Required practical 19 · Physics · Physics Paper 2, both tiers
Force, mass and acceleration
Investigate the effect of varying the force on the acceleration of an object of constant mass, and the effect of varying the mass with a constant force.
The exam trap. Adding new masses to the hanger changes the mass being accelerated too. a = Δv ÷ t: divide by time once.
Variables
- Independent (you change)
- Resultant force (hanging weight), or the mass of the system
- Dependent (you measure)
- Acceleration, from light gates or ticker tape
- Control (you keep the same)
- Total mass of the system when force is varied
- Force when mass is varied
- The same runway, angle and light gate positions
Apparatus
- Trolley and runway
- Pulley, string and slotted masses
- Two light gates and data logger (or ticker timer)
- Balance
- Card to break the light beams
Method, and why each step matters
Attach the trolley to a string over a pulley with a hanging mass.
Why The weight of the hanging mass provides the accelerating force.
Set two light gates a measured distance apart to record the trolley's velocity at each.
Why a = change in velocity ÷ time between the gates.
Release the trolley and record the acceleration.
Why Light gates remove human reaction time.
To vary force, move masses from the trolley to the hanger.
Why This changes the force but keeps the total mass of the system constant.
To vary mass, add masses to the trolley and keep the hanging mass the same.
Why Only one variable changes at a time.
Tilt the runway slightly so the trolley just keeps moving on its own.
Why This compensates for friction.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Falling masses. | Place a cushion or box under the hanger. |
| Trolley running off the bench. | Put a stop at the end of the runway. |
A worked set of results
| Force in N | Acceleration in m/s² |
|---|---|
| 0.2 | 0.40 |
| 0.4 | 0.79 |
| 0.6 | 1.21 |
| 0.8 | 1.58 |
| 1.0 | 2.02 |
- Acceleration is directly proportional to force at constant mass (F = m a, here m = 0.5 kg).
- At constant force, acceleration is inversely proportional to mass.
Mistakes that cost marks
Squaring the time because acceleration is measured in m/s².
Instead a = Δv ÷ t: divide by time once.
Changing the hanging mass without keeping the total mass constant.
Instead Move masses between trolley and hanger so the system mass is unchanged.
Ignoring friction, so accelerations come out lower than F ÷ m.
Instead Tilt the runway to compensate, and say so in the evaluation.