Potential Kinetic Energy Chapter 4 1 2242021 1

Potential & Kinetic Energy Chapter 4 1 2/24/2021 1

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What is energy ? • Wherever you are sitting as you read this, changes are taking place—light bulbs are heating the air around them, the wind might be rustling leaves, or sunlight might be glaring off a nearby window. Every change that occurs— large or small—involves energy. Energy makes change possible. 2/24/2021 3 3

5 Different Forms of Energy • electrical energy • chemical energy • radiant energy • thermal energy • kinetic energy • Is the chemical energy stored in food the same as the energy that comes from the Sun or the energy stored in gasoline? 4 2/24/2021 4

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Radiant Energy Electrical energy Nuclear Energy 7

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Different Forms of Energy 10 2/24/2021

Different Forms of Energy • Radiant energy from the Sun. • Radiant energy travels a vast distance through space to Earth, warming the planet and providing energy that enables green plants to grow. 11 2/24/2021

Kinetic Energy • Kinetic energy is the energy a moving object has because of its motion. • The kinetic energy of a moving object depends on the object’s mass and its speed. 12 2/24/2021

Kinetic Energy is energy that is in motion. • Moving water and wind are good examples of kinetic energy. • Electricity is also kinetic energy because even though you can't see it happen, electricity involves electrons moving in conductors. 13

Potential Energy • Even motionless objects can have energy. This energy is stored in the object. • A hanging apple in a tree has stored energy 14 2/24/2021

Potential Energy • Stored energy due to position is called potential energy. • If the apple stays in the tree, it will keep the stored energy due to its height above the ground. 2/24/2021 15

Potential Energy is measured in the amount of "work" it does. Potential Energy is stored energy. • Examples of potential energy are oil sitting in a barrel, or water in a lake in the mountains. This energy is referred to as potential energy, because if it were released, it would do a lot of work. 16

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Gravitational Potential Energy • Anything that can fall has stored energy called gravitational potential energy. • Gravitational potential energy (GPE) is energy stored by objects due to their position above Earth’s surface. 19 2/24/2021

Gravitational Potential Energy • On Earth the acceleration of gravity is 9. 8 m/s 2, and has the symbol g. • Like all forms of energy, gravitational potential energy is measured in joules. 20 2/24/2021

Gravitational Potential Energy 21 2/24/2021

• A 5 kg book is perched on a shelf 2 meters above the floor. How much stored energy does that book possess? Shelf GPE = (5 kg) X ( 9. 8 m/s²) X (2 m) GPE = 98 Joules Floor Gravitational potential energy – (GPE) is energy stored by objects due to their position above Earth’s surface 2/24/2021 22

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Elastic Potential Energy • The stretched rubber band had energy stored as elastic potential energy. • Elastic potential energy is energy stored by something that can stretch or compress, such as a rubber band or spring. 24 2/24/2021

Chemical Potential Energy • Gasoline stores energy in the same way as food stores energy-in the chemical bonds between atoms. • Energy stored in chemical bonds is chemical potential energy. Chemical Energy 25 2/24/2021

Chemical Potential Energy • Energy is stored in the bonds that hold the carbon and hydrogen atoms together and is released when the gas is burned. In this chemical reaction, chemical potential energy is released 26 2/24/2021

Potential to Kinetic Energy Conversions 2/24/2021 27

Potential - kinetic Energy Conversions 28

Pendulum Energy Conversions 29

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Energy Transformations in Projectile Motion • Energy transformations also occur during projectile motion when an object moves in a curved path. 32

Energy of a Slinky 33

Energy conversions of a tossed ball 34

Roller coaster Energy Conversions 35

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Roller coaster Energy Conversions 37

Energy Conversions 38

The Law of Conservation of Energy • The law of conservation of energy states that energy cannot be created or destroyed. 39

Energy conservations Energy Conservation Total energy is the sum of both types of energy. 40

The Law of Conservation of Energy • Energy can change from one form to another, but the total amount of energy never changes. 41

Conservation of Energy • A light bulb converts electrical energy into thermal energy and light energy. • Gasoline in a car converts chemical potential energy into kinetic energy of a moving car. • Green plants convert light energy into chemical energy potential energy. 42

Conservation of Energy Mechanical energy = Potential energy + Kinetic Energy in a system. • As an object falls, its potential energy changes to Kinetic energy. However the mechanical energy stays the same. 43

Law of Conservation of Energy • The total amount of Energy in the Universe always stays the same. • “Energy cannot be created or destroyed but it can change form but the total amount of energy remains the same. ” • When energy changes form, some of the energy is converted to heat energy because of friction. This heat energy that can’t be used do to work. 44

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Work & Power 46

Work & Power • Work: in order to do work on an object, the object must move. • • Work = Force X Distance Unit (SI) for work = Joule The Joule is the name for: N-m 1 N = kg/m/s² 47

Work 48

Power • Power is the rate at which work is done. • To do work quickly it requires more power. • Power = Work / Time • Unit (SI) for Power is the Watt. • Watt = Joule / Second 49

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Using Machines Conserving Energy • The amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine. • A machine cannot create energy, so Wout is never greater than Win.

Using Machines Conserving Energy • When a machine is used, some of the energy transferred changes to heat due to friction. • The energy that changes to heat cannot be used to do work, so Wout is always smaller than Win.

Work Makes Something Move • Remember that a force is a push or a pull. In order for work to be done, a force must make something move. • Work is the transfer of energy that occurs when a force makes an object move. • If you push against the desk and nothing moves, then you haven't done any work. 2/24/2021 Physical Science 53

Doing work • For example, when you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books. 2/24/2021 Physical Science 54

Work Force and Direction of Motion • When you carry books while walking, you might think that your arms are doing work. • However, in this case, the force exerted by your arms does no work on the books. 2/24/2021 Physical Science 55

• Work done is zero when displacement is zero. This happens when a man pushes a wall. There is no displacement of the wall. Thus, there is no work done. 2/24/2021 Physical Science 56

Work and Energy • When work is done, a transfer of energy always occurs. • This is easy to understand when you think about how you feel after carrying a heavy box up a flight of stairs. • You transferred energy from your moving muscles to the box and increased its potential energy by increasing its height. 2/24/2021 Physical Science 57

Calculating Work • The amount of work done depends on the amount of force exerted and the distance over which the force is applied. • When a force is exerted an object moves in the direction of the force, the amount of work done can be calculated as follows. 2/24/2021 Physical Science 58

Work 2/24/2021 Physical Science 59

Solve for work Solve force Solve for distance Solve for total work W = work Wtotal = total work F = force d = distance m = mass vinitial = initial velocity vfinal = final velocity 2/24/2021 Physical Science 60

2/24/2021 Physical Science 61

Power 2/24/2021 Physical Science • Power is the amount of work done in one second. It is a rate— the rate at which work is done. 62

Calculating Power • To calculate power, divide the work done by the time that is required to do the work. • The SI unit for power is the watt (W). One watt equals one joule of work done in one second. 2/24/2021 Physical Science 63

• Power is the rate of work done in a unit of time. The unit of the power from the equation given above, joule/s, however, we generally use the unit of power as watt. 1 joule/s = 1 watt 2/24/2021 Physical Science 64

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Work & Power • Work: in order to do work on an object, the object must move. • • Work = Force / Distance Unit (SI) for work = Joule The Joule is the name for: N-m 1 N = kg/m/s² 2/24/2021 Physical Science 66

Work Power and Energy • Just as power is the rate at which work is done, power is also the rate at which energy is transferred. • When energy is transferred, the power involved can be calculated by dividing the energy transferred by the time needed for the transfer to occur. 2/24/2021 Physical Science 67

Work & Power 2/24/2021 Physical Science 68

Using Machines What is a machine? • A machine is a device that makes doing work easier. • Machines can be simple. • Some, like knives, scissors, and doorknobs, are used everyday to make doing work easier. 2/24/2021 Physical Science 69

Using Machines Ideal Machines • Suppose the ideal machine increases the force applied to it. • This means that the output force, Fout, is greater than the input force, Fin. • Recall that work is equal to force times distance.

Using Machines Ideal Machines • If Fout is greater than Fin, then Win and Wout can be equal only if the input force is applied over a greater distance than the output force is exerted over.

Using Machines Mechanical Advantage • The ratio of the output force to the input force is the mechanical advantage of a machine. • The mechanical advantage of a machine can be calculated from the following equation.

Using Machines Efficiency • Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine. • A machine with high efficiency produces less heat from friction so more of the input work is changed to useful output work.

Using Machines Calculating Efficiency • To calculate the efficiency of a machine, the output work is divided by the input work. • Efficiency is usually expressed as a percentage by this equation:

Using Machines Calculating Efficiency • To calculate the efficiency of a machine, the output work is divided by the input work. • Efficiency is usually expressed as a percentage by this equation: