. x — is the graph a straight line through the origin?
2. What is your spring constant k (in N/m)? If you used a stiffer spring, how would the graph change?
3. Did the spring return exactly to its starting position? If not, what happened (elastic limit)?

Try it yourself: A spring stretches 4.0 cm under a 2.0-N load. Find k, then predict the stretch under 5.0 N. Answers: k = F/x = 2.0/0.040 = 50 N/m; x = 5.0/50 = 0.10 m = 10 cm.

Real-world application: Bathroom scales, car suspensions, and spring balances all rely on Hooke's law.

Key vocabulary: Hooke's law — F = kx; spring constant — stiffness, k; elastic limit — the stretch beyond which a spring stays deformed.

Experiment 15: Shadows, Reflection, and the Pinhole Camera

Big idea: Light travels in straight lines. That single fact explains shadows, pinhole images, and why a mirror swaps left and right. In this experiment you will build a pinhole camera — a light-tight box with a tiny hole — and watch it project an upside-down image of the world.

Hypothesis: The pinhole camera will form an inverted image that gets larger when the screen moves farther from the pinhole.

Materials: an empty cereal box or oatmeal canister, aluminum foil, a pin, tracing paper or wax paper, tape, a ruler, a bright window or lamp, a small mirror.

Procedure:

1. Cut a hole in one end of the box and cover it with foil. Poke a single clean pinhole in the foil's center.
2. Cover the other end with tracing paper (the screen). Seal all light leaks with tape.
3. In a dim room, point the pinhole at a bright window or lamp. Observe the screen.
4. Sketch the image: is it upright or inverted? Left-right reversed?
5. Move the screen farther from the pinhole (use a longer box or slide the screen) and note how the image size changes.
6. With the mirror, check the law of reflection: shine light at the mirror and compare the angles of incoming and outgoing beams (use a flashlight and a protractor if available).

Analysis questions:

1. Why is the image upside down? (Trace straight-line rays from the top and bottom of the object through the pinhole.)
2. What happens if you make the pinhole bigger — brighter or sharper? Why is there a trade-off?
3. Why does a mirror image seem to swap left and right but not top and bottom?

Try it yourself: A tree 6.0 m tall stands 12 m from a pinhole camera whose screen is 0.20 m behind the hole. How tall is the image? Answers: Similar triangles: image height = 6.0 × (0.20/12) = 0.10 m = 10 cm, inverted.

Real-world application: Camera lenses, the human eye, and solar-eclipse viewers all work on the pinhole principle.

Key vocabulary: Rectilinear propagation — light travels in straight lines; pinhole camera — image formed by light through a tiny hole; law of reflection — angle of incidence = angle of reflection.

Experiment 16: Heat Conduction Race — Which Material Insulates Best?

Big idea: Some materials let heat flow quickly (conductors) while others slow it down (insulators). Metals feel cold to the touch not because they are colder, but because they conduct heat away from your hand faster. In this experiment you will race heat through different materials and rank them.

Hypothesis: Metal will conduct heat fastest, and foam, fabric, or cardboard will insulate best.

Materials: identical cups (metal, plastic, foam), hot water (adult supervision required), a thermometer or kitchen thermometer, a stopwatch, samples of materials to test (foil, cloth, cardboard, bubble wrap), an adult helper for hot water.

Procedure:

1. Pour equal amounts of hot water into the three cups. Record the starting temperature of each.
2. Record the temperature every 2 minutes for 20 minutes. Do not stir or move the cups.
3. Plot temperature vs. time for each cup on the same axes.
4. Optional: wrap identical cups in different materials and repeat, comparing cooling rates.
5. Control variables: same water volume, same starting temperature, same room, same timing.

Analysis questions:

1. Which cup cooled fastest? Rank the materials from best conductor to best insulator.
2. Why do foam cups work — what is trapped inside foam that insulates so well?
3. Why does a metal spoon in hot soup burn your fingers while a wooden spoon does not?

Try it yourself: Two identical cups of hot water: one wrapped in foil, one in a towel. Predict which cools slower and why. Answers: The towel — trapped air in fabric is a poor conductor, while foil is metal and also lets heat radiate. (Test it!)

Real-world application: Thermos flasks (vacuum insulation), winter coats (trapped air), and building insulation all exploit conduction control.

Key vocabulary: Conduction — heat transfer by particle collisions; conductor / insulator — materials that let / resist heat flow; control variable — a factor kept the same for a fair test.

Experiment 17: Static Electricity — Balloon and Paper

Big idea: Rubbing a balloon on hair transfers electrons, leaving the balloon negatively charged. Like charges repel, opposite charges attract — and a charged object can attract a neutral one by polarizing it. With a balloon, you can bend water, pick up paper, and make hair stand up, all without a battery.

Hypothesis: A rubbed balloon will attract small neutral objects and repel another identically charged balloon.

Materials: 2 balloons, a wool sweater or hair, small bits of paper, a thin stream of water from a tap (adult permission), an empty soda can, a ruler.

Procedure:

1. Rub a balloon on wool or hair for 10 seconds. Hold it near (not touching) small paper bits. Record what happens.
2. Hold the charged balloon near a thin stream of water. Record the deflection.
3. Charge two balloons the same way. Hang them by strings and bring them close. Record what happens.
4. Place an empty can on its side on a smooth table. Bring the charged balloon near it (no contact). Record what happens.
5. Repeat on a humid day vs. a dry day if possible, and compare.

Analysis questions:

1. Why does the balloon attract neutral paper? (Hint: charges inside the paper shift slightly — polarization.)
2. Why do the two balloons repel each other?
3. Why does static electricity work better on dry days? What does humidity do to charge?

Try it yourself: If you rub a balloon and it picks up 10 paper bits at 1 cm away, predict what happens at 3 cm. Answers: Fewer bits, or none — the force weakens rapidly with distance (Coulomb's law: F ∝ 1/r²).

Real-world application: Photocopiers, electrostatic precipitators (cleaning smoke), and lightning rods all use static-charge physics.

Key vocabulary: Static charge — an imbalance of electric charge; polarization — shifting charges inside a neutral object; Coulomb's law — force ∝ q₁q₂/r².

Experiment 18: Build and Test an Electromagnet

Big idea: An electric current creates a magnetic field. Coil a wire around an iron nail and the fields add up into a magnet you can switch on and off — an electromagnet. Its strength depends on the current, the number of turns, and the core.

Hypothesis: More coil turns and higher current will each make the electromagnet stronger (able to lift more paper clips).

Materials: a long iron nail, ~2 m of insulated copper wire (enameled or plastic-coated), 1 AA battery (or 2), paper clips (count them), tape, sandpaper (to strip wire ends — adult supervision), a switch if available. Caution: disconnect the battery when not measuring; the wire can get warm — if it gets hot, disconnect immediately.

Procedure:

1. Strip about 2 cm of insulation from each wire end.
2. Wrap the wire tightly around the nail 20 times. Tape the coil so it stays put.
3. Touch one wire end to each battery terminal for a few seconds. Count how many paper clips the nail tip can lift.
4. Rewind with 40 turns and repeat. Then try 2 batteries in series (with adult permission) and repeat.
5. Test with the nail removed (air core) vs. with the nail (iron core).

Analysis questions:

1. How did clips lifted change with turns? With batteries? With the core? Which factor mattered most?
2. Why does the iron core help? (Iron's magnetic domains align with the coil's field.)
3. Why must you disconnect the battery between trials? What happens to the magnetism when current stops?

Try it yourself: Predict: 60 turns with 1 battery vs. 20 turns with 2 batteries — which lifts more? Test it. Answers: Strength roughly ∝ turns × current; doubling turns and doubling current have similar effects, so the two may be close — the experiment decides.

Real-world application: Scrapyard cranes, MRI machines, door locks, and maglev trains all use electromagnets.

Key vocabulary: Electromagnet — a magnet made by current in a coil; core — the material inside the coil; solenoid — a coil of wire.

Experiment 19: Water Waves in a Tray — Reflection and Refraction

Big idea: Water waves in a shallow tray behave like light waves: they reflect off barriers, refract when the depth changes, and pass through gaps to diffract. A ripple tank lets you see wave behavior that is invisible with light.

Hypothesis: Waves will reflect off a straight barrier at the same angle they arrive, and will bend (refract) when they cross into shallower water.

Materials: a shallow baking tray or clear plastic container, water, a ruler or pencil (to make waves), a small block or book (to create a shallow region), a straight barrier (ruler or cardboard), a flashlight (optional, for shadows on paper below).

Procedure:

1. Fill the tray with about 1 cm of water on a white surface. Let it settle completely.
2. Dip a pencil tip rhythmically to make straight waves. Sketch the wavefronts.
3. Place a straight barrier diagonally in the tray. Send waves at it and sketch the reflected wavefronts. Estimate the incoming and outgoing angles.
4. Place a flat block under the water to make one half of the tray shallower. Send waves across the depth change and sketch what happens to wavelength and direction.
5. Put two small gaps in a barrier (double slit) and send waves through. Sketch the pattern beyond the barrier.

Analysis questions:

1. Did the angle of reflection equal the angle of incidence? What does this predict for light on a mirror?
2. In shallower water, waves slow down. Did the wavelength shrink? Did the frequency change? (Frequency is set by the source.)
3. Behind the double gap, where do waves add up and where do they cancel? How is this like light's double-slit experiment?

Try it yourself: If water waves slow from 20 cm/s to 10 cm/s crossing into shallow water while the frequency stays 2 Hz, find the wavelength on each side. Answers: Deep: λ = v/f = 10 cm. Shallow: λ = 5 cm. Wavelength halves; frequency unchanged.

Real-world application: Harbor design (breakwaters diffract waves), ultrasound imaging, and seismic surveys all use wave reflection and refraction.

Key vocabulary: Wavefront — a line of equal wave phase; refraction — bending when wave speed changes; diffraction — spreading through a gap; interference — waves adding or canceling.

Experiment 20: Design Your Own Experiment — Science Fair Capstone

Big idea: You now have the full toolkit: variables, fair tests, data tables, graphs, and conclusions. In this capstone you choose your own question, design a fair test, run it, and report it like a scientist.

Hypothesis: Your chosen experiment will have a clear, testable prediction — for example, "If the drop height increases, then the bounce height will increase, because the ball starts with more energy." You will state yours in step 1 of the procedure.

Your task: Pick one question from the list (or invent your own with an adult's approval), write a hypothesis, identify your variables, and run the experiment with at least 3 trials per condition. Suggested questions: Does the drop height change a ball's bounce height? Does water temperature change how fast sugar dissolves? Does string length change a pendulum's period? Does wing size change a paper airplane's flight?

Materials: whatever your chosen experiment needs (household items only; no fire, no mains electricity, no sharp hazards without adult supervision).

Procedure:

1. Write your question and hypothesis: "If [independent variable] changes, then [dependent variable] will [prediction], because [reason]."
2. List your variables: independent (what you change), dependent (what you measure), controlled (what you keep the same).
3. Design a data table with room for 3 trials per condition and an average column.
4. Run the experiment. Record every result — including surprises and mistakes.
5. Graph your averaged results (bar graph for categories, line graph for a changing quantity).
6. Write a conclusion: Was your hypothesis supported? What would you change next time? What new question did this raise?

Analysis questions:

1. What was the biggest source of error, and how could you reduce it?
2. Did any trial disagree with the others (an outlier)? What might explain it — and should you keep it or repeat it?
3. If a friend repeated your experiment from your report alone, could they? What detail would they need that you forgot?

Try it yourself: Write a hypothesis for this question: "Does the mass of a pendulum bob change its period?" Then state the independent, dependent, and two controlled variables. Answers: "If the bob's mass increases, the period will stay the same, because gravity accelerates all masses equally." Independent: bob mass. Dependent: period. Controlled: string length, release angle.

Real-world application: This is exactly how professional science works — a clear question, a fair test, honest data, and a conclusion that invites the next question.

Key vocabulary: Independent / dependent / controlled variables — what you change / measure / hold constant; outlier — a result far from the rest; replication — repeating an experiment to check a result.