Energy

Chapter 15

Chapter 15 Pretest1. How much work is done when a

weightlifter holds a barbell motionless over his head?– No work is done!

2. Calculate the work done on a 2-N mass when it is lifted to a height of 2 m.– 4 J

3. Calculate the average speed of a bicycle that travels 100 m in 20 s.– 5 m/s

Chapter 15 Pretest

4. Is weight a force? – yes

5. What is the formula for calculating weight?

– W = mg

6. How does the temperature of an object change when it is acted on by friction?

– The temperature increases.

Chapter 15 Pretest7. True or false: In a closed system, the

loss of momentum of one object equals the gain in momentum of another object.– True

8. How is power related to work?– Power is the rate at which work is done

9. True or false: The amount of work done on a machine (work in) always equals the amount of work done by the machine (work out).– False

How is Energy Related to Work?• Energy is defined as the ability to do work.

Recall that work is the product of force and distance. If a force acts through a greater distance, it has done more work. You can use work to measure changes in energy.

• Place two identical books on the table so there is a gap of about 8 cm between books. Place a sheet of notebook paper on the books so it covers the gap shown. Now drop a penny from a height of 10 cm onto the paper above the gap. Note what happens. Next, drop the penny from a height of 30 cm and observe the results.

How is Energy Related to Work?

1. How did the height of the penny affect the distance the paper moved?– The paper moved farther when the penny was

dropped from a greater height.

2. How did lifting the penny affect the work it did on the paper?– Lifting the penny allowed it to do more work on the

paper.

3. How did lifting the penny affect its energy?– Lifting the penny increased the penny’s energy.

15.1- Energy and Its Forms• Energy is the ability to do work.

– Energy is transferred by a force moving an object through a distance.

• Work and Energy are closely related.– When work is done on an object, energy is transferred

to that object.– Work is a transfer of energy.

• Energy is typically measured in joules (J) just like work.

15.1 – Kinetic Energy• The energy of motion is called kinetic energy.

• The kinetic energy of any moving object depends upon its mass and speed.

KE = ½ mv2

• KE = joules (J), mass (m) = kg, speed (v) = meters/sec (m/s)

– Double the mass, double the KE– Double the speed, quadruple the KE

Kinetic Energy Math Practice

1. A 70.0 kilogram man is walking at a speed of 2.0 m/s. What is his kinetic energy?– KE = 1/2mv2 = 0.5(70kg)(2 m/s)2= 140 J

2. A 1400 kilogram car is moving at a speed of 25 m/s. How much kinetic energy does the car have?– KE = 1/2mv2 = 0.5(1400kg)(25 m/s)2= 437,500 J

3. A 50.0 kilogram cheetah has a kinetic energy of 18,000 J. How fast is the cheetah running?– v = √2(KE)/m = √2(18,000 J)/50.0 kg = 27 m/s

15.1 – Potential Energy

• Potential energy is the energy that is stored as a result of position or shape.– PE has the potential to do work– Examples: plucking a guitar string, lifting a

book

• Two forms of potential energy are:– Gravitational Potential Energy– Elastic Potential Energy

15.1 – Gravitational Potential Energy• Potential energy that depends on an object’s

height is called gravitational potential energy.– Gravitational Potential Energy increases when an

object is raised to a higher level.

• An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity.

PE = mgh• PE = joules (J), mass (m) = kilograms (kg), acceleration due to

gravity (g) = 9.8 m/s2 on Earth, height (h) = meters• Note: height is measured from the ground or floor, and GPE is

measured relative to that same reference level.

15.1 – Elastic Potential Energy

• The potential energy of an object that is stretched or compressed is known as elastic potential energy.

• Something is said to be elastic if it springs back to its original shape after it is stretched or compressed.

• Examples: rubberband, spring, basketball, shock absorber, wind-up toy

15.1 – Forms of Energy• All energy can be considered to be kinetic

energy, potential energy, or the energy in fields such as those produced by electromagnetic waves.

• The major forms of energy are:– mechanical energy– thermal energy– chemical energy– electrical energy– electromechanical energy– nuclear energy.

15.1 – Mechanical Energy• The energy associated with the motion and position

of everyday objects is mechanical energy.

• Mechanical energy is the sum of an object’s potential energy and kinetic energy.

• Examples: speeding trains, bouncing balls, sprinting athletes

• Mechanical energy is NOT limited to machines.

• Mechanical energy does not include thermal energy, chemical energy, or other forms of energy associated with the motion or the arrangement of atoms or molecules.

15.1 – Thermal Energy• The total potential and kinetic energy of all the

microscopic particles in an object make up its thermal energy.– Almost all of the matter around you contains atoms.– These particles are always in random motion and thus

have kinetic energy.

• When an object’s atoms move faster, its thermal energy increases and the object becomes warmer.

• Examples: molten metal, toasting marshmallows

15.1 – Chemical Energy

• Chemical energy is the energy stored in chemical bonds.– When bonds are broken, the released energy

can do work.

• All chemical compounds store energy.

• Examples: fuels such as coal or gasoline, wood

15.1 – Electrical Energy

• Electrical energy is the energy associated with electric charges.– Electric charges can exert forces that do

work.

• Examples: batteries, lightning bolts

15.1 – Electromagnetic Energy

• Electromagnetic energy is a form of energy that travels through space in the form of waves.

• Examples: visible light, x-rays, sun

15.1 – Nuclear Energy

• The energy store in atomic nuclei is known as nuclear energy.

• Examples: heat and light of the sun, nuclear power plant operations– nuclear fission – process that releases energy by

splitting nuclei apart – nuclear fusion – releases energy when less massive

nuclei combine to form a more massive nuclei

15.2 – Energy Conversion

• Energy can be converted from one form to another.

• The process of changing energy from one form to another is energy conversion.

• Examples: wind-up toys, light bulbs, striking a match

15.2 – Conservation of Energy

• When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur.

• The law of conservation of energy states that energy cannot be created or destroyed.– Energy can be converted from one form to another.– In a closed system, the amount of energy present at

the beginning of a process is the same as the amount of energy at the end.

15.2 – Energy Conversions

• One of the most common energy conversions is between potential energy and kinetic energy.

• The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls.

• Examples: avalanche, compressed spring, gull/oyster shell

15.2 – Energy Conversion

• A pendulum consists of a weight swinging back and forth from a rope or a string.

• Pendulum Examples: rope swing, clock

15.2 – Energy Conversion Calculations

• Mechanical Energy = KE + PE

• *Conservation of Mechanical Energy (KE + PE)beginning = (KE + PE)end

* Friction is neglected

Conservation of Mechanical Energy

• A 10 kg rock is dropped and hits the ground below at a speed of 60 m/s. Calculate the gravitational potential energy of the rock before it was dropped. You can ignore the effects of friction.

PEbeginning = KEend = 1/2mv2

=(0.5)(10kg)(60m/s)2 = 18,000 J

Conservation of Mechanical Energy• A diver with a mass of 70.0 kg stands motionless

at the top of a 3.0 m high diving platform. Calculate his potential energy relative to the water surface while standing on the platform, and his speed when he enters the pool. (Hint: Assume the diver’s initial vertical speed after diving is zero).PEbeginning = mgh

At the beginning, KE = 0 and at the end, PE = 0,

so PEbeginning = KEend = 1/2mv2

2100 J = (0.5)(70.0kg)v2

v = 7.7m/s

Conservation of Mechanical Energy

• A pendulum with a 1.0 kg weight is set in motion from a position of 0.04 m above the lowest point on the path of the weight. What is the kinetic energy of the pendulum at the lowest point? (Hint: Assume there is no friction.)

PEbeginning=mgh

=(1.0 kg)(9.8 m/s2)(0.04m) = 0.4 J

At the beginning, KE = 0; at the lowest point, PE = 0;

PEbeginning = KEend = 0.4 J

15.2 – Energy and Mass• Special theory of relativity developed by

Albert Einstein in 1905.• E = mc2 c = 3.0 x 108 m/s• E = energy, m = mass, c = speed of light

• Einstein’s equation says that energy and mass are equivalent and can be converted into each other.– Energy is released as matter is destroyed– Matter can be created from energy

15.3 – Energy Resources

• Energy resources can be classified as renewable or nonrenewable.

• Nonrenewable – exist in limited supply and cannot be replaced quickly– Examples: oil, natural gas, coal, uranium– Fossil Fuels: oil, natural gas, coal

• Usually readily available and inexpensive, but their use creates pollution

15.3 – Renewable Energy Resources

• Renewable – energy resources that can be replaced in a relatively short period of time.– Most originate directly or indirectly from the

sun– Examples: hydroelectric, solar, geothermal,

wind, biomass, nuclear fusion

15.3 - Definitions

• Hydroelectric energy – energy obtained from flowing water

• Solar energy – sunlight that is converted into usable energy

• Geothermal energy – thermal energy beneath Earth’s surface

• Biomass energy – chemical energy stored in living things

• Hydrogen fuel cell – generates electricity by reacting hydrogen with oxygen.

15.3 – Conserving Energy Resources

• Energy resources can be conserved by reducing energy needs and by increasing the efficiency of energy use.

• Things you can do:– Turning off lights, etc. when not in use– Walk or bike on short trips– Carpool or mass transportation– Fuel-efficient automobiles– Energy-efficient purchases