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Mechanical Energy. Energy. Energy is the thing that enables an object to do work. Measured in joules It appears in many forms Two most common forms of mechanical energy are Potential and kinetic energy. What is the difference between potential and kinetic energy? Examples of potential?
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Energy • Energy is the thing that enables an object to do work. • Measured in joules • It appears in many forms • Two most common forms of mechanical energy are Potential and kinetic energy.
What is the difference between potential and kinetic energy? • Examples of potential? • Examples of kinetic? • When ever work is done energy is exchanged! • Only changes in potential energy are meaning full.
Gravitational potential energy • The potential energy due to elevated position. • The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity in lifting it. • W=Fd • GPE=weight x height • GPE=mgh
Question • How much work is done in lifting the 100-N block of ice a vertical distance of 2 m? • W=Fd=100 N x 2 m= 200 J
Question • How much work is done in pushing the same block of ice up a 4-m- long ramp? • The force needed is only 50 N which is the reason ramps are used. • W=Fd= 50 N x 4 m = 200 J
Kinetic Energy • Is energy in motion and can do work. • It is equal to the mass multiplied by the square of the speed, multiplied by the constant ½. • Kinetic energy=1/2 mass x speed2 • KE=1/2mv2 • The kinetic energy of a moving object is equal to the work required to bring it from rest to the speed, or the work the object can do while being brought to rest. • Work= kinetic energy • Fd=1/2mv2
Kinetic and potential energy Pendulum • http://media.pearsoncmg.com/aw/aw_0media_physics/hewittfigures/IF_energy_trans_pendulum.swf
Work Energy Theorem • When a car speeds up, its gain in kinetic energy comes from the work done on it. • Or, when a car slows, work is done to reduce its kinetic energy . • We can say: Work= ∆KE • Work-energy theorem is net work equals change in kinetic energy.
Net force • Example: • If you push a box across a floor there is also friction acting upon the object in the opposite direction. • The change in Kinetic energy is equal to the work done by the net force. • The net force is your push minus friction. • Thus only part of your total work changed the objects kinetic energy. • The rest is used to overcome friction and goes into heat.
Applies to decreasing speed also . • When you slam on the brakes of a car that skids, the road does work on the car. • This work is the friction force multiplied by the distance over which the friction force acts. • When an automobile is braked, the drums and tires convert kinetic energy to heat.
Applies to all energy types • The work-energy theorem can apply to more than changes in kinetic energy. • Work =∆E • Work is not a form of energy but a way of transferring energy from one place to another or one form to another.
Conservation of energy • Every for of energy can be transformed into every other form. • Energy is nature’s way of keeping score. • The law of conservation of energy states: • Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. • It may change form or it may be transferred from one place to another, but the total energy score stay the same.
Question • Rows of wind-powered generators are used in various windy locations to generate electrical power. Does the power that is generated affect the speed of the wind? That is, would locations behind the wind generators be windier if the generators weren’t there? • Wind-powered generators take KE from the wind, so the wind is slowed by interactions with the blades. • So, yes it would be windier behind the wind generators if they weren’t there.
