# Homework Help: I need help solving three problems (2 for friction, 1 in hooke's law)

1. Jan 13, 2009

### accol

1. http://www.prep101.com/MCAT/102MCATPhysicsAnswers.pdf [Broken] Numbers 4 7 and 14

2. As you can see the answers are already available, I'm just having trouble trying to get to them.

3. For number 4 I've tried plugging in the k value (8000) and distance (.05) in to get a force value. I'm kind of lost from there as I've tried. Number 7 and 14 are kind of the same. However, for 7 I did get the answer when I plugged I plugged it into a Fc = v/r. So I did in fact get 10m/s but when I ignored the squared sign from the original equation (Fc = v^2/r). If I did do something correct then why did the squared sign disappear? For number 14 I don't really know how to tackle this if I don't have mass.

Once again if you can tell me how to reach the answers to one of these questions that would be great.

Last edited by a moderator: May 3, 2017
2. Jan 13, 2009

### LowlyPion

Welcome to PF.

In 4 you have determined the force between the 2 blocks. That means that m2 is being accelerated with 400 N. Doesn't that give you then the acceleration for the whole system?

For 7 I'd say it's coincidence. Work out the equations for mv²/r = μmg. Solve for v and then plug in your numbers.

For 14 you don't need the mass. F = ma = μmg ... hence a = μg then solve by ordinary kinematic means.

3. Jan 13, 2009

### accol

Dude thanks so much....I have trouble remembering all the little substitutions you've showed me.

Thanks