How Do You Calculate the Maximum Kinetic Energy of a Dropped Sign?

[joepro]

Homework Statement

A 500N sign is dropped off a building. % seconds later it strikes the ground. Calculate teh Max KE and the origional PE of the sign

Homework Equations

The Attempt at a Solution

I know I need to find at least three of these varriables
Vo = 0
Vf
T = 5 seconds
S
a

Would a be 9.81 m/s2 Both answers are to be in Jewls Thanks for the help!

 

[cepheid]

1. You should be able to use basic kinematics to figure out what the starting height must be for a drop in free fall to take 5 seconds. (Btw, yes it is free fall, so to answer your question: a = -g = -9.81 m/s²)

2. Once you know the height, you know the initial PE.

3. Once you know the initial PE, you know the final KE (energy is conserved), and the final KE is equal to the max KE, since its speed will be highest at the bottom of the fall.

By the way, that's JOULES for the unit of energy.

EDIT: For step 1 above, use the kinematic equation that expresses how the displacement for motion in 1D under constant acceleration depends upon the time elapsed, the initial velocity, and the acceleration itself.

 

[joepro]

I found the dispancement to be 122.6 m?

But I do not know how to find the kinetic energy and potential energy in joules?

 

[cepheid]

I found the dispancement to be 122.6 m?

Good.

But I do not know how to find the kinetic energy and potential energy in joules?

Really? Let's define the gravitational potential energy to be 0 at the base of the building. What is the potential energy of an object of mass m at height h above the base of the building?

Or more simply: how much work is done by gravity during the fall?