# Find time if boxes moved X distance

1. Sep 20, 2006

### sweetdaisy186

Hey guys! Here's another question

A mass, m1=5 kg, is on a frictionless ramp. The mass is connected to a cable that passes over a pulley which is then attached to a second hanging mass, m2 = 10kg. How long will it take the boxes to move 3 m if they are released from rest?

My problem with these questions, is that I don't know how to approach them. I need help determining how I should approach this problem, what I should be thinking about, how do I know which formula to use ect. Basically, I don't know how to start the problem.

Any help would be greatly appreciated!!

I have attached a picture for this as well.

Thanks

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2. Sep 21, 2006

### SGT

The forces involved are the weights of the two masses, the tension in the cable and the normal force from the ramp to the mass m1.
equate this to obtain the acceleration acting on the masses. Knowing the acceleration, you can calculate the time to move 3m.

3. Sep 21, 2006

### sweetdaisy186

I see. that makes a lot of sense. Thanks!

4. Sep 21, 2006

### sweetdaisy186

Calculate Friction

Hey guys! I think I have half of the question figured out

The question:

Skid is driving his car at 80 mi/hr. While reading the Springfield Reivedw of Books, he looks up and notices he is about to crash into a wall. When he is 100 m from the wall, he puts on the brakes. If Skid does not lock his brakes, calculate the coefficient of friction he needs so that he stops just before he hits the wall. What type of friction does he use?

My work:

I know that this involves kinetic friction. I thought I should calculate acceleration so that I can use the force equation.

I used the V^2 = V0^2 + 2a*delta X

I got an acceleration of 0.8. In order to use the SumFx = m*a equation, don't I need a mass so that I can somehow solve for the kinetic friction?

5. Sep 21, 2006

### sweetdaisy186

whoops! Sorry, my mistake!