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## Homework Statement

a wrecking ball, with a mass of 315kg, hangs from a crane on 10.0m of cable. if the crane swings the wrecking ball so that the angle that the cable makes with the vertical is 30', what is the potential energy of the wrecking ball in relation to its lowest position?

What will be the kinetic energy of the wrecking ball when it falls back to the vertical position?

What will be the speed of the wrecking ball?

## Homework Equations

i think i will need to find the distance between the ball and the wall sin30'(10m)

Ek = 1/2mv^2

Eg = mgh

the two questions before i am having a hard time with cause i believe i need the final velocity before hitting wall (the speed in this case) before i can solve the others.

but i could be totally wrong (wuldnt be the first time, ;)

## The Attempt at a Solution

sin30'(10) = 5m (this is the distance from the ball to the wall.

after that i do: Ek' + Eg' = Ek + Eg

and i am lost because i know there is something i am not doing but dont know what.

Ek' + Eg' = Ek + Eg

1/2mv^2 + mgh = 1/2mv^2 + mgh (mass canceled)

1/2v^2 + (-9.81)(5) = 1/2(0) + 9.81(0) (1/2 canceled)

v^2 = 49.1

v = 7m/s

this is the speed of the wrecking ball that i get.

the answers in my textbook are:

- Eg = 4140 J

- Ek = 4140 J (these two make sense because the two equal one another always

- V = 5.12m/s

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any help is always appreciated