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Chapter 8 Work and Energy

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Definition Work is the way that energy is transferred between objects. The amount of work done equals the amount of energy transferred. Work is a scalar. Positive work on an object makes it gain energy. Negative work on an object makes it lose energy.

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Check Questions 1.A baseball thrown at 80 mi/h has about 350 J of kinetic energy. In its trip to the batter from the pitcher, air resistance does -80 J of work on it. How much kinetic energy does it now have? 2.The same baseball had no kinetic energy before the pitcher threw it. How much work did he do on the baseball?

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Equation for Work Work = Force x distance W=Fd (Note: Only the component of the force in the direction of motion does work!) Units—Joule(J) = Nm

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Check Question How much is work done: 1.If the 10-kg baby lifts himself to the top of the 1-meter high stairs? W=Fd=(10kg)(9.8m/s 2 )(1m)=98J 2.If he sits at the top of the stairs? W=0J

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Definition Energy is the ability to do work. An object with 10 J of energy can do up to 10 J of work on something else. Energy can take many forms (light, heat, sound, mass). For now, we will look at mechanical energy which is energy objects have because of their position or motion.

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Potential Energy Potential Energy (PE) --stored energy an object has due to its position. Gravitational potential energy —the energy an object has because of its position above the earth or another planet. PE = mgh Units—Joule(J) =( kg x m/s 2 ) x m = Nm

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Example Find the potential energy of a 50-kg person that is lifted to a height of 2 m. Find the same person’s PE if he is lifted twice as high.

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Kinetic Energy Kinetic Energy (KE) is energy of motion. Kinetic energy = ½ mass x velocity 2 KE = ½ mv 2 Units—J = (kg)(m/s) 2 = (kg)(m 2 /s 2 ) = (kg m/s 2 )(m)=Nm

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Example Find the KE of a 1000-kg car that is moving at 5 m/s. Find its KE when it is moving twice as fast.

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Work-Energy Theorem The amount of work done on a system equals the system’s change in mechanical energy (PE + KE). Some forces (gravity, electromagnetic forces, and springs) do not change the total energy, just change it between PE & KE and other forces (applied, friction, air resistance, normal, tension) do. Practice

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Which path requires the most energy?

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No change in mechanical energy. What force acts on both of these?

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Change in mechanical energy because work is done. What force does work?

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Review

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Review 2

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The Law of Conservation of Energy Energy can be neither created nor destroyed. It changes form and is transferred between objects (through work), but the total amount of energy is constant.

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Definition Power is the rate at which work is done. Power = Work / time P = W/t Units—Watt(W) = J/s

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Powerlifting! Facts: Your Physics book weighs about 10 N and the lab table is about 1 m high. Calculate how much work you do to lift 5 Physics books from the floor to the top of the lab table. Time yourself lifting the books all together and again lifting them one at a time. Calculate your power for each one.

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Are you as powerful as a horse? 1.Find your power in J: P=W/t=(your mass)(g)(height of stairs)/time 2. Convert to hp. One hp=745.7 W

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