# Calculate the work done by friction

1. Oct 31, 2011

### cc2hende

1. The problem statement, all variables and given/known data
You have a level track. You push a cart with mass = 1.12[kg].
You measure the initial velocity to be 0.57[m s-1].
2 seconds later, you measure the velocity to be 0.427[m s-1].

2. Relevant equations
What is the work (reported in mJ) that friction did on the cart?

3. The attempt at a solution

W(f)=F(f) x d
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 31, 2011

### SammyS

Staff Emeritus
Welcome to PH.

You need to show us your work so we can help you.

Hint: What does the Work-Energy Theorem say?

3. Nov 1, 2011

### cc2hende

W= deltaK = 1/2m(Vf^2 - Vi^2)

W=1/2 (1.12kg) [((0.427m/s)^2) - ((0.57m/s)^2)]

W= -0.0798 kg m/s

Is this the work done by friction though? or the cart?

4. Nov 1, 2011

### SammyS

Staff Emeritus
On what object would the cart do work?

Friction slows the cart, so it does negative work on the cart, so your answer seems reasonable.

5. Nov 1, 2011

### cc2hende

Okay that makes sense. thank you.

So if i were to calculate work done by friction, but this time I'm given distance and delta K, would I use Wncf=fd where f=(deltaK)/d or work energy theorem?

6. Nov 1, 2011

### SammyS

Staff Emeritus
That gives you the friction (force) itself.