# Homework Help: Force exerted on an object

1. May 11, 2010

### fcb

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

A warehouse worker applies a force of 420 N to push two crates across the floor. One crate is 30kg and the other is 40kg. The boxes are touching each other.

The friction forces opposing the motion of the crates is a constant 2.0N for each kilogram..

Edit: The question is; Calculate the force exerted by the 40kg crate on the 30 kg crate

2. Relevant equations

F=ma

3. The attempt at a solution

Just substituted the answers in correctly taking into consideration the friction of 2.0N and got 240N but the answer at the end of the book saids 280N

I am not sure where i went wrong.

Last edited: May 12, 2010
2. May 11, 2010

### thebigstar25

is it 70kg or 40kg?

3. May 12, 2010

### fcb

Sorry, It was 30kg and 40kg

4. May 12, 2010

### phyzmatix

Draw a force diagram (it doesn't have to be very complicated) with the force exerted by the warehouse worker on the boxes in one direction and the total force of the resistive friction in the other (as these are opposing forces). Since the two boxes are touching each other, you can view them as a single entity.

What is the total friction for the system? How can you use this, in light of your diagram, to calculate the force of box 'a' on box 'b'?