# Energy in spring (slingshot problems)

**Sorry I just noticed I posted this is the wrong forum, I don't see any delete button could a moderator move this to coursework/homework forum?

This is a problem that I kept getting wrong and never understood why. I'm not asking for answers just help on tackling the question. I have tried it before and I only got a. right. I am paraphrasing the question by the way.

11. There is a slingshot. It can be stretched 2.00 meters from its rest position with a force of 760N. They place a 6.50kg bowling ball in the sling and pull it down 2.90 meters from rest position. There is only vertical movement.

A)Find spring constant.
I got k=380N/m.

B)What is the total energy stored in the sling when the bowling ball is ready to be fired.
I tried Energy=0.5(k)(x)^2 where k is from A) and x is 2.90 meters. I got 1597.9 J

C)Total energy of bowling ball when it leaves the slingshot.
I tried Energy stored in spring. 1597.9 J

D)What is the kinetic energy when the ball leaves the sling?
I tried Energy stored in spring - potential energy used to gain 2.90 meters. 1413.17 J

F)Ignoring air Resistance what is maximum height.
I tried kinetic energy = potential energy (Solving for height in potential energy) ~22.184m + 2.9m from rest point.

Last edited:

gneill
Mentor
What other form of potential energy is operating here?

I think gravitational potential energy and elastic potential energy are the two I have to work with. Are you hinting at me missing one in my calculations? Would it happen to deal with what my x=0 is set at?

it seems right to me, but i think its probably easier to go right from elastic potential to graviational potential instead of converting your energy twice. is it possible that the question wants you to use g = Gmm/(r+h) as your gravitational acceleration, or wants you to take into account drag forces?

gneill
Mentor
Can you define the difference between the "total energy" of part C and the "kinetic energy" of part D? Since the question did not specify zero references for the potential energies involved, it's a bit ambiguous.