Chapter 8

Dr. Jie Zou PHY 1151G Department of Physics

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Chapter 8

Potential Energy and Conservative Forces

Dr. Jie Zou PHY 1151G Department of Physics

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Gravitational Potential Energy

Gravitational potential energy U: The energy of a body due to elevated positions is called gravitational potential energy. U = weight height = mgh The gravitational potential

energy is relative to the reference level and depends only on mg and the height h.

Dr. Jie Zou PHY 1151G Department of Physics

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Examples Example 1: Find the gravitational

potential energy of a 65-kg person on a 3.0-m-high diving board. Let U = 0 be at water level.

Example 2: An 82.0-kg mountain climber is in the final stage of the ascent of 4301-m-high Pikes Peak. What is the change in gravitational potential energy as the climber gains the last 100.0 m of altitude? Let U = 0 be (a) at sea level and (b) at the top of the peak.

Dr. Jie Zou PHY 1151G Department of Physics

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Conservation of Mechanical Energy

Mechanical energy E: The sum of the potential and kinetic energy of an object. E = U + K.

Conservation of mechanical energy: In systems with conservative force (such as gravity) only, the mechanical energy E is conserved. Ui + Ki = Uf + Kf.

Dr. Jie Zou PHY 1151G Department of Physics

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Example 8-6 A player hits a 0.15-kg

baseball over the outfield fence. The ball leaves the bat with a speed of 36 m/s, and a fan in the bleachers catches it 7.2 m above the point where it was hit. Assuming frictional forces can be ignored, find (a) the kinetic energy of the ball when it is caught and (b) its speed when caught.

Dr. Jie Zou PHY 1151G Department of Physics

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Example 8-7 A 55-kg skateboarder

enters a ramp moving horizontally with a speed of 6.5 m/s, and leaves the ramp moving vertically with a speed of 4.1 m/s. Find the height of the ramp, assuming no energy loss to frictional forces.

Dr. Jie Zou PHY 1151G Department of Physics

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Homework Chapter 8, Page 227, Problems: #

7, 13, 14.