Calculating Work Done by Gravity on a Falling Skydiver

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Homework Help Overview

The discussion revolves around calculating the work done by gravity on a skydiver falling at terminal velocity. The original poster presents a scenario involving a skydiver with a weight of 700N and a terminal velocity of 45 m/s, seeking to understand the rate at which gravity does work on her.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the relationship between weight, mass, and kinetic energy, questioning how to apply the given weight to find kinetic energy. Some express confusion regarding the calculations and the concepts involved.

Discussion Status

There is an ongoing exploration of how to relate weight to mass and subsequently to kinetic energy. Some participants have provided insights into the formulas involved, while others are still seeking clarity on the connections between the concepts and calculations.

Contextual Notes

Participants note the lack of explicit information about the mass and the confusion surrounding the interpretation of the question, particularly regarding the distinction between work done and power. The discussion reflects a mix of understanding and uncertainty about the physics principles at play.

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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...
 
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Well gravity would be cause the work done to be entirely kinetic energy. So you just need to apply that formula.
 
i don't see how i could apply that as i don't have the mass to find out the kinetic energy gained.
 
synkk said:
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.
 
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.
 
synkk said:
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)

synkk said:
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.
 
weight (force) = mass times gravity

mass = weight (force) divided by gravity

Her weight is 700N and gravity is 9.81m/s^2
 
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. :\
 
synkk said:
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 :redface:
 
  • #10
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?
 
  • #11
synkk said:
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.
 
  • #12
Lol, i had the answer from the start, sure feel like a idiot now for not understanding the question. Thanks anyway :)
 

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