Question

A bungee jumper hangs from a bungee cord with zero velocity. If the bungee cord has a spring constant of 54 N/m and the jumper has a mass of 68 kg, how stretched is the bungee cord past its natural length?

A. 12.3 m
B. 8.32 m
C. 1.26 m
D. 15.2 m

Answer 1

Use force balance in equilibrium position.

Answer 2

is that an equation? @LastDayWork

Answer 3

I don't know what other term can be used for it.

Consider Newton's 2nd law F=ma
At equilibrium a=0
Hence, the vector sum of all the forces will also be equal to 0

Answer 4

what would be equal to 0?
if i did f=ma
it would =0.
But it's natural length couldn't be 0? @LastDayWork

Answer 5

Consider only the bungee jumper and try to tell the forces acting on him (we call this free body diagram)

Answer 6

^^ When he (jumper) is at equilibrium.

Answer 7

the only force would be gravity, the cord, and friction? @LastDayWork

Answer 8

how would that help me solve this? @LastDayWork

Answer 9

Friction, really?

Answer 10

uh, yeah?
gravity is the force that pulls the jumper down.
the cord is what keeps him from falling all the way down.
& friction would be due to the air that would slow him down a little? @LastDayWork

Answer 11

You can neglect friction due to air as
- the question is silent about it
- friction due to air (more commonly called air drag) depends on its velocity (w.r.t air)

So we are left with two forces.
Now write an equation which shows that their vector sum is zero.

Answer 12

what would i need to include in the equation?
i dont even think i know one that equals 0.
I didn't see any in my notes either.
does it have a specific name? @LastDayWork

Answer 13

Do you know vectors?

Answer 14

that's not even mentioned in the section im studying.
i know what a vector is but thats about it @LastDayWork

Answer 15

Do you know co-ordinate geometry (straight lines to be precise)?

Answer 16

yeh, i took geometry a couple years ago. @LastDayWork

Answer 17

Check this - http://gscim.com/phy/Physics_Basics/vectors.html

Answer 18

^^ Tell me when you are done.

Answer 19

alright, i read it all @LastDayWork

Answer 20

Did you try the little program at the end?

Answer 21

i watched the videos, and looked at the thing at the end but i didnt see directions so i left it alone cx @LastDayWork

Answer 22

does the equation for the problem have to do with vectors or is it on that site? @LastDayWork

Answer 23

This one is simple, although in general questions of mechanics require a basic understanding of vectors.

Now, tell me if gravity and the force exerted by the cord is in the same direction or opposite?

Answer 24

direction as in which way the jumper is going? @LastDayWork

Answer 25

Have you seen such a diagram before?

Answer 26

doesn't mg=-kx ? or am i close? @LastDayWork

Answer 27

The sign depends on how you write the vector.

So, have you seen a diagram like above; in school (or books) ?

Answer 28

yes, i have @LastDayWork

Answer 29

So how do you solve it? (Find x)

Answer 30

is this how its set up?
(68)(9.8)=(54)(x)? @LastDayWork

Answer 31

Yes

Answer 32

i got it. thanks c:

Answer 33

:)