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Work, Potential Energy, Kinetic Energy & Power

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Work, Potential Energy, Kinetic Energy & Power

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  1. Work, Potential Energy, Kinetic Energy & Power Whoa!!! Now I do!!!  I just don’t have any energy  The force, F, pushes the box for a short distance. This causes the box to start moving!!!!!! The force gives the object energy, it does work!!!!! F Energy is the ability to use a force and move something a distance. Energy is the ability to do work.

  2. Work If a force, F, is used to move an object a distance, d, then the force does work on the object. W = Fd F d

  3. Units for Work • The units for force are Newtons (N) and the units for distance are meters (m). • Since Work is equal to force times distance, we could use a Newton-meter (N·m). • But the more common unit is the Joule (J) • 1 J = 1 N·m • 1 J is defined as the amount of work for 1- N of force to move an object 1-m

  4. The weight lifter in the video lifts 280-kg a distance of 0.75-m. How much work does he do during one lift? The force needed to lift the barbell is equal to its weight!!!! F = w = mg = (280)(9.8) = 2744-N d = 0.75-m W = Fd = (2744)(0.75) =2058-J If he lifts it twice, how much work would he do? F = 2744-N W = Fd = (2744)(1.5) =4116-J d = 2(0.75-m) = 1.5-m Watch Video

  5. Work • Work is the product of force and distance. • W = Fd • If work is done on an object its energy is changed

  6. 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? 5-m F = 19.6-N d = 5-m W = Fd = (19.6)(5) = 98-J 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. 19.6-N

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

  8. 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. Pile Driver 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!!!!!!

  9. 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? m = 2-kg h = 5-m PE = mgh = (2)(9.8)(5) = 98-J The same as if we lifted it straight up!!!! 5-m

  10. 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? m = 2-kg h = 5-m PE = mgh = (2)(9.8)(5) = 98-J The same as if we lifted it straight up!!!! 5-m

  11. 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!!!

  12. Lifting a Ball • When you lift a ball to a certain height you do work on it. • This work (W) is equal to the weight of the ball (w=mg) times the height (h). • The work done to lift the ball is stored as potential energy. PE = W = mgh h W = Fd =wh = mgh

  13. Lifting a Ball • The ball now has potential energy stored in it. • This means the ball has the potential to do work if you drop it on something. • But before it can do that work it must convert the PE to another form of energy called kinetic energy – energy of motion PE = mgh h Kinetic Energy Energy of Motion

  14. Kinetic Energy • Kinetic Energy (KE) is energy of motion. • Kinetic Energy depends on the moving object’s mass (m) and the square of its speed (v).

  15. Kinetic Energy Example Find the kinetic energy of a 5000-kg Asian elephant running at 4-m/s. How could the elephant increase its kinetic energy? The elephant could increase its mass, increase its speed or both. But changing its speed would be better since KE depends more on speed.---Speed is squared!!!!!!

  16. Kinetic Energy Example The kinetic energy of a 5000-kg Asian elephant running at 4-m/s was calculated to be 40,000-J. What would its kinetic energy become if its mass doubled? What would its kinetic energy become if its speed doubled?