Quick answer: Newton's laws link force and motion. The first law says zero net force means constant velocity, the second says ΣF = ma, and the third says forces come in equal and opposite pairs acting on different objects. This AP Physics 1 lab on Newton's laws 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.
A hanging mass pulls a trolley along a bench. By timing a one-metre run for different pulling forces while the total mass stays constant, you test Newton’s second law and measure the mass of the system and the friction force.
Level: AP Physics 1 (algebra-based) · Curriculum: AP P1 Unit 2
Aim
To show that the acceleration of a system is proportional to the net force on it when the total mass is kept constant, and to find the mass of the system from a graph.
Equipment
- dynamics trolley with a track or smooth bench
- pulley with clamp and string
- mass hanger with 10 g slotted masses
- metre rule and stopwatch (or a phone video with a frame counter)
- balance
Safety
Place a cushion on the floor under the hanging masses and a block at the end of the track so the trolley cannot fall off the bench.
Method
- Use the balance to set the total moving mass (trolley, extra masses and hanger) to M = 0.600 kg.
- Mark start and stop lines 1.00 m apart.
- Put m = 0.020 kg on the hanger and the remaining slotted masses on the trolley, so the total stays 0.600 kg.
- Release the trolley from rest and time the motion from start to stop. Repeat three times and take the mean.
- Move 0.020 kg from the trolley to the hanger and repeat, up to 0.100 kg.
Results
Mean times for the 1.00 m trip. Calculate a using a = 2d ÷ t².
| m (kg) | F = mg (N) | t (s) | a (m/s²) |
|---|---|---|---|
| 0.020 | 0.196 | 2.77 | 0.26 |
| 0.040 | 0.392 | 1.85 | 0.58 |
| 0.060 | 0.589 | 1.48 | 0.91 |
| 0.080 | 0.785 | 1.27 | 1.24 |
| 0.100 | 0.981 | 1.13 | 1.57 |
Analysis questions
Question 1 Acceleration
Show that a = 2d ÷ t² follows from constant acceleration starting from rest, and check the value for m = 0.060 kg.Question 2 The graph
Calculate the gradient of the a against F graph using the first and last points.Question 3 System mass
Use the gradient to find the mass of the system and compare with 0.600 kg.Question 4 The intercept
Find the force at which the line crosses the F axis and interpret it.Question 5 Constant total mass
Explain why masses were moved from the trolley to the hanger instead of simply adding masses to the hanger.Question 6 String tension
Calculate the tension in the string for m = 0.100 kg and compare it with the weight of the hanging mass.Question 7 Improvement
Suggest two ways to reduce the effect of friction and of timing error.
