Force on Block A: 100N Left & Right?

[eprex]

So this was a question that I had on a previous test and I'm looking for a definitive answer.

The question is what horizontal force/s is/are acting on block A. I thought it was 100N to the left and 100N to the right, however some other students had argued it was only 100N to the right.

I'm confused as to why block A would remain on block B if there was only a force acting to the right.

 

[Doc Al]

The question is what horizontal force/s is/are acting on block A. I thought it was 100N to the left and 100N to the right, however some other students had argued it was only 100N to the right.

I'd say neither is correct. The 100 N force acts on block B, not on block A. (What force acts on block A?)

I'm confused as to why block A would remain on block B if there was only a force acting to the right.

The entire system (assuming they move together) is accelerating to the right. So there had better be a force acting to the right on block A.

 

[eprex]

I forgot to mention that there is no acceleration. Okay so you agree there us a force directed to the right on A. You disagree with the aforementioned conclusions though?

 

[Doc Al]

I forgot to mention that there is no acceleration.

Ah, big difference.

Okay so you agree there us a force directed to the right on A.

No. But since there's no acceleration, there must be a force to the left acting on B. Friction from the surface, I presume?

You disagree with the aforementioned conclusions though?

Yes.

(Perhaps you should state the entire problem, word for word as it was given to you. That way you can get a definitive answer.)

 

[eprex]

I don't have it on me but basically a force of 100N is pushing block B to the right at a constant speed. The question asks what the proper free body diagram for A is. The answers are:

100N to the right
100N to the left and 100N to the right
Two 100N forces to the right
No force at all

Seems to me it's either one or two.

 

[Doc Al]

If it were "100N to the right", block A would accelerate.
If it were "100N to the left and 100N to the right", what would produce those forces on A?

 

[eprex]

I thought it was 100n to the right and 100n to left. The pull to the right being 100n and the friction being 100n to the left, hence no acceleration. It seems me there has to be a counter force of some kind and not simply the initial pull only

 

[Doc Al]

I thought it was 100n to the right and 100n to left. The pull to the right being 100n and the friction being 100n to the left, hence no acceleration. It seems me there has to be a counter force of some kind and not simply the initial pull only

The 100 N pulling force shown acts on block B, not block A. There must be another force on block B, from friction perhaps, to give a net force of 0 on block B.

Block A is moving along at constant speed. There's no horizontal force acting on A.

 

[eprex]

Wow, so we were all wrong haha. That's pretty interesting. I understand why the net force is zero, but it's hard to conceptualize that it's moving without any force.

 

[Doc Al]

Wow, so we were all wrong haha. That's pretty interesting.

Yep. The best answer is "no force at all".