Two blocks attached

1. Jan 18, 2014

thonwer

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

Two blocks with masses m1 and m2, m2>m1, are attached, I push from the left and from the right side, which force is bigger? The surface is frictionless.

http://imagizer.imageshack.us/v2/800x600q90/194/wkft.png [Broken]

2. The attempt at a solution

I think it's bigger when applied from the left because of the reaction of m2 is bigger than the reaction of m1 because m2>m1, but I don't know any equations to corroborate it.

Last edited by a moderator: May 6, 2017
2. Jan 18, 2014

tiny-tim

hi thonwer! welcome to pf!
i don't understand the question, there seems to be something missing

why can't the forces be whatever you want?

3. Jan 18, 2014

thonwer

The force I have to consider is the arrow drawn in each case because it is what they ask me, the picture is part of the formulation.
What I mean on the question is, in one case I push from the left, in other case I push from the right. In which case the force is bigger so as to move the blocks?

4. Jan 18, 2014

tiny-tim

hi thonwer!
but the picture shows the same letter, F, for both forces, which presumably means that they are equal

do you mean which acceleration is bigger?

5. Jan 18, 2014

thonwer

Ok sorry, I did not explain myself quite well, the Force applied is the same in both cases.

The correct question would be, which reaction force is bigger?

6. Jan 18, 2014

tiny-tim

ahhh!

well, as you probably know, the acceleration is the same in both cases

so do good ol' Newton's second law for the block that the force F isn't on in each case …

what do you get?

7. Jan 18, 2014

thonwer

I didn't understand you sorry, English is not my first language,
what did you want to say with "do good ol' Newton's second law for the block that the force F isn't on in each case" ?

8. Jan 18, 2014

tiny-tim

sorry!

in the first picture, the force F is on the left, pushing on the smaller block

so do the equation "force = mass times acceleration" (ie Newton's second law) for the larger block

(and then do the same for the smaller block in the second picture)

9. Jan 18, 2014

thonwer

For 1: F-R=m1*a -> R=F-m1*a

For 2: F-R=m2*a -> R=F-m2*a

F is the same, a is the same => m1*a<m2*a so R in 1 is bigger

Is it correct?

10. Jan 18, 2014

tiny-tim

yes!

(though it would have been slightly quicker to do the other block in each case:

For 1: R=m2*a

For 2: R=m1*a )​