# Friction (Sliding)

## Homework Statement

I'm positive Part 1 is right, 2 and 3 I'm sketchy with.

1. A race car with mass 1000 kg moves at speed 40 m/s. What is the kinetic energy of the car in kiloJoules? Do not write the unit in your answer.

For this I got 1/2(1000)(40^2) = 800000 convert to kilojoules = 80

2. The car in the previous problem locks its brakes and slides to a stop due to a constant frictional force. How much work was done by this force? Express your answer in kiloJoules but as usual do not write the unit as part of your answer.

= 1/2(1000)(40^2) - 1/2(1000)(0^2) = 80000 = 80kilojoules?

3. If the car in the previous two problems slides to rest with a constant frictional force of magnitude 10,000 N, how far does it slide? Express your answers in meters, but do not write the unit in your answer.

This one is below.

## Homework Equations

1/2m(v^2)
vf^2 = vi^2 - 2*a*d

## The Attempt at a Solution

I'm really not sure how to do this. I tried finding acceleration which I believe is 10,000N / 1000kg = 10m/s

Then I just tried to use an equation of kinematics...

= 0^2 = 25^2 + 2(10)(d) and I get -31.25 for that, but I don't think that's right ^_^;.

I appreciate any help

Delphi51
Homework Helper
1 and 2 look good.
For 3 you have
vf^2 = vi^2 - 2*a*d
0^2 = 25^2 + 2(10)(d)
Of course when you put in a = -10, you'll then get a minus sign.
And the 25 should be 40.

Also can be done using the work formula - a little less "work" involved.

1 and 2 look good.
For 3 you have
vf^2 = vi^2 - 2*a*d
0^2 = 25^2 + 2(10)(d)