# Homework Help: Three blocks newtons law help have I got my answers and sign correct ?

1. Jan 22, 2013

### aruji73

Three blocks, each of mass 7.0 kg are on a frictionless table. A hand pushes on the left most box (A) such that the three boxes accelerate in the positive horizontal direction as shown at a rate of a = 1.1 m/s2.

What is the net horizontal force on block B? =15.4 N
What is the horizontal force on block B due to block C?= -7.7N

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2. Jan 22, 2013

### Staff: Mentor

Careful. They want the net force, not the force that A exerts on B.
Good, but just say 7.7 N. (Block C exerts a force of 7.7 N on Block B--and vice versa.)

3. Jan 22, 2013

### aruji73

wouldnt it be -7.7 because of negative direction

so is the horizontal force on block B 7.7 ?

4. Jan 22, 2013

### Staff: Mentor

Just give the magnitude of the force.

Yes, that's the net horizontal force on B.

5. Jan 22, 2013

### aruji73

in the same problem i had a question
What is the horizontal force on block A due to block B?
i got -15.4 and I got it right
so how come that one can be -15.4 and this one is just 7.7

6. Jan 22, 2013

### haruspex

Each of the three must have a net force of +7.7N. The "horizontal force on block A due to block B" is somewhat ambiguous, but it seems they mean the actual force exerted by B on A. As far as A is concerned it is pushing B and C, so the force between A and B must be enough to accelerate both B and C at 1.1m/s2.

7. Jan 23, 2013

### Staff: Mentor

OK. So apparently they want you to include the direction (via the sign), and they are taking positive to mean 'to the right'.
I assume you are talking about "What is the horizontal force on block B due to block C?". Using the same convention as above, that force would have to be negative as well.