# Homework Help: Forces and friction

1. Feb 5, 2006

### Xenocite

Maybe I'm thinking way too deep for this problem, but here goes:

-A car is traveling 11.7 m/s on a flat road.
-Acceleration of gravity = 9.8 m/s^2
-The coefficient of friction between the road and the tires is 0.156

-What is the minimum distance in which the car will stop?

I tried to start this problem, but I really need to know the car's weight to figure out the normal force (Fn - force exert on the car by the road). The car's weight will also give me the horizontal force, which I could use it to find the car's deceleration. Then find the time period from start to stop to find the distance it travelled. Is there another aspect of this problem that I fail to notice? Any hints would help.

2. Feb 5, 2006

### stunner5000pt

you dont need the mass of the car to proceed

what is the formula for any force on an object of mass m
what is the force of friction on thisc ar... from this can you calculate the acceleration due to this firction?

3. Feb 5, 2006

### Xenocite

Well, to my current knowledge, the frictional force requires the mass of the car to be calculated:

frictional force = coefficient of friction * mass

Sorry if I'm not getting anywhere by your input.. I'm really trying here :(

4. Feb 5, 2006

### stunner5000pt

firctional force = coefficient x (NORMAL force)
here since the car is on a flat surface... the normal force is equal to the force of gravity (that is not always the case, however)
also you can write ANY force as mass x acceleration
so u could write firctional force as mass x acceleration.
now you already know an expression for frictional force which you can equate to mass x acceleration... the mass will cancel out.. and you are left with the accleration