Calculating Work Done by Gravity on a Falling Skydiver

[synkk]

Homework Statement

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?

Homework Equations

P = W / t
P = F * (d/t) = f*v
v = d/t
w = F * d

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...

 

[rock.freak667]

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

 

[synkk]

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

 

[rock.freak667]

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

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

 

[synkk]

uhm I'm not quite sure what you mean, i don't 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.

 

[rock.freak667]

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

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)

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.

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.

 

[Mr. Johnson]

weight (force) = mass times gravity

mass = weight (force) divided by gravity

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

 

[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. :\

 

[rock.freak667]

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. :\

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

Your initial way is correct

 

[synkk]

No worries, i learned 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?

 

[rock.freak667]

No worries, i learned 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?

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

 

[synkk]

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