Consider a box that is placed on different surfaces. a) In which situation (s) i

Question

Consider a box that is placed on different surfaces.

a) In which situation (s) is there no force acting on the box?
b) In which situation (s) is there a static friction force acting on the box?
c) In which situation (s) is there a kinetic friction force acting on the box?

Various possible situations are as under:

I. The box is at rest on a rough horizontal surface.
II. The box is at rest on a rough tilted surface.
III. The box is on the rough-surfaced flat bed of a truck- the truck is moving at a
constant velocity on a straight, level road and the box remains in the same
place in the middle of the truck bed.
IV. The box is on the rough-surfaced flat bed of a truck- the truck is speeding up
on a straight, level road, and box remains in the same place in the middle of
the truck bed.
V. The box is on the rough-surfaced flat bed of a truck- the truck is climbing a
hill, and the box is sliding towards the back of the truck.

Explanation / Answer

a) none. even though the box is not accelerating, it can still have some force on it, just no Net Force. Even in Situation I, there is still gravity and normal force.

b) static friction means that the force of friction is equal to the external movement force and the box remains still. this occurs in II, IV.

c) Kenetic friction will act at a constant rate for any force that exceeds static friction, hence the box will move. this occurs in V.

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