# Homework Help: Work Energy and Power

1. Jan 11, 2006

### skull

Ok I need this answered by whoever can.
The force on an object is F=C/x^2
A) what are the units of C?
b)How much work is done by this force when the object is moved from x=a to x=3a.
How much work is done by F when the object is moved from x=3a to x=infinite

units of x are in meters

I have tried this countless time, and still have no answer. All help is greatly appreciated.

2. Jan 11, 2006

### stunner5000pt

What are the units of Force? what are the units of x^2? CAn you find the units of C from this?
what is the formula for work? Are you familiar with this definition
$$W = \int_{i}^{f} F \bullet dx$$ or the non calculus version?

3. Jan 11, 2006

### skull

units of force are newtons
units of x^2 are meters
the formula that I was given is W=Fdcosx
where f is force, d is distance, and x is the angle

4. Jan 11, 2006

### skull

anyone know?
By the way, this is Gade 11 physics, so keep that in mind when answering

Last edited: Jan 11, 2006
5. Jan 11, 2006

### G01

If F is Newtons like you said, and x is m^2 like you said. C divided by m^2 must give you force in Newtons. If this is the case what must the units of C be so that the answer comes out in Newtons?

6. Jan 11, 2006

### skull

the question says that c=a constant, if the helps

7. Jan 11, 2006

### G01

Well you don't need that fact to get the units at least. Maybe later. To get the units for c all you need are the units of the other two variables. Solve for c and what do you get as the units? That gives you the answer for part a.

Tell me what you get when you do that.

8. Jan 11, 2006

### skull

solving for c would simply be c=n/m^2
where n is newtons and m is metres
I dont really understand what you are saying

9. Jan 11, 2006

### G01

No. solve for c from this equation:
$$F = \frac{c}{x^2}$$ gives:

$$c = Fx^2$$

Now fill in the units:

$$c = Nm^2$$ C then, has the units N times m^2. You screwed up your algebra. Where you should have multiplied both sides by x^2, you divided.

10. Jan 11, 2006

### civil_dude

Don't you have to use calculus to solve for this since F(x) is not a constant?

11. Jan 11, 2006

### G01

Yeah I was just thinking that. But you can find the units of C without calc. All you need is dimensional analysis.

12. Jan 11, 2006

### skull

ok, and how can I get units with the equation you gave?

13. Jan 11, 2006

### G01

I just did that. do you agree that c = Fx^2? If so then c must have the same units as Fx^2 since it is equal to it. What are the units of F times x^2

You MUST have done this at some time during you physics studies. Dimensional Analysis is something physics teachers never let you forget. That is what were doing here to find the units.

14. Jan 11, 2006

### skull

I said F is constant

15. Jan 11, 2006

### G01

You said c was constant not F. I don't know how F can be constant if it equals C/x^2. If thats true then F is not constant it changes with respect to x.

16. Jan 11, 2006

### skull

17. Jan 11, 2006

### G01

Anyway do u understand how to find the units?

18. Jan 11, 2006

### skull

I thought your earlier post was implying that i did not give enough information. Can I still find the units?

19. Jan 11, 2006

### G01

I've shown you how to find the units MULTIPLE TIMES including the post I just quoted. I've told you how to do it and actually given the answer in one post which I'm not supposed to do since you didn't so any work at all.

I will tell you the method ONE more time and thats it. Take the formula for force and solve for c. You won't get a numeric answer just work with the variables. What is c equal to? It must have the same units as whatever it is equal to.

20. Jan 11, 2006

### skull

Ok, I got joule metre as the unit. Is this right? Is this also know as a newton?