Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equations: F = ma, a = Δv/t
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Aim
To investigate the effect of varying the force on the acceleration of an object of constant mass, and the effect of varying the mass of an object on its acceleration with a constant force.
Method
- Attach a trolley to a string that runs over a pulley at the end of the bench to a hanging mass holder.
- Measure the acceleration using light gates (or a stopwatch and marked distances).
- Varying force: start with masses on the trolley. Move them one at a time to the hanger, measuring the acceleration each time. This keeps the total mass constant.
- Varying mass: keep the hanging mass the same and add masses to the trolley.
- Plot acceleration against force (or against mass).
| Independent variable | Force (weight on the hanger) – or mass |
| Dependent variable | Acceleration |
| Control variables | Total mass (when varying force); force (when varying mass); same runway |
Questions
Q1 (F) When varying the force, name the independent and dependent variables.
Show answer
Independent: the force (weight of the hanging masses). Dependent: the acceleration of the trolley.
Q2 (F) Why are masses moved from the trolley to the hanger, rather than new masses added?
Show answer
So the total mass being accelerated stays the same – only the force changes, making it a fair test.
Q3 (F) State one hazard and how to reduce the risk.
Show answer
Falling masses could hurt your feet – put a box of soft material under the hanger. Stop the trolley before it reaches the pulley so it doesn’t fall off the bench.
Q4 (F) The trolley and masses have a total mass of 0.50 kg. The accelerating force is 1.0 N. Calculate the acceleration.
Show answer
F: F = m a
I: 1.0 = 0.50 × a
F: a = 1.0 ÷ 0.50
A: 2.0 m/s²
Q5 (F/H) Light gates record the trolley’s velocity as 0.20 m/s and then 0.80 m/s, 0.50 s later. Calculate the acceleration.
Show answer
F: a = Δv / t
I: a = (0.80 − 0.20) ÷ 0.50
A: 1.2 m/s²
Q6 (F/H) What results would you expect for each part of the practical?
Show answer
Acceleration is directly proportional to the resultant force (at constant mass). Acceleration is inversely proportional to the mass (at constant force) – doubling the mass halves the acceleration.
Q7 (F/H) Why are light gates better than a stopwatch?
Show answer
They remove the error caused by human reaction time, so the timings (and so the acceleration) are more accurate.
Q8 (H) The measured accelerations are all lower than F = ma predicts. Suggest why, and how to improve the experiment.
Show answer
Friction acts on the trolley (and at the pulley), so the resultant force is less than the weight on the hanger. Tilt the runway slightly so the trolley just keeps moving at a steady speed when pushed – this compensates for friction.
