General Physics February 6, 2017 Potential Energy General Physics February 6, 2017

Work & Kinetic Energy – A Review Work is ONLY done when a force moves an object a distance in the direction of that force. Work = ForceDistance Work-Energy Theorem: It takes Energy to do Work! Kinetic Energy is the energy of motion. KE = ½ massvelocity2 KE = ½ mv2

WE WILL FOCUS ON THESE TWO TODAY! How is Energy divided? WE WILL FOCUS ON THESE TWO TODAY!

Sports – Potential and Kinetic Energy

What is Potential Energy? Potential Energy is energy that is “stored” & “waiting” to do work later.

What is Gravitational Potential Energy? Gravity gives potential energy to an object. This potential energy is a result of gravity pulling downwards.

How is Gravitational Potential Energy Calculated? We have already practiced this when we calculated the work done with and against gravity! GPE = mass  accelerationgravity  height GPE = mgh Joules GPE meters kilograms m h g = 9.81m/s2

Example #1: Spiderman has an object suspended in the air. It has a mass of 50 kilograms and is 50 meters above the ground. How much work would the object do if it was dropped?

Example #1: Answered Solution: 24525 Joules PE = mgh PE of the object is (50 kg)(9.81 m/s2)(50 m) PE = (2500 kg m)(9.81 m/s2) PE = 24525 kg m2/s2 or 24525 Nm or 24525 Joules If it was dropped it could do 24525 Joules of work.

Example #2: Jean Grey dropped an object from 10 meters. She knows it did 50 joules of work. How much is its mass?

Example #2: Answered Solution: 0.51 kilograms PE = mgh PE of the object is 50 joules 50 kg m2/s2 = m(9.81 m/s2)(10 m) Solve for m m = 50 kg m2/s2 / (9.81 m/s2)(10 m) m = 0.51 kg The mass of the object was about 0.51 kilograms.

Example #3 Superman drops the Incredible Hulk onto the ground! If the Hulk has a mass of 200-kg and Superman did 3924 Joules of work, then how far did the Hulk drop?

The height from which the Hulk was dropped is 2 meters. Example #3: Answered Solution: 2 meters PE = mgh PE of the Hulk is 3924 joules 3924 kg m2/s2 = (200 kg)(9.81 m/s2)h Solve for h h = 3924 kg m2/s2 / (9.81 m/s2)(200 kg) h = 2 m The height from which the Hulk was dropped is 2 meters.

What is Elastic Potential Energy? This is energy that is stored due to the compression (pushing together) or expansion (pulling apart) of an elastic object.

How is Elastic Potential Energy Calculated? We have already practiced part of this when we learned about Hooke’s Law! EPE = ½ Forceon the elasticdistanceto move the elastic EPE = ½ Fd However, we also learned about the spring constant of elastic objects! Spring constant (k) = Force / distance Force = distancespring constant F = d/k Therefore: EPE = ½ Fd and EPE = ½ kd2

Example #4 Spiderman swings on a web with a force of 250 Newtons while expanding the web a distance of 10 meters. How much elastic potential energy is in the web?

Example #4, Answered Given: F = 250 N and d = 10 m Find: EPE = ½ Fd Solution: EPE = ½ (250 N)(10 m) = 1250 J

Example #5 Spiderman swings on a web with a spring constant of 25 N/m while expanding the web a distance of 10 meters. How much elastic potential energy is in the web?

Example #5, Answered Given: k = 25 N/m and d = 10 m Find: EPE = ½ kd2 Solution: EPE = ½ (25 N/m)(10 m)2 = 1250 J