Three blocks Newtons law help have I got my answers and sign correct ?

[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

 

[Doc Al]

What is the net horizontal force on block B? =15.4 N

Careful. They want the net force, not the force that A exerts on B.

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

Good, but just say 7.7 N. (Block C exerts a force of 7.7 N on Block B--and vice versa.)

 

[aruji73]

wouldnt it be -7.7 because of negative direction

so is the horizontal force on block B 7.7 ?

 

[Doc Al]

wouldnt it be -7.7 because of negative direction

Just give the magnitude of the force.

so is the horizontal force on block B 7.7 ?

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

 

[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

 

[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.

 

[Doc Al]

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

OK. So apparently they want you to include the direction (via the sign), and they are taking positive to mean 'to the right'.

so how come that one can be -15.4 and this one is just 7.7

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.