Forces on blocks with 3 different masses.

  • #1

Homework Statement



Blocks with masses of 4.0 kg, 8.0 kg, and 9.0 kg are lined up in a row on a frictionless table. All three are pushed forward by a 15 N force applied to the 4.0 kg block.

1.) How much force does the 8.0 kg block exert on the 9.0 kg block?

2.)How much force does the 8.0 kg block exert on the 4.0 kg block?

Homework Equations



F=ma

The Attempt at a Solution



I found the acceleration of the blocks to be .71 m/s^2 because I did F/m=a or 15/(4+8+9)=.71

I don't know where to go from there or if that is even correct so far. Please let me know when you get a chance! Thank you!
 

Answers and Replies

  • #2
LowlyPion
Homework Helper
3,090
4
OK. You found the acceleration of the system.

Now they are asking what force is accelerating the individual blocks.

Look at the blocks in isolation. You know that net of whatever forces there are, the block is being accelerated by the common acceleration of the system.

So how much is needed to accelerate the 4 kg block? What is the excess of the 15N that goes to accelerating the other blocks to the other side? And so on. And so on.
 
  • #3
Thank you for the response, but I guess I don't know what I should be doing mathematically.
 
  • #4
LowlyPion
Homework Helper
3,090
4
Thank you for the response, but I guess I don't know what I should be doing mathematically.
You found the acceleration of the system.

So net force F = m*a on the first block. What is that?

If that is the net force and you know how much is applied (15N) then just subtract that from 15 and that's what it's transmitting to the the next block.

Rinse and repeat.
 
  • #5
Ohhhhhh! I get it now, lol! Thank you! =]
 
  • #6
303
0
I have a very similiar problem that I am working on but I guess I do not understand what is going on. I see how you go acceleration.

For the first part is all I do is take the acceleration that I found and multiply by the mass of the last block, then take that force and subtract it from the given force?
 

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