A large 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

Please explain the right answer! Thanks.

Question

A large 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

Please explain the right answer! Thanks.

anonymous · Answer

@saifoo.khan hi.

saifoo.khan · Answer

I think it's C!

anonymous · Answer

"Please explain the right answer! Thanks."

anonymous · Answer

Well, the engine make a force, this force make the weels spin, the friction of the weels with the ground make the truck go forward, now some interaction with the truck makes the crate stay in place, what kind of force can the truck be making on the crate? 
Come on guys, do not give just answers let the guy get them by himself!

anonymous · Answer

Drawing a picture and labeling all those forces on it helps.

anonymous · Answer

This drawing tool sucks but I'll try.

anonymous · Answer

Well the truck provides the crate with Normal force and friction force, but i'm not sure which one it is, because when the truck moves forward, doesn't inertia cause the crate to move backwards?

anonymous · Answer

@some1 inertia tends to make the crate move backwards, but that motion is being resisted by something. Normal force acts perpendicular to the surface, so the normal force acts upward; it can't influence it's sideways motion directly.

anonymous · Answer

@ivanmlerner , you don't have to draw it here. Do it on paper, so you can see it right in front of you.

anonymous · Answer

Oh all right, so the crate pushes on the truck's floor while the truck's floor pushes back on the crate, allowing the crate to move forward along with the truck. Correct?

anonymous · Answer

Yes inertia does that, thats why we need a force to counter inertia. And inertia makes the thing remain stopped, whats moving is hte truck. Think about the direction of those two forces that involve the truck and the crate. What creates them?

anonymous · Answer

Yes, there is an action-reaction interaction, but what is the action force?

anonymous · Answer

Dont give answers! Let him think!

anonymous · Answer

The crate pushing on the floor. the reaction force is the floor pushing back on the crate. So it's the force of friction between the crate and the floor of the truck. Right?

anonymous · Answer

Right on! The acceleration is horizontal, so the force must be horizontal. The normal force is up, the gravity (weight) force is down, and "the 'ma' force" is meaningless.

anonymous · Answer

All right, thanks! I have another problem, so i will post a new question.