Proof involving friction, kinda stumped

1. Sep 23, 2006

the_quack

I that nerd that does those extra problems in the back, the really hard ones, just for the heck of it.

Well, there is one that has me really stumped.

Show that the minimum stopping distance for an auto travelling at speed 'V' is equal to '(v^2)/(2Mg)'.
(I use M for the coefficient of static friction)

Maybe I'm just dumb, but I don't know where to start.

Can someone give me a little hint? It's not actually a homework problem, but I literally go CRAZY if I can't solve a problem I try...

Does it maybe have something to do with the basicx equation of 'V^2=V.^2+2ad'?

Last edited: Sep 23, 2006
2. Sep 23, 2006

big man

Well draw a block diagram first off.

There are going to be two energy equations and you need to apply the conservation of energy. One energy is going to be the kinetic energy as the car is moving. The other energy is going to be working in the opposite direction and it is due to kinetic friction (not static).

3. Sep 23, 2006

Andrew Mason

Use conservation of energy. Kinetic energy lost must be due to force of friction acting over the stopping distance. To find the minimum stopping distance use the maximum value of static friction, which is $\mu_sN = \mu_smg$

The coefficient of static friction is used because the tires are not skidding. If you skid, it takes longer to stop because the coefficient of kinetic friction is smaller. The question asks for the minimum stopping distance. [The force of static friction does not actually do the work in stopping the car. Rather the forces of kinetic friction on the brakes of the car do this. But those forces are limited by the force of static friction.]

AM

Last edited: Sep 23, 2006
4. Oct 2, 2006

the_quack

Man, I guess I am just dumb but I couldn't figure it out...

5. Oct 2, 2006

Andrew Mason

If you take my suggestion and use an energy approach, you have to understand what energy is: Work = Energy = Force x distance. What is the energy that has to be expended by application of the stopping force? (what is the kinetic energy of the car?)

Just use: kinetic energy of car = stopping force x stopping distance.

AM