Question 7
In the trolley experiment, why must the rubber band be stretched by the same amount every time?
The right answer is the one marked above.
To make sure the force applied stays the same. How far a rubber band is stretched shows how much force it applies. Same stretch, same force.
In the second part of the experiment we change the mass and watch what happens to the acceleration. For that, the force must stay fixed — otherwise there is no telling what caused the acceleration to change.
In a good experiment only one thing changes at a time. The trolley's mass is controlled by what you put on it, not by the rubber band.