# Not really a equation? finding friction

## Main Question or Discussion Point

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

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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

Depends, whether it is static friction on kinetic friction

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BruceW
Homework Helper
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).

I like Serena
Homework Helper
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.)

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

why do you find kinetic and not static??

I like Serena
Homework Helper
An estimate of the static coefficient of friction could be made when braking maximally without locking the wheels.

I like Serena
Homework Helper
why do you find kinetic and not static??
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.

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

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

BruceW
Homework Helper
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!

I like Serena
Homework Helper
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.

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.
haha thanks! that helps alot.

would you use cruise control for you initial velocity??

I like Serena
Homework Helper
would you use cruise control for you initial velocity??
That's probably best to start from an accurate initial speed.
Or at least from a reproducible initial speed.

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

I like Serena
Homework Helper
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.
$$\mu_k = {v^2 \over 2 \cdot g \cdot x}$$

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

I like Serena
Homework Helper
yes. im actually doing it. i have to. its a project.
Sounds like fun!

When will you do it?

after thanksgiving break. so the week after next.

I like Serena
Homework Helper
Will you tell us how it goes?

if i remember to!

I like Serena
Homework Helper
I'll remind you!

BruceW
Homework Helper
is this going to be an actual person sitting inside a real car? Isn't there a danger of serious neck injuries? Because you need to brake pretty hard to make the wheels slip?!

Better check your health insurance covers this kind of thing!

I like Serena
Homework Helper
I think you're exagerating here.
Where I live it is required to do an emergency brake test when you get your driver's license.
It can happen that the wheels lock.
The maximum acceleration is "only" less than 1G.
This is very different from being unexpectedly hit from behind with 30+ miles per hour.