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Work, Power and Energy

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Work When a force is applied and an object moves in the direction of the force, work has occurred. Ex. Picking up a box –you do positive work on the box and the box does negative work on you Work = Force X distance Work is measured in joules.

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Example problem A weight-lifter picks up a 45 N set of weights and lifts them a distance of.75 m. How much work was done? W=F X d W= 45N X.75m W=_________ J

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Energy Energy is the ability to do work. It is also measured in joules. Kinetic energy is the energy an object has due to its motion. K E =.5 mv 2 Energy is used to do work.

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Work energy theorem The change of kinetic energy is a system is equal to the work done on a system. Change in Ke = work This is the work energy theorem. Ex. If a 2kg object moves at 3m/s, what is its kinetic energy? This is the amount of work that has been done by the object.

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Through the process of doing work, energy can move between the environment and the objects or systems involved.

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Work when forces are applied at an angle. When finding the amount of work done on an object where the force is applied at an angle, you must use the formula below. W= F x D x cos theta

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Ex. A boat is being pulled by a rope that makes a 30 degree angle with the horizontal. The force applied at that angle is 300N. If the boat is pulled a distance of 2 m, how much work was done?

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Work= F X d X cos theta W= 300N X 2 m X cos (30) 520 J

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sonplans/grades.cfm/grade=3http://apps1.eere.energy.gov/education/les sonplans/grades.cfm/grade=3

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Power Power is measured in watts. A watt is a J/sec. Power = work/time If a machine does 25 J of work in 20 seconds, what is its power?

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A weight lifter lifts 30 kg of weight a distance of 1.3 meters in 15 seconds. What is his power? P = (F X d) / time P= (30kg X 9.8 X1.3m) / 15 sec watts

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Potential Energy Potential energy is the energy an object had due to its position. GPE= m X g X h The higher the object is from the ground the more GPE it has.

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Example How much GPE does a 55kg rock have sitting on top of a 190 m high cliff? GPE= 55kg X 9.8 X 190m = 1.02 X 10 5 Joules

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Examples Use this principle to determine the blanks in the following diagram. Knowing that the potential energy at the top of the tall platform is 50 J, what is the potential energy at the other positions shown on the stair steps and the incline ?

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GPE = mgh A=40 J B=30 J C=20 J D=10 J E and F = 0 J Use this principle to determine the blanks in the following diagram. Knowing that the potential energy at the top of the tall platform is 50 J, what is the potential energy at the other positions shown on the stair steps and the incline?

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The loss of GPE is always equal to the gain in KE. This is an example of the Law of Conservation of Energy. nergy/u5l2a.cfmhttp://www.physicsclassroom.com/Class/e nergy/u5l2a.cfm

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Mechanical Energy the mechanical energy of an object can be the result of its motion (i.e., kinetic energy) and/or the result of its stored energy of position (i.e., potential energy). The total amount of mechanical energy is merely the sum of the potential energy and the kinetic energy. This sum is simply referred to as the total mechanical energy (abbreviated TME ).kinetic energypotential energy TME= GPE + KE

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Example of TME The diagram below depicts the motion of Li Ping Phar (esteemed Chinese ski jumper) as she glides down the hill and makes one of her record-setting jumps.

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More… A horizontal force can never change the potential energy of an object. Horizontal forces cannot cause vertical displacements. The only means by which an external force can contribute to a potential energy change is if the force has a vertical component. Potential energy changes are the result of height changes and only a force with a vertical component can cause a height change.

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More Examples nergy/u5l1b.cfmhttp://www.physicsclassroom.com/Class/e nergy/u5l1b.cfm

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