Kinetic energy tells you how much energy a moving object has, but it can’t tell the whole story. Today you’ll meet a second quantity that can, and see exactly where it agrees with energy and where it doesn’t.
🎯 Goals
- Calculate momentum from mass and velocity, and solve for velocity or mass.
- Use + and − signs for direction and find the total momentum of two objects.
- Measure the momentum of a rolling cart as its speed and mass change.
- Compare how momentum and kinetic energy scale, and explain why they can rank objects differently.
🧰 Materials
🚗The cart or toy car, ramp, and 1.5 m timing zone from Activity 4-1-2
🪙Loads to add mass (coins or washers) and tape
📐Meter stick or tape measure, plus masking tape
⏱️Stopwatch or phone timer
⚖️A scale that reads in grams
📊Optional: your Lab A and Lab B data from Activity 4-1-2, or the sample data on this page
🧮Calculator
📓Physics notebook
Section 1 of 4
Gut Check: Who’s Hardest to Stop?
1
Rank the five moving objects by how hard they would be to stop.
These are the same objects from Activity 4-1-2. Without calculating, write them in your notebook from easiest to hardest to stop.
p = m × v
Momentum is mass times velocity. Mass is in kilograms and velocity is in meters per second, so momentum is in kg·m/s.
2
Calculate the momentum of each object.
Enter each answer. A correct answer is within 2%.
Momentum Builder
Calculate p for each object.
📓 Physics Notebook
Compare this ranking with the kinetic-energy ranking you made in Activity 4-1-2. Which two objects swapped places? Explain in a sentence why momentum and kinetic energy disagree about them.
Section 2 of 4
Direction Matters
3
Choose a positive direction.
Momentum has a direction, because velocity does. Pick one direction as positive, usually right or forward. Anything moving the other way gets a negative velocity, and so a negative momentum. To find the total momentum of several objects, add their signed momenta.
4
Practice with signs.
Enter each answer with its sign. Zero is a valid answer.
Sign Practice
Take right as the positive direction.
✅ Midpoint check: the Momentum Builder and all four Sign Practice rows are green. Next you’ll measure momentum on a real cart.
Section 3 of 4
Cart Lab: How Does Momentum Scale?
5
Set up the same ramp, cart, and 1.5 m timing zone you used in Activity 4-1-2.
If you still have your Lab A and Lab B data from that activity, reuse it. Otherwise, run each setting twice and average the times, or use the sample data below.
Sample data (use only if you don’t have your own)
Table A: cart mass
0.120 kg. Release 5 cm →
0.85 m/s; 10 cm →
1.20 m/s; 20 cm →
1.70 m/s.
Table B: release 10 cm. Mass
0.120 kg →
1.20 m/s;
0.170 kg →
1.18 m/s;
0.220 kg →
1.16 m/s.
6
Table A: change the speed, keep the mass.
Release the cart from 5, 10, and 20 cm. For each, find the speed (v = 1.5 m ÷ time), then the momentum p = mv and the kinetic energy KE = ½mv².
7
Table B: change the mass, keep the release height.
Release from 10 cm with no load, then one load, then two loads. Measure the total mass each time, find the speed, then p and KE.
8
Find the scaling rules.
Work out how momentum and kinetic energy change for each case, then check your answers against your tables. The 5 cm and 20 cm rows of Table A are the key test for speed.
Momentum vs. Kinetic Energy
For each change, how many times as big do the momentum and the kinetic energy become? Enter a number or a fraction like 1/4. Check your answers against your Table A and Table B.
📓 Physics Notebook
In Table A, the release height went up 4 times from 5 cm to 20 cm. By about how many times did the speed go up, the momentum, and the kinetic energy? Why do the momentum and the kinetic energy change by different amounts?
Section 4 of 4
Connect It: Why Momentum Matters
9
Compare a truck and a baseball.
A loaded truck rolls slowly through a parking lot. Calculate its momentum and kinetic energy, then compare them with the baseball from Section 1 (p = 5.8 kg·m/s, KE = 116 J).
Truck vs. Baseball
A 10,000 kg truck at 2.0 m/s.
📓 Physics Notebook
Which would take more effort to bring to a stop, the truck at 2.0 m/s or the baseball at 40 m/s? Use both momentum and kinetic energy in your answer. Then predict: what two things could you change to make a hard-to-stop object easier to stop? (You’ll test your idea in Activity 5-1-2.)
Question 1
Use Table A to explain how the momentum and the kinetic energy of the cart changed when its speed doubled. Include your numbers.
Question 2
Use Table B to explain why momentum grew with mass even though the speed stayed about the same.
Question 3
A baseball and a bowling ball have almost the same kinetic energy. Compare their momenta, and explain which you would rather try to stop.
Question 4
Two carts moving toward each other can have a total momentum of zero. Does that mean they have no energy? Explain.
📤 Turn In
- Your gut ranking, your Momentum Builder results, and your note on which objects swapped places
- Your Sign Practice results
- Completed Table A and Table B, with your note on how momentum and kinetic energy scale
- Your truck-vs.-baseball comparison and your idea for making something easier to stop
- Written answers to the four Conclusion questions