1. Nov 17, 2004

### Raza

Hello, I just want to make sure I did my problem right.
Q: A 41.0N force is applied to a 300 kg crate to make it just move. Calculate the coefficient of static friction.

My Work:

Is that right?

This question I really don't get.
Q:The driver of a car applied the brakes so hard that the car skids at a constant speed on wet asphalt road. If the mass of the car is 2000kg, calculate the force of kinetic friction of the car.

Thank you

2. Nov 17, 2004

### Diane_

1) Looks good to me.

2) This is an odd one. If the car is skidding "at constant speed", and we presume in a constant direction, then what can you say about the acceleration? If the acceleration has that value, then what must the net force be? And, finally, what does that tell you about the force due to friction?

As I say, an odd question, but I don't see any other plausible answer.

3. Nov 17, 2004

### cepheid

Staff Emeritus
Diane,

Thanks for confirming I wasn't going crazy...I looked at the question too and thought...net force zero...yet there is a friction force...what could possibly be counteracting it? After all, the driver hit the brakes...so the engine is not going...it is a stange scenario.

4. Nov 17, 2004

### Diane_

My only thought is that the wet asphalt acts like a frictionless surface. That's a little silly, but not too much so.

If your teacher has something else in mind, I'd appreciate your posting it when you find out. You have me curious now.

5. Nov 17, 2004

### cepheid

Staff Emeritus
Well, I'm not the thread author, just someone who was trying to help out as well, and came to that same conclusion. Hopefully he/she will get back to us on whether that was was the teacher was looking for

6. Nov 18, 2004

### Raza

I am going to ask my teacher about that question. And I really appreciate you guys helping me. Sometime, my teacher acts like an ass and doesn't help me.