Calculating Impact Speed of 4.00kg Cannon Ball Dropped from 55.0m

[redbutterfly]

Homework Statement

Using energy conservation and assuming negligible air resistance, calculate the impact speed of a 4.00 kg cannon ball after being dropped from a height of 55.0 m.

Homework Equations

KE sub total=KE sub 1+KE sub 2
speed=distance/time
gravity=9.8

The Attempt at a Solution

I'm completely lost as to how to get this with energy conservation. Can someone please help?

 

[Delphi51]

Welcome to the physics forums! Hope you ask lots of questions here.

As the ball drops, its gravitational potential energy is converted to kinetic energy. Start with:
Potential energy at the top = kinetic energy at the bottom

Fill in the detailed formulas, solve for the quantity you want to find, then put in the numbers you know.

 

[redbutterfly]

Thank you!

TE=KE+PE sub g
PE sub g=mgh
PE sub g=4*9.8*55=2200 J
KE=0
TE=0+2200
2200=KE+0
KE=2200=1/2*4*v^2
4400=4*v^2
1100=v^2
v=33 m/s

 

[Delphi51]

That's it!
Maybe simpler to write mgh = 1/2m*v^2, cancel the m's, then solve for v:
v = sqrt(2gh) = sqrt(2*g*55) = 32.8 m/s