6 (a) A student investigates the relationship between force and acceleration for a trolley on a runway - Edexcel - GCSE Physics Combined Science - Question 6 - 2018 - Paper 1

The mass of the moving system can be kept constant by transferring equivalent amounts of mass between the trolley and the weight hanger. This means that if the student adds weight to the weight hanger, they should remove the same amount of mass from the trolley. This ensures that the total mass of the system remains unchanged.

To compensate for the effects of frictional forces acting on the trolley, the student could raise one end of the runway. By doing so, the trolley would roll down the incline at a constant speed, reducing the additional forces of friction acting on it. This adjustment will help ensure that the only significant force is that of gravity, allowing for more accurate measurements of acceleration.

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. Before the collision, the total momentum is given by the momentum of Q plus the momentum of R. During the collision, forces act on the objects but are equal and opposite, ensuring that the change in momentum of Q is equal to the negative change in momentum of R. Hence, the total momentum before the collision is equal to the total momentum after the collision, described mathematically as:

$ext{Change in momentum of Q} = - ext{Change in momentum of R}$

This conservation holds true as long as no external forces act on the system, adhering to the principles outlined in Newton's second law.