# Homework Help: How much work does the brake system have to do to stop the car at each speed?

1. Jun 4, 2012

### laylay918

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

Investigate the amount of kinetic energy involved when your car's speed is 60 km/h, 50 km/h, 40 km/h, 30 km/h, 20 km/h, and 10 km/h. (Find your car's mass in the owner's manual) How much work does the brake system have to do to stop the car at each speed?

2. Relevant equations
KE = 1/2 mv^2

3. The attempt at a solution

I figured out the KE for each of those speeds for a 1500 kg car. The problem is the second portion of the question. How much work does the brake system have to do to stop the car at each speed? I have no idea how to solve that part because no equations are given. Anyways, here is my work for the first portion.

KE = 1/2(1500kg)(60km/h)^2 = 2,700,000 J
KE = 1/2(1500kg)(50km/h)^2 = 1,875,000 J
KE = 1/2(1500kg)(40km/h)^2 = 1,200,000 J
KE = 1/2(1500kg)(30km/h)^2 = 675,000 J
KE = 1/2(1500kg)(20km/h)^2 = 300,000 J
KE = 1/2(1500kg)(10km/h)^2 = 75,000 J

2. Jun 4, 2012

### rock.freak667

At 10 km/hr, the KE is 75 kJ. When the car is at rest how much kinetic energy does it have?

Hence how much work will the braking system need to do to get the KE to this value?

3. Jun 4, 2012

### Villyer

Make sure you check your units.

You are calling a kg (km/hr)2 a Joule. What are the units for Joules?

4. Jun 4, 2012

### laylay918

No, i'm calliing ".5mv^2" a Joule.

5. Jun 4, 2012

### SammyS

Staff Emeritus
Hello laylay918. Welcome to PF !

That's kinetic energy. If the mass, m, is in units of kg and the velocity, v, is in units of meters/second, the unit for (1/2)mv2 will be a Joule.

6. Jun 4, 2012

### Villyer

I don't mean mathematically, I mean in units.