9 (a) A student investigates the relationship between force and acceleration for a trolley on a runaway - Edexcel - GCSE Physics - Question 9 - 2018 - Paper 1

The mass of the moving system can be kept constant by ensuring that the weight attachable to the trolley, such as the weight hanger, remains unchanged throughout the experiment. This can involve consistently using the same weights throughout the entirety of the experiment.

To improve the procedure for compensating for the effects of friction, the student could raise one end of the runway to allow the trolley to roll freely. Additionally, using a low-friction surface for the runway or adding lubricant to minimize friction could also enhance the accuracy of the measurements. Finally, measuring and subtracting the frictional force from the total force could provide a clearer understanding of the net forces acting on the trolley.

In the collision between objects Q and R, momentum is conserved due to Newton's third law, which states that for every action, there is an equal and opposite reaction. Therefore, the momentum lost by object Q will be equal to the momentum gained by object R. This can be represented mathematically as:

$m_Q v_Q + m_R v_R = m_Q v_{Q}' + m_R v_{R}'$

where the initial and final momenta of the objects account for their respective masses and velocities before and after the collision.