# Given some body with a constant force F acting on it upwards

1. Jun 5, 2013

### xmjolx

Given some body with a constant force F acting on it upwards
(The body is subject to the gravitational force due to the earth)
What's the correct statement?

Or:

K - Kinetic Energy

Last edited: Jun 5, 2013
2. Jun 5, 2013

### Staff: Mentor

Think about how the potential energy changes with $\Delta{x}$. The change in total energy, which includes both kinetic and potential energy, will equal the total amount of work done on the body.

You might also find it helpful to consider the cases: F is only very slightly larger than mg; and F is so large relative to mg that we can ignore the effects of gravity.

3. Jun 5, 2013

### xmjolx

what if F is small?
how come gravity has no affect on the change in kinetic energy?

4. Jun 5, 2013

### Staff: Mentor

If you want to apply the "Work - KE" theorem, you must use the net force on the body. That net force must include gravity.

5. Jun 5, 2013

### xmjolx

So is the right statement?

6. Jun 5, 2013

Yes.

7. Jun 5, 2013

### xmjolx

Thank you very much!! :)
i knew it was the right answer but i had an argument with my high school teacher(she claims to have Ph.D in physics) about that.

8. Jun 9, 2013

### Philip Wood

Multiply out the rhs, then
$$\Delta K = F \Delta x - mg \Delta x.$$
Identifying the terms on the right as work or energy:
$$\Delta K = work\;done\;by\;F - \Delta (grav\;potential\;energy).$$