What Is the Jumper’s Speed at 12.5 m Above the Water?

[sillydude]

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.

The Attempt at a Solution

so I decided that Eg(at the top of the platform)=Ek+Ee+Eg'(12.5 above water floor)

so for h initial i used 48m...h'= 12.5m and x(stretch of the cord)= 25.5m
therefore
mg(48)=.5k(25.5^2)+.5mv^2+mg(12.5)

i solved for V and got 5.96m/s, the answer is 6.37m/s...please help!

 

[Nabeshin]

I think your error is merely computational. I get the stretch of the cord to be 25.4m, not 25.5, which apparently has a large difference because the value is squared.

 

[sillydude]

I think your error is merely computational. I get the stretch of the cord to be 25.4m, not 25.5, which apparently has a large difference because the value is squared.

bingo!..thanks a lot!