Unit 2: EnergyTHERMAL ENERGYHEAT TRANSFERPOTENTIAL VS. KINETIC ENERGYWORKPOWERSIMPLE MACHINES

Bellringer Day 01

1. What is energy?

2. There are different forms of energy. Name two.

What is Energy?

• Energy is the ability to do work-in other words, to cause change

• In order for something to change, there must be a transfer of energy from one thing to another

• This unit we will be discussing energy in its different forms, including thermal, potential/kinetic, and the ability to do work.

What Energy Is…And Is NOT

• Energy is not a substance…

• It cannot be weighed

• It does not take up space

• Energy is a condition!

• The Law of Conservation of Energy says: Energy cannot be created or destroyed. It can only be transformed from one type to another or passed from one object to another.

Thermal EnergyHEAT AND TEMPERATURE

What is Thermal Energy?

• Thermal Energy is the energy that comes from heat

• Heat is generated by the movementof tiny particles within an object• The faster these particles move, the

more heat is generated

• We measure heat by recording the temperature of something

Thermal Energy and Particles

• When the motion of particles in an object increases, so does the temperature

• When the motion of particles in an object decreases, so does the temperature

Measuring Thermal Energy

• Temperature indicates the average speed of particle motion in a substance

• The Celsius Scale is the common scientific measurement of temperature using the unit degrees; based on the freezing and boiling points of water

• Water boils at a temperature of 100o Celsius and freezes at 0o Celsius

• Other degrees of temperature include Fahrenheit and Kelvin

Absorbing and Releasing Thermal Energy

• Depending on the material, heat may be absorbed or released

• Insulation is the process of trapping thermal energy to keep warm

• Objects releasing heat energy will be getting colder, and objects absorbing heat energy will be getting warmer

• Keep in Mind: Thermal energy always travels from the warmer object to the cooler object

Thermal Energy Absorption Experiment

• Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?

Shiny Metal Dull Metal Shiny Black Dull Black

The dull black container would be the warmest after 10 minutes. It is an insulator, and good at absorbing heat. The shiny metal would be the coolest. It’s surface is reflecting heat.

Bellringer Day 02

1. What is the difference between heat and temperature?

2. If you zoomed in on the molecules of a hot object vs. a cold object, what difference would you see?

3. Name one way that homes are designed to insulate heat energy.

Heat TransferCONVECTION, CONDUCTION, AND RADIATION

Heat Transfer

• Thermal energy always flows from warm to cool

• Hot objects in a cooler room will cool to room temperature

• Cold objects in a warmer room will heat up to room temperature

Methods of Heat Transfer

• Heat transfers from one object to another in three ways:

• Conduction

• Convection

• Radiation

Conduction

• Heat energy travels when two objects at a different temperature are in direct contact with each other (touch)

• During heat conduction, vibrating atoms/molecules excite the atoms/molecules next to them.

Case Study: Why does metal feel colder than wood, if they are both at the same temperature?

• Metal is a conductor, wood is an insulator.

• The metal feels colder because it is a better conductor-heat moves easily out of your warmer hand into the colder metal

• The wood feels warmer because it is a poor conductor. Little heat moves out of your hand into the wood, so your hand does not sense that it is touching something cooler

Convection

• The transfer of heat through a fluid, such as liquid or gas

• The hot, less dense fluid rises while cold, more dense fluid sinks, resulting in the transfer of heat

Case Study: Convection in a Refridgerator

• Freezers are generally located on the top of the fridge. This creates a convection system, where the cold air from the freezer sinks and cools the lower units

Radiation

• Heat energy travels as electromagnetic waves from the sun

Bellringer Day 03

1. If a cup of hot coffee and a red popsicle were left on the table in this room what would happen to them? Why?

2. Create a chart comparing the three methods of heat transfer.

Thermal EnergyPHASE CHANGES

Heat

• Recall that heat is the thermal energy that is transferred from particles at a higher temperature to particles at a lower temperature

Phase Changes

• A phase change diagram shows the relationship of the particles in matter as heat energy is transferred

• Notices as particles heat up and temperature increases, they change phase (IceWaterWaterVapor)

Specific Heat

• Specific Heat is the amount of heat energy required to raise 1 g of the material 1°C

• Water has a high specific heat, meaning it takes a lot of energy to raise the temperature of water

• Thermal energy is determined through the formula Q = mCΔT

• Q=Thermal energy (measured in Joules)

• m=Mass of the liquid being heated

• C=The specific heat, in oC

• ΔT=Change in temperature (Tf-Ti)

You do not have to know this formula-just understand that the amount of heat transferred is related to how large the object is, how much energy it requires to heat, and the temperature!!

Bellringer Day 04

• Label the phase change diagram below to the best of your ability, showing how heat is transferred. You can use your notes to help.

Kinetic vs. Potential EnergyKINETIC AND POTENTIAL ENERGY, MECHANICAL ENERGY, AND SYSTEMS

Types of Energy

• We’ve discussed energy in the form of heat (thermal energy), but energy comes in many forms.

• The seven forms of energy are: thermal, chemical, electrical, nuclear, sound, radiant, and mechanical-we will be focusing for the remainder of the unit on mechanical!

• Energy can change between these forms. Remember the Law of Conservation of Energy: Energy cannot be created or destroyed. It can only be transformed from one type to another or passed from one object to another.

Kinetic Energy

• Kinetic energy is energy of motion

• The kinetic energy of an object is dependent on its mass and velocity

KE = ½ mv2

KE=Kinetic Energy

m=mass

v=velocicy

Energy is measured in a

unit called Joules (J)

Solving for Kinetic Energy

Find the kinetic energy of a 10 kg bowling ball that is rolling at 3 m/s

GIVEN

• KE = ½ mv2

• KE=?

• m=10kg

• v=3m/s

SOLVE

• KE = ½ mv2

• KE = ½ (10) (3)2

• KE = ½ (10) (9)

• KE = ½ (90)

• KE = 45 J

Potential Energy

• Potential Energy is energy stored in an object

• It is related to the object’s position

Gravitational Potential Energy

• Gravitational Potential Energy (PEg) is energy stored in objects that can fall

PEg = mghPEg=Potential Energy

m=mass

g=gravity

h=height

Remember, on Earth g=9.8

m/s2

Solving for Potential Energy

Tanner’s Coke can sits on his desk, 1 m above the floor. The mass of the can is 0.5 kg. Find its PEg

GIVEN

• PEg = mgh

• PEg=?

• m=0.5kg

• g=9.8m/s2

• h=1m

SOLVE

• PEg = mgh

• PEg = (.5)(9.8)(1)

• PEg = 4.9 J

Comparing Potential and Kinetic Energy

• Remember, energy is converted from one form to another

• For a falling object, the potential energy will decrease as the height decreases

• For that same falling object, the kinetic energy will increase because gravity increases

• The energy changes from potential to kinetic!

Mechanical Energy

• Mechanical energy is the total amount of kinetic and potentialenergy in a system

• A system is a group of objects that work together

Conserving Energy

• The law of the conservation of energy essentially means that the amount of energy in the universe does not change

• It describes what happens to energy as it is transformed from one type to another-even in situations where energy seems like it is not being conserved, it is! It is always transformed. The total amount of energy always stays the same.

Friction

• So what is causing us to lose mechanical energy? Friction!

• Friction changes kinetic energy to heat energy, which is lost to the surroundings

• Example: A swing set-if the energy of the swing decreases, the energy lost must be gained in another object. In this case, friction with the chains and hooks of the swing and air resistance cause mechanical energy to change to thermal energy

• So, while it may appear that the swing is losing energy, it’s really just transferring it to a different form!

Conserving Energy

• Some systems are designed to reduce friction and conserve energy

• Pendulums and rollercoasters are systems that generally conserve mechanical energy • Energy is transformed between gravitational potential and kinetic

Unit Project: Mouse Trap Racecars!

Bellringer Day 05

• What is the kinetic energy of a 150kg object that is moving with a speed of 15m/s?

• Formula: KE = ½ mv2

Work and PowerCOMPARING WORK AND POWER

Work

• In physics, work is the transfer of energy that occurs when a force makes an object move

• In order for work to be done, an applied force must make an object move, and the force must be in the same direction as the object• Example: You pick up a stack of books of the

floor. You are applying force in the direction the books are moving. Work is being done.

• If you are holding the books in your arms, force is being applied but the books are not moving. No work is being done.

Work and Energy

• When work is done on an object, the result is an increase in its energy and is accompanied by a decrease in energy somewhere else.

• If something has energy, it can transfer energy to another object by doing work on that object

• Example: You carry a box. Chemical energy from your muscles is transferred to the box.

• Energy is always transferred from the object doing the work to the object on which work is done.

Calculating Work

• The amount of work done depends on the amount of force exerted and the distance over which the force is applied.

W=FdW=Work (in Joules)

F=Force (in Newtons)

d=distance (in meters)

Work is measured in Joules-it is a

measure of energy!

Solving for Work

Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back?

GIVEN

• W=Fd

• W=?

• F=30N

• D=1.5m

SOLVE

• W=Fd

• W=(30)(1.5)

• W=45J

Bellringer Day 06

• A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift?

• Formulas: W=Fd; F=ma

Case Study: Power

• Imagine you and a friend are both pushing boxes up a ramp. The boxes weigh the same. Your friend is able to push the box faster than you, taking 30 seconds to push compared to your 45 seconds. What is happening here?

• You and your friend both moved the equal weight boxes the same distance-you both performed the same amount of work on the box

• The difference here is the time-your friend has morepower than you do!

Power

• Power is the amount of work done in one second.

• It is a rate-the rate at which work is done!

Calculating Power

• Power is the relationship between the work done and the time it takes to do the work.

P=W/tP=Power (in Watts)

W=Work (in Joules); W=Fd

t=time (in seconds)

The unit for Power is Watts!

Solving for Power

You do 900 J of work in pushing a sofa. If it took 5s to move the sofa, what was your power?

GIVEN

• P=W/t

• W=900 J

• t=5s

SOLVE

• P=W/t

• P= (900)/(5)

• P= 180 W

Bellringer Day 07

• The power produced by an electric motor is 500 W. How long will ittake the motor to do 10,000 J of work?

• Formula: P=W/t

Simple MachinesINCLINED PLANES, LEVERS, ANDMECHANICAL ADVANTAGE

Machines

• A machine is a device that makes doing work easier

• Changes the size and/or direction of the exerted force

• They do not make less work; they make it easier to work

• All simple machines can be categorized as an inclined plane or a lever

Inclined Plane

• An inclined plane is a sloping surface used to raise objects

Variations of the Inclined Plane

WEDGE

• A wedge is a moving inclinedplane with 1 or 2 sloping sides

SCREW

• A screw is an inclined plane wrapped in a spiral around a cylinder

Lever

• A lever is a bar that is free to pivot around a fixed point (fulcrum)

Variations of the Lever

PULLEY

• A grooved wheel with a rope or chain running along thegroove

WHEEL AND AXEL

• Two wheels of different sizes that rotate together; a pair of rotating levers

Work Input vs. Work Output

• The force applied to the machine is called the input force• What you do

• The input force is represented by the abbreviation Fin

• The force applied by the machine is called the output force• What the machine does

• The output force is represented by the abbreviation Fout

• The work the machine applies to the object cannot be more than the work you apply to the machine-remember, energy cannot be created or destroyed!• However, work input is always more than work output, because some of that

energy transfers to heat energy due to friction

Mechanical Advantage

• The ratio of the output force to the input force is the mechanical advantage of the machine

• There are two types of mechanical advantage:• Ideal Mechanical Advantage (IMA): the mechanical advantage of a machine

without friction

• Actual Mechanical Advantage (AMA): number of times a machine increases the effort force

IMA=dE/dR AMA=FR/FE

Effort/Resistance Resistance Force/Effort Force

Solving for Mechanical Advantage

You use a 160 cm plank to lift a large rock. If the rock is 20 cm from the fulcrum, what is the plank’s IMA?

GIVEN

• IMA=dE/dR LE/LR

• LE (effort) = 160

• LR (resistance) =20

• IMA = ?

SOLVE

• IMA= LE/LR

• IMA = 160/20

• IMA = 8

Solving for Mechanical Advantage

A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s AMA?GIVEN

• AMA= FR/FE

• FR = 500 N

• FE= 20 N

• AMA = ?

SOLVE

• AMA= FR/FE

• AMA = 500/20

• IMA = 25

Bellringer Day 08

• Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10.

• Formula: AMA= FR/FE

EfficiencyEFFICIENCY OF MACHINES

Efficiency of Machines

• Efficiency is a measure of how much work put into a machine is changed into useful output work by the machine

• High efficiency machines produce less heat and friction

• In an ideal machine, the input work equals the output work and there is no friction. An ideal machine has an efficiency of 100%

• In a real machine, friction always causes the output work to be less than the input work. The efficiency of a real machine is always less than 100%

Calculating Efficiency

• Efficiency is a relationship between the output work and input work.

E=Wout/Win x 100E=Efficiency

Wout=output work (in J)

Win=input work (in J)

Remember, no machine is

100% efficient!

Solving for Efficiency

Find the efficiency of a machine that does 800J of work if the input work is 2,400 J.

GIVEN

• E = Wout/Win x 100

• E = ?

• Wout = 800 J

• Win = 2,400 J

SOLVE

• E = Wout/Win x 100

• E = (800)/(2400) x 100

• E = 33.3%

Bellringer Day 09

1. Explain why no machine can be 100% efficient.

2. Find the mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N.

• Formula: AMA= FR/FE

Bellringer Day 10

1. Explain the Law of the Conservation of Energy.

2. A person places a metal spoon that is at room temperature into a bowl of hot soup. How will the thermal energies of the spoon and the soup be affected?

a) The spoon will increase in thermal energy, and the soup will decrease in thermal energy.

b) Both the spoon and the soup will increase in thermal energy.

c) The spoon will decrease in thermal energy, and the soup will increase in thermal energy.

d) Both the spoon and the soup will decrease in thermal energy

Mouse Trap Racecars!

Bellringer Day 11

1. How much potential energy does a 55-kg person gain when she walks to the top of a hill 20 m tall?

• Formula: PEg = mgh

2. An object has a mass of 12.8 kg and a velocity of 8.4 m/s. What is the kinetic energy of the object?

• Formula: KE = ½ mv2

Day 12: Test Day

• Thermal Energy

• Energy Transfer

• Kinetic vs. Potential Energy

• Mechanical Energy

• Simple Machines

• Efficiency