# Not really a equation? finding friction

1. Nov 14, 2011

### physicsgurl12

how would you find the coefficent of friction on a car using a stopwatch??
i was trying to use this webite ( i didnt really help explain anything) to help it says you should get about .9 but i really want to know how.
http://regentsprep.org/regents/physics/phys01/friction/default.htm

Last edited by a moderator: Apr 26, 2017
2. Nov 14, 2011

### Darth Poison

Depends, whether it is static friction on kinetic friction

Last edited by a moderator: Apr 26, 2017
3. Nov 14, 2011

### BruceW

I don't see how a stopwatch would help.

While the wheels are not slipping (which is usual for a car), the acceleration times mass of the car must be provided by the friction force between the tyres and the road. We have a limit for the max force provided by static friction, which is $f_{max} = \mu_s F_N$, where $F_N$ is the normal force (equal to the weight of the car usually). So if the acceleration times mass of the car breaks this limit, the force of static friction is no longer enough to keep the tyres from slipping.

So to measure the coefficient of static friction, you could measure the maximum of 'mass times deceleration' without the tyres slipping, and divide by the weight of the car.
Of course, there will be much more accurate ways of measuring static friction. But this is one way that any tom, dick or harry could do. (Although I don't advise wearing out your tyres by doing this experiment).

4. Nov 14, 2011

### I like Serena

Take the time it takes for the car to stop from an initial speed with brakes maximally engaged.
$$\mu_k = {v \over g \cdot t}$$

(This measures the coefficient of kinetic friction.)

5. Nov 14, 2011

### BruceW

Oh, I see. Yes, you could use a stopwatch to measure the coefficient of kinetic friction.

6. Nov 14, 2011

### physicsgurl12

why do you find kinetic and not static??

7. Nov 14, 2011

### I like Serena

An estimate of the static coefficient of friction could be made when braking maximally without locking the wheels.

8. Nov 14, 2011

### I like Serena

Kinetic friction is when the wheels slip on the pavement.
Static friction is when the wheels do not slip.
The maximum static friction is higher than the kinetic friction.

9. Nov 14, 2011

### BruceW

Yes. There are two equations.
1) if the wheels are not slipping: $f_{max}=\mu_s F_N$
2) if the wheels are slipping: $f=\mu_k F_N$

10. Nov 14, 2011

### physicsgurl12

okay well how do you know if the wheels slip??

11. Nov 14, 2011

### BruceW

For the wheels to not slip, the horizontal force on the entire car must be provided by the friction force. In other words, as long as $$f_{max} \geq m \frac{d^2x}{dt^2}$$
then the wheels will not slip. the horizontal acceleration will be due to braking. So as long as you don't brake too hard, the wheels won't slip!

12. Nov 14, 2011

### I like Serena

You will know because suddenly you start gliding, and it makes you feel out of control! Assuming you do not have an anti-blocking-system (ABS) of course.

13. Nov 14, 2011

### physicsgurl12

haha thanks! that helps alot.

14. Nov 14, 2011

### physicsgurl12

would you use cruise control for you initial velocity??

15. Nov 15, 2011

### I like Serena

That's probably best to start from an accurate initial speed.
Or at least from a reproducible initial speed.

16. Nov 15, 2011

### BruceW

Is she actually doing this 'experiment'??! Haha, be careful!

17. Nov 15, 2011

### I like Serena

If you drive 10 miles per hour and lock the wheels, you're guaranteed to stop within 2 meters.
That should be safe enough.
After that you can try it a little faster to be able to make a better time measurement.

Btw, you can also use the stopping distance to calculate the coefficient of kinetic friction.
$$\mu_k = {v^2 \over 2 \cdot g \cdot x}$$

18. Nov 16, 2011

### physicsgurl12

yes. im actually doing it. i have to. its a project.

19. Nov 16, 2011

### I like Serena

Sounds like fun!

When will you do it?

20. Nov 16, 2011

### physicsgurl12

after thanksgiving break. so the week after next.