# Homework Help: Physics help (Difficulty EASY)

1. Sep 25, 2005

### jtm

I just forget how to do this, its been 2 years.

I forgot the equation for friction F=ma * mew?? Anyway The driver of a 1560kg Toyota traveling at 24m/s hits breaks for red light. What is the distance needed to stop if the coefficient of friction is 0.80?

I'm guessing I have to use some kinematics and find the net force and divide the mass for the acceleration and use that to figure out the distance where v_f = 0 etc etc, anyway I just can't find this formula I forget. Can someone help out? I've tried google.

2. Sep 25, 2005

### whozum

Nope, this is a change in momentum problem. The toyota has a momentum of [itex] 1560kg \times 24m/s [/tex], the friction force would be measured by [itex] F_f = \mu_s N [/tex] where N is the gravitational force on the car.

From there you'll note that when the car is stopped it will have no momentum, and the relationship between the force and chang ein momentum is given by

$$\Delta mv = F \Delta t$$ with t = time, m = mass, and v = velocity.

Last edited: Sep 25, 2005
3. Sep 25, 2005

### jtm

This class hasn't done momentum yet, its a low level physics class, I am just taking it for a pre-req I know all this stuff, its just annoying. They are doing Newton's 2nd law and kinematics. Nothing hard.

4. Sep 25, 2005

### jtm

Here's the problem written exactly in the book:

The driver of a 1560-kg Toyota Avalon traveling at 24 m/s on a level, paved road (i.e. no sin or cos needed), hits the brakes to stop for a red light. Determine the distance needed to stop the car if the coefficient of friction between the car tires and road is 0.80.

Then they tell me to sketch it, choose a system, draw a free body diagram, apply newton's second law, combine the results and USE kinematics to determine the unknown quantity.

5. Sep 25, 2005

### jtm

The answer is 36m btw, but I need to show work.

6. Sep 25, 2005

### Leong

newton's 2nd law, frictional force and kinematic

assuming the horizontal force that acts on the car is frictional only,
then $$F=\mu*R$$ where R = normal force.
Find a using newton's 2nd law.
Find s using kinematic formula.