1. Jan 8, 2015

### bomberkid

1. The problem statement, all variables and given/known data
. question is as attached. answer is 2f/5..how do u get this?
2. Relevant equations
F=ma
F= (5m)a
a= F/(5m)

3. The attempt at a solution
gotten a=F/5m , but why the answer is 2f/5?

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2. Jan 8, 2015

### Joffan

What force is necessary to produce your calculated acceleration in the second block?

3. Jan 8, 2015

F?

4. Jan 8, 2015

### Joffan

No... you calculated the specific $a$, now put that back into $F=ma$ along with the second block's mass

Unfortunately the confusion between the abstract "F" and the specific "F" in this problem is perhaps making understanding a little harder than it should be.

5. Jan 9, 2015

### bomberkid

I see. Why not use the mass 3m?

6. Jan 9, 2015

### haruspex

Either will do provided you bring all relevant forces acting on the mass into the equation.

7. Jan 9, 2015

### bomberkid

but if i were to use f=3m, and a is F/5m, i wouldnt get 2f/5 as the answer

8. Jan 9, 2015

### haruspex

You get the right answer if you bring in all forces acting on the mass and do the algebra correctly. Please post your working.