# Homework Help: Static friction question with no mass

1. Jan 3, 2008

### kirstynl

1. if a car is speeding off on a surface of asphault, no slipping tires, estimate static friction for a drag racer that covers the quarter mile in 6 seconds

My attempt:

I made a Free Body Diagram: Weight (W) down wards, Normal force (fn) upwards, and friction static friction to the right (Ffs). ou rteacher set that up for us.
I then tried calculating acceleration.

1320=1/2a6^2
a=18.33 m/s/s i think that answer is completely wrong and then i tried figguring out fn

Fn-W= ma
Fn-mg=18.33m
fn-9.8=18.3
Fn=28.1

But i think you have to divide the Fn by the mass times gravity

but i don't have the mass..... so i don'/t know phelp pleasee

2. Jan 3, 2008

### mgb_phys

You don't have enough information in the question
I could have a tractor pulling a sledge do 1/4mi in 6 seconds (with a big enough engine) or a I could have an air hockey puck do the same.

You need at least the mass and power.

3. Jan 3, 2008

### Staff: Mentor

coefficient of static friction

As mgb_phys points out, you don't have enough information to find the friction force. But you can estimate the coefficient of static friction, which might be what your teacher meant.

Hints: When finding the acceleration, convert .25 miles to meters, not feet. How does the friction force relate to the normal force?