# Elementary Physics help.

1. Oct 17, 2005

### jtm

This problem I have to do is really bothering me.

Your 1600 kg PT Cruiser moves around a level 50m radius road. The coefficient of STATIC friction between the car tires and the road is 0.80. Determine the MAX speed of the car so that it does not skid off the road.

I think I get somewhere around.

ma - mg*0.80 = mv^2 / R I'm sure this is wrong because m shouldn't be able to be cancelled. Always ALL the information provided is used in the calculation.

2. Oct 17, 2005

### Tide

jtm,

Why not?

3. Oct 17, 2005

### jtm

Always ALL the information provided is used in the calculation.

Also, it gives the wrong answer ;) I checked.

4. Oct 17, 2005

### Gokul43201

Staff Emeritus
1. What is 'a' ?
2. The answer is independent of the mass, m.

5. Oct 17, 2005

### jtm

Answer from what I have on key is 6.3 m/s I am getting 19.8 m/s with mass cancelling out. We don't have a :) I'm assuming 0.

6. Oct 17, 2005

### Diane_

Think about this one physically first. If the car is going in a curve, there must be a centripetal force acting. Where is that force coming from? What you're looking for is the speed at which the maximum value of the force providing the centripetal force is exactly what's necessary to hold the car on the road. If it goes any faster, that force will not be able to hold it, and the car will skid.

So - at the point in which you're interested, the centripetal force must equal the maximum of the force providing it. You'll find the masses do cancel out. Sometimes teachers will give you information you don't need, to see if you'll find a way to stick it in anyway. I speak from experience.

7. Oct 17, 2005

### Gokul43201

Staff Emeritus
19.8 m/s is correct. The key is wrong. They forgot to multiply by 'g' !

PS : Did you draw a free-body diagram ?