# Homework Help: Car Braking

1. Nov 23, 2009

### JPhysic

1. The problem statement, all variables and given/known data

I am just enquiring about car brakes, and figuring out how much force needs to be applied to stop a moving object (car).

2. Relevant equations

1457Kg – Curb weight of the car (overall mass)

F = M x A

1457kg x 10mph = 14570 (Newtons)
1457kg x 20mph = 29140 N
1457kg x 30mph = 43710 N
1457kg x 40mph = 58280 N
1457kg x 50mph = 72850 N
1457kg x 60mph = 87420 N
1457kg x 70mph = 101990 N

3. The attempt at a solution

So i've figured out the acceleration, but I need to find out, how much force needs to be applied to bring the car to a stand still using 312 Diameter brake disks. Would it be equal to or greater than the Acceleration?

2. Nov 23, 2009

### Staff: Mentor

I believe you will need the coefficient of friction (dynamic) between the brake pads and the rotors. Why would you need that?

And you will probably need to do an integration from the inside to the outside of the swept radius of the rotor, since the braking torque contribution will ratio with the radius.

3. Nov 23, 2009

### JPhysic

I'm sorry my friend, but you just confused the hell out of me...

4. Nov 23, 2009

### Staff: Mentor

Perhaps I misinterpreted your question. I thought you were asking how hard (how much force) you have to squeeze the brake pads together on the rotor in order to brake a car to a standstill. Were you asking something different?

The "brakes" do not apply force directly to a car in a linear fashion. The brake pads sqeezing the rotors apply a braking torque to the wheels, which is what slows the car down...

5. Nov 24, 2009

### JPhysic

I need to know at:

10mph - 70mph with a mass at 1457Kg, what force is needed to bring the car to a stand still

6. Nov 24, 2009

### Staff: Mentor

Well, first of all, you don't multiple mass in kg by speed in mph to get force in Newtons.

And you don't multiply mass by speed to get force anyway.

You wrote the equation F=MxA, which is correct. You also need the equation W=FxD (work equals force multiplied by distance). Now, write the equations, and keep your units consistent (use mks units).