Alarge crate of mass m is placed on the back of a truck but not tied down. as the truck accelerates forward with an acceleration a, the crate remains at rest relative to the truck. what force causes the crate to accelerate forward?
a) normal force
b) force of gravity
c) force of friction between the crate and the floor of the truck
d) the "ma" force
e) none of these

Option C

Explanation: Let's analyze each possible force:

A)normal force, it pushes in the direction in which the box has contact with some surface, in this case, is in the bottom of the truck, so this force pushes upwards.

B) Force of gravity always pushes downwards, so it can not accelerate the box in the same direction in which the truck is moving.

c) The force of friction points in the opposite direction in which the box would move, if the truck moves to the right, the box will move to the left, so the friction pushes also to the right, which is the same direction in which the truck is moving.

D) there is nothing called the "ma" force.

Now, the correct option would be C; the force of friction (the statical one, because we know that the box does not move)