Question

A rock at the top of a 30 meter cliff has a mass of 25 kg. Calculate the rock’s gravitational potential energy when dropped off the cliff. Assuming that energy is conserved and there is no air friction and gravity is 9.8 m/sec., at the bottom of the cliff the rock’s potential energy is completely converted to kinetic energy. Use the formula for kinetic energy to calculate the rock’s speed at the bottom of the cliff. Show all calculations.

Answer 1

Lets see:-

We have our formula for potential energy, which we are trying to solve for, 

PE = mgh  or  potential energy = mass * gravity * height

So we know that PE all depends on these. 

Height :  30 meters
Mass : 25 kg
Gravity (which is always constant) : 9.8 m/s/s

Now add into formula. 

PE = 25*30*9.8 
PE = 7350 Joules

Answer: PE = 7350 Joules

Now calculate speed:-

7,350 joules/12.5 kg  =  speed-squared
 588 m²/s²  =  speed-squared
 about 24.248 m/s