RP7: Force, Mass and Acceleration

Acceleration is the change in velocity divided by the corresponding elapsed time. Suitable light-gate arrangements can estimate a speed from an interrupting flag length and its blocking time, then compare speeds at two positions using the time between the relevant measurements. State what the logger actually measures; the time to block one gate is not automatically the time between gates.

Alternatively, for motion genuinely starting from rest with approximately constant acceleration, a measured travel distance and elapsed time give a = 2s/t². This model is unsuitable if the trolley is pushed on release or if acceleration varies substantially. A speed calculated as distance divided by the whole journey time is an average speed and is not the final speed of uniformly accelerating motion.

Newton’s second law relates resultant external force to the mass of the chosen system. In a trolley–pulley investigation, moving slotted masses from the trolley to the hanging holder can increase driving weight while keeping total moving mass approximately constant. The system includes the trolley, holder and moving loads; it is not just the trolley body. Friction and pulley effects mean the hanging weight is not automatically an exact measured resultant force.

For an investigation of mass at constant driving force, keep the driving arrangement unchanged while adding moving mass in a controlled way. For a ramp investigation, increasing trolley mass also increases the downhill gravitational component at the same slope. In an ideal frictionless model, these increases cancel when calculating acceleration. Do not call the ramp’s gravitational driving force constant simply because the angle stayed the same.

At constant system mass, acceleration is proportional to resultant force in the Newtonian model. At constant resultant force, acceleration is inversely proportional to mass; an a-against-1/m plot can test that relationship. Clearly label calculated and directly measured quantities. A force-against-acceleration slope estimates mass, whereas an acceleration-against-force slope estimates its reciprocal.

Use a consistent release without a push, check track alignment and use appropriate motion sensors. Repeat measurements at each setting and retain anomalous results with an explanation rather than deleting inconvenient points. A repeatable frictional force can create a systematic departure from the simplest model; faster timing equipment alone does not remove that force. Follow the centre’s track-end and falling-load precautions.

AQA RP7 requires the effect of varying force on an object of constant mass and the effect of varying mass at constant force. Write two variable tables and explain which mass and which resultant force belong to each system. The horizontal trolley–pulley example develops these choices; follow the centre’s approved arrangement and evaluate friction.

For a constant-total-mass force comparison, transfer masses within the same moving system rather than adding extra mass from outside it. For the constant-force comparison, keep the driving arrangement unchanged while adding the specified system mass. Measure suitable velocities and elapsed times, or justify another valid acceleration method, before plotting the relationship.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.