# Crate on a box problem

1. Aug 22, 2010

### homyungtj

1. The problem statement, all variables and given/known data

You and a friend have just loaded a 200 kg crate filled with priceless art objects into the back of a 2000 kg truck. As you press down on the accelerator, force F(surface on truck) propels the truck forward. To keep things simple, call this Ft. What is the maximum magnitude Ft can have without the crate sliding? The static and kinetic coefficients of friction between the crate and the bed of the truck are 0.8 and 0.3.

That is the original question.

My question is when I look at the free body diagram of the truck on the x axis, Ft is moving forward (towards east). That I understand.
But friction by crate on truck is moving backward (towards west of FBD).

I'm thinking the crate is on the truck. IN terms of Newton's third law, yes it makes sense.
But when you're thinking the crate is on the truck, how could it pull (force definition) the
truck backwards, affecting Ft?

2. Relevant equations

3. The attempt at a solution

2. Aug 23, 2010

### ehild

If the crate does not slip it has to move together with the truck. The truck is accelerating. So does the crate, and its acceleration is the same as that of the truck. What force accelerates the crate? The same force, but with opposite sign acts at the truck from the crate.

ehild

3. Aug 23, 2010

### homyungtj

First of all, thanks for the reply ehild.
your explanation helps me better to understand this problem,

let me just paraphrase what you explained just so I understand your point.

1) crate moves together with the truck. So it has to have same acceleration
2) because it's accelerating, there must be force.
3) so what is that force?

and I don't think I understand
"The same force, but with opposite sign acts at the truck from the crate."

because what I don't understand is how can a crate "on the truck"
possibly pull the truck backwards (according to FBD of the textbook)
when it makes sense explained by your way (Newton's third law).

4. Aug 23, 2010

### ehild

Yes, it is Newton's third law. There is an interaction between the truck and the crate through static friction. This force acts against the accelerating force in case of truck. At the same time, the force of static friction accelerates the crate.

ehild