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Conservation of energy

  • Thread starter jasonbans
  • Start date
  • #1
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Homework Statement


A bungee jumper of mass 64.5 kg (including safety gear) is
standing on a platform 48.0 m above a river. The length of
the unstretched bungee cord is 10.1 m. The force constant
of the cord is 65.5 N/m. The jumper falls from rest and just
touches the water at a speed of zero. The cord acts like an
ideal spring. Use conservation of energy to determine the
jumper’s speed at a height of 12.5 m above the water on
the first fall.

Homework Equations


Ek=1/2mv^2
Ep=mgh
Ee=1/2kx^2

The Attempt at a Solution


so i did mgh = 1/2mv^2 + 1/2kx^2 and solve for v but somehow i am not getting the answer 6.3m/s
 

Answers and Replies

  • #2
PeterO
Homework Helper
2,425
46

Homework Statement


A bungee jumper of mass 64.5 kg (including safety gear) is
standing on a platform 48.0 m above a river. The length of
the unstretched bungee cord is 10.1 m. The force constant
of the cord is 65.5 N/m. The jumper falls from rest and just
touches the water at a speed of zero. The cord acts like an
ideal spring. Use conservation of energy to determine the
jumper’s speed at a height of 12.5 m above the water on
the first fall.

Homework Equations


Ek=1/2mv^2
Ep=mgh
Ee=1/2kx^2

The Attempt at a Solution


so i did mgh = 1/2mv^2 + 1/2kx^2 and solve for v but somehow i am not getting the answer 6.3m/s
You may have substituted in some incorrect values, or you may have "mis-used" you calculator.

Show us your substitution / simplifications so we can see what went wrong.
 
  • #3
45
0
this is what i did
 

Attachments

  • #4
PeterO
Homework Helper
2,425
46
this is what i did
Two problems:

When 12.5 m above the water, the jumper has not descended 48m, so you h value is not appropriate, and the bungee has not been stretched 12.5 m, so your x value is not appropriate.
 
  • #5
45
0
12.5 above the water meaning 48 subtract 12.5 = 35.5m for the height?
i dont know how much it stretched by am i suppose to calculate it?
 
  • #6
45
0
thx got the answer now
 
Last edited:
  • #7
PeterO
Homework Helper
2,425
46
is the height 35.5 m and it stretched 25.5m right?
if it was 10.0 m long to start with it will have stretched 25.5m

edie: the h value is 35.5, yes. best called the change in height.
 

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