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ENERGY Part I

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**Learning Goals for Section 1 The Nature of Energy**

Distinguish between kinetic and potential energy. Calculate kinetic energy. Describe different forms of potential energy. Calculate gravitational potential energy.

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What is Energy? the ability to do work the ability to cause a change

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**Work, Power, Efficiency Work = (Force) (distance) Power = Work / time**

Work measured in Nm 1 Nm = 1 Joule Power = Work / time Power measured in J/s 1 J/s = 1 Watt 1 HP = 764 Watts Efficiency = (Wout / Win) x 100 Efficiency is a percentage (%)

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**Work Power Efficiency Problems**

Amy uses 20N of force to push a lawn mower 10 meters. How much work does she do? Frank does 4.8 E4 J of work in climbing a set of stairs. If he climbs the stairs for 2 minutes, what is his power output? A block and tackle does 1.25 E5 J of useful work, but friction limits the block and tackle to an efficiency of 45%. What is the amount of input on work block and tackle?

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**Kinetic Energy Kinetic Energy Equation energy of motion**

depends on mass and speed depends on speed more than it depends on mass – LOOK AT THE EQUATION!!! Kinetic Energy Equation kinetic energy (joules) = ½ mass (kg) × [speed (m/s)]2 KE = ½ mv2

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**Kinetic Energy Equations**

KE = ½ mv (Joules) m = 2 KE (kilograms) v2 v = √2KE (m/s) m

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**KE Problems What is the KE of an 8 kg mass at 5 m/s?**

What is the mass of an object that has 100 J of KE when moving at 5 m/s? Determine the velocity of a 6 kg mass has 75 J of KE. A rocket with a mass of 1.5 E4 kg accelerates at 220 m/s2 for 29s from an initial speed of 5200 m/s. Determine the kinetic energy of the rocket before and after the acceleration.

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**stored energy Potential Energy stored energy in an object**

means the object has potential to cause change

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**Gravitational Potential Energy**

stored energy due to position depends on both mass and height Gravitational Potential Energy Equation GPE (J) = mass (kg) × acceleration due to gravity (m/s2) × height (m) GPE = mgh remember… mass x acceleration due to gravity = weight acceleration due to gravity is 9.8 m/s2

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**GPE Equations GPE = mgh (Joules) m = GPE (kg) gh h = = GPE (m) mg**

g = GPE (m/s2) mh

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GPE Problems What is the GPE of a 3 kg ball that is 2 m above the floor? How high do you have to lift a 5 kg box to give it 98 J of GPE? What is the weight of an stone that has 1200 J of GPE when setting on a 275 m cliff

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**Elastic Potential Energy**

energy stored by something that can stretch or compress – rubber band, spring PEE = ½ kd2 K is the spring constant for the material Measured in N/m d is the compression/expansion distance of the material Measured in meters

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PEE Equations PEE = ½ kd2 (Joules) k = 2PEE (N/m) d2 d = √2PEE (m) k

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PEEProblems The elastic force constant of a spring in a toy is 550 N/m. If the spring is compressed 1.2 cm, compute the potential energy stored in the spring. The spring constant (k) of a car’s front coil is 1800 N/cm. When the front tire rolls over a rock, the spring is compressed 15 cm from its equilibrium position. How much PEE is stored in the spring?

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**PEEProblems continued**

A 100.0 g arrow is pulled back 30.0 cm against a bow, which has an elastic force constant of 125 N/m. If the entire potential energy stored in the bow is converted to the kinetic energy of the arrow, compute the speed of the arrow.

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