2. Potential Energy

3. Review of Work Work is the product of force and distance. W = Fd If work is done on an object its energy is changed If the work done is positive, the energy of the object is increased. If the work done is negative, the energy is decreased.

4. Lifting a Ball This ball has a mass of 2-kg. What is its weight? w = mg = (2)(9.8) = 19.6-N What force must be used to lift this ball? Its weight: F = w = 19.6-N How much work will be done to lift this ball to a height of 5-m? F = 19.6-N d = 5-m W = Fd = (19.6)(5) = 98-J 19.6-N 5-m Since 98-J of work was done on the ball its energy was increased by 98-J. This was added to a form of energy called Potential Energy.

5. Potential Energy Potential Energy (PE) is energy stored in an object due to its position, and is stored by doing work. For our ball, its position is its height, 5-m, above the ground. This form of potential energy is technically called Gravitational Potential Energy Gravitational Potential Energy is found by multiplying the mass (m) of the object by gravity (g) and height (h). 5-m PE = mgh 98-J of work was done lifting this ball. It now has 98-J of potential energy stored in it. It now has the ability to do 98-J of work. PE = (2)(9.8)(9) = 98-J

6. Pile Driver A pile driver is a machine that uses the conversion of potential energy into work to drive beams and poles into the ground. The block at the top of the pile driver has a mass of 275-kg and is 9-m above the pole it is driving into the ground. What is the potential energy of the block? m = 275-kg h = 9-m PE = mgh = (275)(9.8)(9) = 24,255-J This means the pile driver (block) can do 24,255-J of work on the pole to drive it into the ground!!!!!!

7. A 2-kg ball is rolled up a ramp until it reaches a height of 5-m. What is the potential energy of the ball? 5-m m = 2-kg h = 5-m PE = mgh = (2)(9.8)(5) = 98-J The same as if we lifted it straight up!!!!

8. A 2-kg ball is rolled up a ramp until it reaches a height of 5-m. What is the potential energy of the ball? 5-m m = 2-kg h = 5-m PE = mgh = (2)(9.8)(5) = 98-J The same as if we lifted it straight up!!!!

9. When you are calculating the potential energy of an object, all you need to know is the mass and height of the object. How the object gets to the height doesn’t matter!!!!! 5-m No matter what path the 2-kg ball takes to get to a height of 5-m, it still has a potential energy of 98-J!!!

10. 4-m A 3-kg ball is at a height of 4-m when it starts to fall. At 4-m its PE = (3)(9.8)(4) = 117.6-J PE = 117.6-J At 3-m its PE = (3)(9.8)(3) = 88.2-J 3-m 2-m 1-m 0-m PE = 88.2-J At 2-m its PE = (3)(9.8)(2) = 58.8-J PE =58.8-J At 1-m its PE = (3)(9.8)(1) = 29.4-J PE =29.4-J At 0-m its PE = (3)(9.8)(0) = 0-J PE =0-J As the ball falls it loses potential energy because its height is decrasing. Where does this potential Energy go??? It is transferred into Kinetic Energy – Energy of Motion. I see more notes in your future!!!!!!!!!!!!!!