Quick answer: Torque is the turning effect of a force, τ = r F sin θ, measured in newton metres. This AP Physics 1 lab on torque 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.
How does the turning effect of a load change with its distance from a pivot? You balance a fixed 0.200 kg load against a lighter 0.100 kg mass on a metre rule pivoted at its centre, change the position of the load, and test whether the torques balance each time.
Level: AP Physics 1 (algebra-based) · Curriculum: AP P1 Unit 5
Aim
To test that a rule balances when the clockwise and anticlockwise torques about the pivot are equal, and to find how the balancing distance depends on the load distance.
Equipment
- Metre rule with a knife-edge pivot or a ruler resting on a pencil at the 50.0 cm mark
- Load of 0.200 kg and a counterweight of 0.100 kg, each on a loop of thread
- Tape to hold the threads in place
Safety
Work over a table, not near the edge, so falling masses cannot hit your feet. Keep the masses small and stand clear while the rule is adjusted.
Method
- Balance the unloaded rule on the pivot and check that it stays horizontal.
- Hang the 0.200 kg load at r = 0.050 m from the pivot on the left.
- Slide the 0.100 kg mass along the right side until the rule balances. Record its distance d from the pivot.
- Repeat for r = 0.100, 0.150, 0.200 and 0.250 m.
- Calculate the torque on each side using τ = m g × distance.
Results
Sample measurements. Complete the two torque columns using g = 9.81 N/kg.
| r (m) | d (m) | Left torque (N m) | Right torque (N m) |
|---|---|---|---|
| 0.050 | 0.101 | ||
| 0.100 | 0.199 | ||
| 0.150 | 0.305 | ||
| 0.200 | 0.398 | ||
| 0.250 | 0.503 |
Analysis questions
Question 1 Left and right torques
Complete the table by calculating the torque of the load (0.200 kg at r) and of the counterweight (0.100 kg at d) for each trial.Question 2 Percentage difference
Calculate the percentage difference between the two torques for the first and last trials and state whether the rule is in equilibrium within experimental error.Question 3 Graph of d against r
Describe the graph of d against r. Calculate the gradient using the first and last points and compare it with the ratio of the masses.Question 4 Reading uncertainty
The distances are read to ±0.002 m. Calculate the percentage uncertainty in the torque of the load for r = 0.050 m.Question 5 Why the pivot is at the centre
Explain why the pivot is placed at the centre of the rule and what would happen if it were not.Question 6 Predict a new balance
Predict the distance d needed to balance the 0.200 kg load placed at r = 0.180 m.Question 7 Improving the method
Propose two changes that would reduce the uncertainty in the balance distances.
