Crate motion question

1. Jul 7, 2006

Hollysmoke

Just wanted to make sure if I did this right:

Two crates of mass 12kg and 20kg are pushed across a smooth floor together, the 20kg crate in front of the 12kg crate. Their acceleration is 1.75m/s^2. What is the action reaction force between the crates?

F=ma
F=(32)(1.75)
F=56N

then F for the 12kg block = 21N, 56-21=35N, therefore the force between the crates is 35N

2. Jul 7, 2006

arildno

It is not "F", the applied force on the 12kg crate that is 21N. The applied force F on this crate is 56N!!

However, m*a for the 12kg crate equals 21N, and hence, the reaction force R between the two crates has magnitude 56N-21N=35N

3. Jul 7, 2006

borisleprof

Hi Hollysmoke,
Since the 20 kg crate is in front of the 12 kg crate, the 12kg crate must push the other crate with a force that is sufficient to communicate an acceleration of 1.75 m/s^2.
Force that must act on the 20 kg crate must be equal to it's mass times its acceleration = 20kg x 1.75 m/s^2 = 35N.

bye