Direction of the friction force

[joej24]

Homework Statement

A crate is located in the center of a flatbed truck. The truck accelerates to the east and the crate moves with it, not sliding at all. What is the direction of the friction force exerted by the truck on the crate?

a) it is to the west
b) it is to the east
c) No friction force exists be cause the crate is not sliding.

Homework Equations

F net= ma

The Attempt at a Solution

Since the crate is accelerating to the east, I thought that the friction force must be in the opposite direction. But the answer is B) east

This is the book solution. I don't quite understand why still after reading it.

​

The crate accelerates to the east because the only horizontal force

​

acting on it is the force of static friction between its bottom surface

​

and the truck bed, that force must also be directed to the east.

I understand that the only horizontal force acting on the crate is static friction but not why the direction of friction is east.

 

[panchoman]

The crate exerts a westward force on the truck to prevent it from sliding. Now, Newtons 3rd law suggests that there must be an equal and opposite force, in this case, the truck will be applying an eastward force on the crate with the same magnitude. It's one of the weirs things that are hard to conceptualize. Think of this scenario, you have something really heavy on your head. And you move the right. The object will be exerting a friction force to the left so that it can stay on your head, but you'll feel yourself applying a force to the right so that your head and body stay together.

 

[PhanthomJay]

Homework Statement

A crate is located in the center of a flatbed truck. The truck accelerates to the east and the crate moves with it, not sliding at all. What is the direction of the friction force exerted by the truck on the crate?

a) it is to the west
b) it is to the east
c) No friction force exists be cause the crate is not sliding.

Homework Equations

F net= ma

The Attempt at a Solution

Since the crate is accelerating to the east, I thought that the friction force must be in the opposite direction. But the answer is B) east

This is the book solution. I don't quite understand why still after reading it.

​

The crate accelerates to the east because the only horizontal force

​

acting on it is the force of static friction between its bottom surface

​

and the truck bed, that force must also be directed to the east.

I understand that the only horizontal force acting on the crate is static friction but not why the direction of friction is east.

The book answer explained it well. Since F_net =ma, and the net force must always be in the direction of the acceleration, and static friction is the only force acting, in what direction must be the static friction force? A couple of tips here: The crate and truck are accelerating east at the same rate with respect to the ground. Always use Newton's first 2 laws with respect to the ground. Note also that friction always opposes the relative motion or pending motion between the 2 surfaces in contact. The pending motion of the crate to move with respect to the truck is backwards if the acceleration is high enough, thus, friction acts forward in this case, even though with respect to the ground, the crate's motion is forward. This can oft be confusing.

 

[joej24]

Note also that friction always opposes the relative motion or pending motion between the 2 surfaces in contact. The pending motion of the crate to move with respect to the truck is backwards if the acceleration is high enough

Why does the crate move west if the acceleration is high enough?

 

[PhanthomJay]

As long as there is friction between the crate and truck bed, the crate always moves east with respect to the ground, whether there is no acceleration, low acceleration, or high acceleration. It moves west relative to the truck bed, only when the truck's acceleration is high enough such that the friction force acting on it exceeds the maximum available static friction force, and the crate starts to slip relative to the truck bed. Then kinetic friction force now acts on it, keeping it accerating to the east with respect to the ground, at an acceleration that is different than the truck's acceleration.

 

[joej24]

Thanks! I understand now