Is there no force acting on the box?

[Rashid101]

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.

 

[Kreamer]

Considering the force of gravity acting on the box in all situations, I would assume that answer A (no force acting on the box) is not an answer for any situation unless the box is floating off in space far away from any other objects.

However if the answer meant "In which situation (s) is there no frictional force acting on the box?" I would say any object at rest that would remain at rest if it were placed on a frictionless surface.

Static friction is the friction that holds and object at rest where Kinetic friction is the friction that acts to slow down an object already set in motion.

In the first situation (I. The box is at rest on a rough horizontal surface) it doesn't really matter if the surface is rough or frictionless. If it is sitting on a horizontal surface that box has no net force that will get it moving.

Second situation (II. The box is at rest on a rough tilted surface. ) is similar to the first, however, now the surface is tilted. If the surface were frictionless then the box would simply slide down the surface no problem. However the Static friction between the box and the rough surface keeps it in place.

Third situation (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.) may be a bit confusing to understand. You may think that if the surface were frictionless the box would slide across the flat bed. However the truck is staying at a constant speed and level road. The box is moving at the same speed as the truck already, there is no net force causing the box to slide. (Think about yourself on a plane. That plane flying through the air is going about 500mph. That means you are also flying through the air that fast. While the plane is at constant speed you do not find yourself fighting to stand in the aisle, the box does not fight to stay in that one place on the truck bed.)

Forth situation (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.) can be thought of kind of like the last problem. Imagine yourself on a plane as it is speeding down the runway (before leaving the ground so we are still horizontal like in the problem). You can feel yourself being pushed back into the seat, you feel that force against you yet the seat keeps you in the same place. The same works for the box. The static friction is like the seat of the airplane keeping it in that position as the truck speeds up.

Last situation (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). For consistency let's go back to our plane example. The plane is taking off (speeding up and climbing in altitude). You were unlucky enough to be caught in the aisle during takeoff. You start to feel the same force others feel while they are in their seats but you do not have that seat holding you still. You fall to the ground and start sliding to the back of the plane while grabbing for anything to slow you down. The box again does the same thing. The acceleration and incline caused the box to start moving which gets that kinetic friction going. That friction does its best to slow the box down.

So if the box feels a force but does not move, static friction kept it in its seat.
If the box felt a force and starts moving then (assuming it is not a frictionless surface) the kinetic friction tries to stop it from sliding to the back of the plane.

I am a very visual learner and examples like this help me a lot. Hope this helps you too.