How Much Force Does a 40kg Crate Exert on a 30kg Crate?

[fcb]

Homework Statement

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

Homework Equations

F=ma

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.

Could someone please help me out.

 

[thebigstar25]

is it 70kg or 40kg?

 

[fcb]

is it 70kg or 40kg?

Sorry, It was 30kg and 40kg

 

[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'?