# Stupid Question

1. Oct 11, 2005

### Cyrus

My friends a pre-med student and hes taking basic phyiscs 1. He asked me to do two problems. Can you double check to make sure I did not tell him the wrong thing!

Problem 1.)

There are two square masses, side by side on a frictionless pond. Mass A is 150kg, and mass b, is 50kg. A force of 100N is applied at mass A, find the reaction force that mass B exerts on mass A.

So I said on block A: simga(F): (150)a = 100N - F

and on block B: sigma(F): F = 50a

since both blocks are in contact, they have to have the same acceleration, so both a's are equal.

so that means a = F/50, and I plug that into block A's equation.

150(F/50) = 100 -F

and so F = 25 N

Does that sound right?

I did not have a solution to check for a mistake. And i feel stupid asking this question, but his test is comming up and I dont want him to fail because of what I told him. I was real stupid the first time I did it because I approached it as a statics problem without thinking, I just said oh, its 100N.........errrrrr whoops......... that was dumb of me.

Last edited: Dec 26, 2005
2. Oct 11, 2005

### big man

Well to me the free-body diagrams look right for sure and the reasoning looks good as well. Your net forces a fine and i they would have the same acceleration. I know it's probably not reassuring to have me say it looks fine, but I'm saying it anyway : )

3. Oct 11, 2005

### Cyrus

Does anyone else agree, no offense big man? Just makin sure.

4. Oct 11, 2005

### DaveC426913

Surely, there's a trick to this question.

Unless it is a given that the pond is frozen, the masses will not sit on the surface.

5. Oct 11, 2005

### Cyrus

HAAHAHAHAHAH, i forgot to type frozen......sorry.

6. Oct 11, 2005

### DaveC426913

Ah. Then never mind. Carry on.

7. Oct 11, 2005

### Cyrus

Does it look correct though?

Theres another problem in his book that is worded funny, and I keep getting a different anwser than what they have. Ill post that problem up too, maybe you can find something im not seeing.