Finding the Force on Block 1 by Block 2: A Homework Problem

[mybrohshi5]

Homework Statement

http://www.public.asu.edu/~gbadams/f06/F424.gif

Block1 = 2kg
Block2 = 3kg
Block3 = 4kg

What is the force on block 1 by block 2

Homework Equations

F=ma

The Attempt at a Solution

i know this is easy but i can't figure it out.

i found the acceleration of the 3 blocks

8=9a
a=0.889

i know the net force on block 1 is 0.889(2kg) = 1.78

I know the answer is 6.22N but i am not sure how to find it.
any help would be great.

thank you

 

[ehild]

"i know the net force on block 1 is 0.889(2kg) = 1.78"

Very good. You know that a force of 8 N acts to the first block. So what other force should be present to make the net force 1.78 N?

ehild

 

[mybrohshi5]

a 6.22N force should be present to add to the 1.78N net force to equal the 8N force in the opposite direction so they cancel out to zero correct?

 

[ehild]

No. 8 N - 6.22 N = 1.78 N causes 0.889 m/s^2 acceleration of the first block. Where comes this 6.22 N come from?

ehild

 

[mybrohshi5]

"where comes this 6.22 N come from?"

i do not quite get what you are asking here?

i understand that Fnet = Force applied + (force of block 2 on block 1)
1.78N = 8N + x

solve for x to be -6.22N or just 6.22N to the left and that is the force on block 1 by block 2

 

[ehild]

OK. You solved the problem.

 

[mybrohshi5]

Thank you for your help and explanations :)