Acceleration, force and mass
GCSE Physics (8463) · Required practical 7 — method, variables, the marks examiners report students losing.
Investigate how the acceleration of an object depends on the resultant force on it (at constant mass) and on its mass (at constant force), verifying F = m x a.
Apparatus
- Dynamics trolley on a runway or linear air track
- Light gates and a data logger (or ticker tape and timer) to measure acceleration
- String over a pulley at the end of the bench, with a mass hanger and slotted masses
- Extra masses to load onto the trolley, and a balance
Method
- 1Set up the trolley on the runway with a string over a pulley to a hanging mass; tilt the runway slightly to compensate for friction (until a tapped trolley moves at steady speed).
- 2To investigate force: keep the total mass of the system constant by moving masses from the trolley onto the hanger; measure the acceleration with the light gates for each force.
- 3To investigate mass: keep the hanging mass (the force) constant and add masses to the trolley; measure the acceleration each time.
- 4Record the acceleration for each force and for each mass.
- 5Plot acceleration against force, and acceleration against mass (or against 1 / mass).
Variables
Independent
Resultant force (part a) or mass (part b)
Dependent
Acceleration of the trolley
Control
- Total mass of the system (when varying force)
- Resultant force (when varying mass)
- The runway and its friction compensation
Results & processing
- Acceleration against force is a straight line through the origin: acceleration is proportional to resultant force at constant mass.
- Acceleration against mass is a curve (acceleration is inversely proportional to mass); a graph of acceleration against 1 / mass is a straight line. Both confirm F = m x a.
Where students lose marks
Not compensating for friction.
Fix: Tilt the runway until the trolley moves at constant velocity when tapped, so friction does not add a hidden systematic error.
Changing the total mass when investigating force.
Fix: Move masses from the trolley to the hanger rather than adding them, so the total mass being accelerated stays constant.
Inaccurate timing of the trolley.
Fix: Use light gates with a data logger (and a card of known length) rather than a hand-held stopwatch.
Improve the method
- Friction-compensate the runway before taking readings.
- Use light gates and a data logger for accurate acceleration measurements.
- Transfer masses (do not add them) when varying the force, and repeat each reading.
Try it — exam-style
A resultant force of 6.0 N acts on a trolley of mass 1.5 kg. Calculate its acceleration.
When investigating how force affects acceleration, explain why masses are moved from the trolley onto the hanger rather than simply added to the hanger.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on acceleration, force and mass?
This practical is really a Newton's second law question with a fair-test twist — I drill the variable control and the F = m x a maths, and your first lesson is free.