Homework Help: Newton Laws

1. Jun 30, 2008

asi123

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

https://www.physicsforums.com/attachment.php?attachmentid=14568&stc=1&d=1214858251"

There is no friction between the masses.
The first part of the question is to find the force F so that m2 will be in rest (in respect to m3).
The second part is, what are the accelerations of the masses when F=0?

Ok, I have my own question, how does F affect m2, it's like on the other way around?

10x in advance.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jun 30, 2008

calef

Think about this problem as if m3 wasn't there.

Once F acts, m2 is, relative to the ground, stationary, and m1 will be accelerated to the right.

Now, think about the problem as if m2 wasn't there.

Once F acts, m1 and m3 are accelerated to the right, and m3 falls to the ground.

Now, put both systems together, but take out F. m2 is now being pulled to the right with a force of m3g.

Now, put back in F:
What force does F need to be to match that acceleration of m2? Alternatively, what happens when F is much greater than the force of gravity? (no the answer isn't infinity, this is just a conceptual hint)

Last edited: Jun 30, 2008
3. Jun 30, 2008

epsilonNaught

Also think about the fact that the pulley is attached to mass 1, and mass 2 is in contact with the pulley

4. Jun 30, 2008

asi123

I still don't get what is the force that makes m2 not to be pull to the right towards m3?
Is it F?
Can I say F = T?

10x again.

5. Jun 30, 2008

calef

That's another tricky part. There's no force "opposing" m2's motion.

Imagine you're standing on an infinite plane of ice, and let's say your feet are frictionless against the surface. Suddenly you're being pulled along by a rope with magnitude, let's saaaay, m3g.

So you're gliding along this ice surface, but suddenly, the whole ice surface starts speeding up in the same direction you're going.

To you, it would feel like you were slowing down, up until the surface is moving at the same speed you are. Once this happens, it feels like you aren't moving at all. But really, you and the ice surface are traveling at the same speed.

This is what's happening with the blocks. m1 is the "surface". F has the same acceleration as m2. m2, analogous to you-standing-on-our-ice-world, is being pulled along with magnitude m3g. The surface of the ice, when it's moving the same speed as "you", must have the same acceleration as m2 for the mass on the pulley not to fall.

6. Jun 30, 2008

asi123

Ok, got u.
But still, what is the right equation?
I cant right T = ma with out any regards to F. In other words, how does F affects the equation?

7. Jun 30, 2008

calef

You get to find the right equation ^_^.

The biggest hint I can give you is that F has the same acceleration as the force acting on m2.

8. Jun 30, 2008

asi123

So T=0?

Is there a "not answering homework" policy here? I've been thinking about this problem for hours, it's not like I'm posting my entire homework in here and telling you solve...

10x anyway.

9. Jul 2, 2008

calef

I never thought about these problems in terms of tension--not at least until the pulley actually had friction acting on it.

But yeah, T=0.

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