# 2 blocks being pulled across the floor

1. Jul 16, 2007

1. The problem statement, all variables and given/known data
A block of mass m1 = 2 kg and a block of mass m2 = 3 kg are tied together and are pulled from rest across the floor by a force of P = 22 N. The coefficient of friction of the blocks with the floor is µ = 0.26.
m1 is on left, pulling force on right

2. Relevant equations

What is the tension in the string between the blocks?

3. The attempt at a solution

I've already got the acceleration of the 2 blocks (first part of prob).
This is [22N/(2kg+5kg)] - [5*9.8*.26] = 1.85m/s/s = acceleration of whole system.

I'm stuck on the tension though...but heres what i did.
F=ma
F=2*1.85 = 3.7N

Then to get the opposing (frictional) force i went
F= µn
F= .26 (2*9.8) = 3.7 = 5.1N

Then i find the difference to get net force: 5.1 - 3.7 = 1.4N

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

2. Relevant equations

3. The attempt at a solution

2. Jul 16, 2007

### Staff: Mentor

OK.

This is the net force on m1.

OK.

That's not the net force!

What other force acts on m1?

Realize that F = ma is the net force. Identify (on a diagram) all individual forces acting on each block. The sum of those forces on each block must equal the net force on that block from F = ma.

3. Jul 16, 2007

all forces acting on m1 are weight, normal, tension and friction
If F=ma is the net force on m1, doesn't that = the pulling tension in the string?

4. Jul 17, 2007

### Staff: Mentor

Right. The weight and normal force cancel, but the others do not.

No. The tension is just one of the forces acting on the block. How can it equal the net force?