Quick answer: Momentum is the product of an object's mass and its velocity, p = mv, measured in kg m/s. This Physics 9-10 lab on momentum gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Does the total momentum of two trolleys stay the same when one hits the other and they couple? You will time a trolley before and after a collision, calculate the momenta, and compare them, including the effect of friction and timing uncertainty. It can be done with toy cars at home.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS2-2, HS-PS2-3
Aim
To test whether the total momentum of two trolleys is the same before and after they couple together.
Equipment
- Two dynamics trolleys or toy cars with Velcro or plasticine on the ends
- Smooth, level runway at least 1.5 m long
- Kitchen scale or balance (to 0.01 kg)
- Metre rule and masking tape
- Phone with slow-motion video, or a stopwatch
- Spare masses to add to a trolley
Safety
Keep trolleys away from the table edge, catch them with a soft stop at the end of the runway, and keep feet clear of the floor path.
Method
- Measure the mass of each trolley and add masses where needed.
- Stick three tape lines on the runway: line 1, line 2 and line 3, each 0.50 m apart.
- Place trolley B at rest on line 2.
- Push trolley A so it moves steadily through line 1 and hits B.
- Film the run and read the time A takes from line 1 to line 2 (time before).
- Read the time the coupled pair takes from line 2 to line 3 (time after).
- Repeat with different masses and speeds, and record every run.
Results
Sample measurements (distance for each timed section 0.50 m, time read to ±0.02 s).
| Run | Mass A (kg) | Mass B (kg) | Time before (s) | Time after (s) |
|---|---|---|---|---|
| 1 | 0.50 | 0.50 | 0.40 | 0.84 |
| 2 | 0.50 | 0.50 | 0.55 | 1.14 |
| 3 | 0.50 | 1.00 | 0.45 | 1.40 |
| 4 | 1.00 | 0.50 | 0.45 | 0.70 |
Analysis questions
Question 1 Speeds in run 1
1. Calculate speed before and after for run 1.2. Repeat for run 3.
Question 2 Momentum before and after
Calculate the total momentum before and after the collision for each of the four runs.Question 3 Percentage retained
Calculate the percentage of the initial momentum found after the collision in each run, and the mean.Question 4 Conclusion
State whether the results support conservation of momentum. Use the percentages to justify your answer.Question 5 Why less than 100 %
Explain why every run gives slightly less momentum after the collision.Question 6 Timing uncertainty
The time is read to ±0.02 s. Calculate the percentage uncertainty in the time before for run 1 and in the time after for run 1.Question 7 Improving the lab
Suggest two changes that would improve the accuracy of the results.
