Question 1

Why does a frictionless trolley not necessarily have zero acceleration?

Accepted Answer

A frictionless track removes only the friction force. Any non-zero resultant from tension, an applied push, or another external force can still accelerate the trolley. Acceleration is zero only when the resultant of all external forces is zero.

Question 2

Which forces belong in the trolley's free-body diagram?

Accepted Answer

Only forces acting on the isolated trolley belong in its free-body diagram. These can include its weight, normal reaction, string tension, applied push, friction, and a bumper force. Forces exerted by one part of the trolley on another are internal and must not be drawn.

Question 3

Why is friction drawn opposite the direction of motion?

Accepted Answer

The relevant direction is not simply the direction of the trolley's motion. Kinetic friction acts opposite the direction of relative sliding of the trolley on the track. In this lab the trolley moves toward positive x, so the friction arrow points toward negative x.

Question 4

Why are impulse and momentum-change arrows coincident?

Accepted Answer

For the isolated trolley, the impulse-momentum theorem gives J = Δp = p_after - p_before. Therefore J and the finite momentum change Δp are the same vector, so their arrows must have equal direction and may be labelled separately only when they are visually distinguishable.

Question 5

Which forces form the collision action-reaction pair?

Accepted Answer

The force exerted by the bumper on the trolley is paired with the force exerted by the trolley on the bumper. They are equal in magnitude and opposite in direction, but they act on different bodies and therefore do not cancel when drawing the free-body diagram of only one body.

Newton's Trolley Lab — Forces, Friction & Impulse

About this lab

Practice