# Homework Help: Power & Work newb question

1. Jul 7, 2011

### synkk

1. The problem statement, all variables and given/known data
A skydiver is falling at a terminal velocity of 45 metres per second. If her weight is 700N, at what rate is gravity doing work on her?

2. Relevant equations
P = W / t
P = F * (d/t) = f*v
v = d/t
w = F * d

3. The attempt at a solution

Pretty confused at this question, this is all i got:

v = 45m/s

p = 45 * 700 = 31500
31500 = 700 * (d/t)

thats about it, i'm not even sure if i'm on the right track.

PS, sorry for asking such a newbie question hope someone can help, thanks...

2. Jul 7, 2011

### rock.freak667

Well gravity would be cause the work done to be entirely kinetic energy. So you just need to apply that formula.

3. Jul 7, 2011

### synkk

i don't see how i could apply that as i dont have the mass to find out the kinetic energy gained.

4. Jul 7, 2011

### rock.freak667

You have her weight which is related to mass and acceleration due to gravity.

5. Jul 7, 2011

### synkk

uhm i'm not quite sure what you mean, i dont see how the 700N can relate to the kinetic energy.

ek = 0.5 * m * v^2

why would m = 700N? and if it does that means the gain in kinetic energy would be 708750J, where would i go from there?

sorry but i'd need more htan a sentance or two explanation as i'm trying teach myself these things, thanks anyway.

6. Jul 7, 2011

### rock.freak667

700 Newtons is her weight, weight is related to mass 'm' by W = mg where g is acceleration due to gravity (on Earth this is 9.81 m/s2)

You will need to calculate m from the 700 N weight.

Her kinetic energy will be the work done by gravity since she is at terminal velocity.

7. Jul 7, 2011

### Mr. Johnson

weight (force) = mass times gravity

mass = weight (force) divided by gravity

Her weight is 700N and gravity is 9.81m/s^2

8. Jul 7, 2011

### synkk

Okay so m = 71.4kg ( 700/9.81 )

using this the gain in kinetic energy is 72292.5J

i've taken a look at the answer and it's 31 500W, i thought i'd be figuring out the work done which is calculated in joules, now my head is going everywhere. :\

9. Jul 7, 2011

### rock.freak667

Sorry sorry, I initially read the question as what is the work done by gravity.

10. Jul 7, 2011

### synkk

No worries, i learnt how weight is related to mass due to gravity so thanks for that.

Anyone have any ideas how to go on for this question then?

11. Jul 8, 2011

### rock.freak667

The same way you did it, using power = force*velocity and in this case the force = 700 N.

12. Jul 8, 2011

### synkk

Lol, i had the answer from the start, sure feel like a idiot now for not understanding the question. Thanks anyway :)