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## Kinetic and Potential Energy

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**Kinetic and Potential Energy**Chapter 11**Definitions:**• Kinetic Energy – energy of motion • Formula: KE = 1/2mv2 • Units of KE =the joule • m = mass in kg • v = velocity in m/s**Potential Energy – energy of position, or stored energy**• Formula: Ug = mgh • Units of Ug are the joule • m = mass in kg • g = gravity (9.8 m/s2) • h = height in meters**Other things…**• With potential energy, the reference level is important. Remember, you may not always start off at 0 meters. • Velocity is what affects KE the most. Why? Look at the formula… • Height is what determines potential energy. Should the mass or gravity of an object change?**Other things continued…**• An object has the most KE when it is moving the (fastest/slowest)? • Ug will be greatest when an object is (highest/lowest) and (moving/not moving)? • Energy is conserved. This means energy is converted from KE to Ug and vice-versa.**Example 1**• A 90 kg rock climber climbs 45 m up to the top of a hill. He then walks 85 m from the top of the hill to the bottom. If the initial height is the reference level, find the Ug at the top of the hill and at the bottom.**Here we go again…**Reference level start finish**At the top:**• m = 90 kg • g= 9.8 m/s2 • h = 45 m • Ug = mgh • Ug = 90 x 9.8 x 45 • Ug = 39,690 J**At the bottom:**• m= 90 kg • g= 9.8 m/s2 • h= 40 m ???? • Ug = mgh • Ug = 90 x -9.8 x 40 • Ug = -35,280 J**Example 2**• A 50 kg artillery shell is shot from a cannon at Earth’s surface to a height of 425 m. • What is the gravitational potential energy of the system when the shell is at this height? • What is the change in potential energy when it falls to a height of 225 m?**Answers…**• a. 208,250 J • b. -98,000 J**Classwork**p. 269 #’s 38- 40, 42, 46-49 • Converting km/hr to m/s : multiply by 1000 then divide by 3600? • Use F = ma to find mass • Quiz Thursday!