# Energy and Work

1. Apr 11, 2008

### StephenDoty

A 1000.0 kg car experiences a net force of 9500N while decelerating from 30.0m/s to 23.4m/s. How far does the car travel while slowing down?

I do not even know how to start to setup the problem. I know that W=Fd but I do not know how to apply this with the velocities.

Any help would be appreciated. Thank you.

Stephen

2. Apr 11, 2008

### PhanthomJay

You can calculate the car's acceleration from the given data using Newton's laws, then use the standard kinematic equations of motion to calculate the distance.

3. Apr 11, 2008

### StephenDoty

since we are on the energy and momentum chapter is there any way to do it with KE and PE?

4. Apr 11, 2008

### PhanthomJay

Oh, sure. Use the work energy theorem
$$W_{total} = W_{net} = \Delta KE$$. In general , you've got to be a bit careful when using this equation, because W_net includes work done by both conservative (like gravity) and non-conservative (friction, etc.) forces, but it this case, those forces are given as one net number, so you don't have to worry about PE change.

Last edited: Apr 11, 2008
5. Apr 11, 2008

### StephenDoty

so...
Fd=1/2mvf^2 - 1/2mvi^2

?????

6. Apr 11, 2008

### PhanthomJay

yes, where the value you use for F is the given F_net. Check it out both ways using the energy method vs. Newton 2 and the kinematic equations.

7. Apr 12, 2008

### StephenDoty

Would vi be 23.4m/s
and vf= 30m/s
???

If it is the other way around then F*D= a negative number

any help would be appreciated.

8. Apr 13, 2008

### subwaybusker

vf is 23.4m/s since you're slowing down, right? so the change in kinetic you get should be a negative number, and keep in mind that the force of friction is always negative, so the work done is negative too.

9. Apr 13, 2008

### StephenDoty

so vi= 30.0m/s
vf=23.4m/s

and the force of 9500N is negative???

-9500N*d=(1/2)(1000kg)(23.4m/s)^2 - (1/2)(1000kg)(30m/s)^2

???

10. Apr 13, 2008

### subwaybusker

yep.

11. Apr 13, 2008

### StephenDoty

d= 18.5m or 19m with sig figs.?

12. Apr 13, 2008

### Snazzy

A general rule of thumb is to use the least amount of sig figs as the values given to you in the question.

13. Apr 13, 2008

### StephenDoty

which would be 2 from 9500N

14. Apr 13, 2008

### StephenDoty

do i have the right number of sig figs