# Frictional force homework help

1. Jun 24, 2008

### c4iscool

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

A 20-kg block on a horizontal surface is attached to a light spring (force constant = 8.0 kN/m). The block is pulled 10 cm to the right from its equilibrium position and released from rest. When the block has moved 2.0 cm toward its equilibrium position, its kinetic energy is 13 J. How much work is done by the frictional force on the block as it moves the 2.0 cm?

do I need a coefficient of friction to solve this?

2. Jun 24, 2008

### kreil

You are told what the blocks kinetic energy is with friction, so if you solve for what it should be without friction then the difference is the work done by the frictional force...no coefficient of friction needed.

3. Jun 24, 2008

### c4iscool

ok, I get that but how would I find the velocity to get the kinetic energy?

ke = 1/2 M*V^2

or am I still not getting it?

4. Jun 24, 2008

### PhanthomJay

You need to apply the total conservation of energy principle. Are you familiar with it?

5. Jun 24, 2008

### c4iscool

6. Jun 24, 2008

### PhanthomJay

In the absence of non conservative forces other than friction, you should discover that $$\Delta K + \Delta U + W_{friction}= 0$$. Now you must apply it.

7. Jun 25, 2008

### c4iscool

I got it. I think it works out to be -1.4J

8. Jun 25, 2008

### PhanthomJay

Looks right. Even your minus sign is correct. I slipped on mine, sorry, I should have said $$\Delta K + \Delta U - W_{friction} = 0$$