Unit 5: Work and Energy

Unit 5: Energy and Work

Chapter 7

Main Ideas

Types of Energy

Potential vs. Kinetic Energy

Energy and Work

Efficiency and Power

Types of Energy

•Energy has the ability to flow between various forms of energy such as from Potential Energy to Kinetic Energy.

•There are 6 different forms of energy

Types of Energy

Mechanical Energy - This is energy that is
possessed by an object due to its motion or stored up
potential energy. Mechanical Energy involves moving
parts

For Example: A simple machines has
mechanical energy. Mechanical energy is the
ability to do work with a machine

Radiant Energy - energy that is
transferred by electromagnetic
rays/radiation.

Light is a Radiant Energy, as well
as Radio waves, microwaves,
infrared light, visible light, UV
light, X-Rays, Gamma Rays

It’s the Radiant Energy from the
sun that helps keep us warm!

Electrical Energy - energy from the movement of electrons.

Electricity that we use in our everyday lives.

Examples: Car battery, lightning, Power lines

Chemical Energy - Chemical
reactions either release or
absorb energy.

•Chemical energy can be stored
in batteries and then converted
into electrical energy.

Food

Gasoline

Nuclear Energy - Energy that comes from the
splitting or combination of atoms known as
fusion and fission.

Nuclear Reactors produce Electricity from
Fission Reactions

Thermal Energy - energy in objects that comes from atom
and molecular motion.

Heat is a form of energy.

–Heating contractor measure the amount of heat in BTU’s or British
Thermal Units. One BTU is the same amount of energy as 1055
Joules.

How is it different from radiant?

Radiant=electromagnetic waves
Thermal=energy in molecular motions

Match

Match the following

• This energy is due to motion or stored up potential energy

• Energy that is transferred by electromagnet rays

• energy from movement of electrons

• energy from the splitting or combination of atoms

• energy from atom and molecular motion

mechanical energy

radiant energy

electrical energy

nuclear energy

thermal energy

• mechanical energy

• radiant energy

• electrical energy

• nuclear energy

• thermal energy

Potential vs. Kinetic

• Potential energy- stored work relative to the position of the object to the Earth

• PE= mgh

  mass X acceleration due to gravity (9.8 m/s²) X height

Units!: PE=J, m=kg, h=m, g=m/s²

Multiple Choice

Does potential energy apply to moving objects?

• Yes

• No

Practice Problems-Potential Energy

What is the mass of a zombie that is perched on top of a cliff that is 140 meters high and has potential energy of 96,000 J?

Practice Problem-Potential Energy

What is the height of the slide if a 38 kg kid has 560 J of potential energy at the top of the slide?

Kinetic Energy

•Energy of motion

•KE= ½ mv² (mass is in kg)

Energy in motion:

• Kinetic energy increases with speed

• Kinetic energy increases with mass

• Kinetic energy and potential energy both have units of Joules

Multiple Select

Kinetic energy increases with...

• speed

• distance

• mass

• weight

Practice Kinetic Energy

What is the kinetic energy of the car if it has a mass of 1,100 kilograms and is
moving at 35 m/s?

Multiple Choice

What is the kinetic energy of the car if it has a mass of 1,100 kilograms and is
moving at 35 m/s?

• 673,750 J

• 19,250 J

• 1,347,500 J

• Cannot answer

Practice Kinetic Energy

A zombie runs by at 12 m/s and has 8200 Joules of kinetic energy. What is the zombie’s mass?

Multiple Choice

A zombie runs by at 12 m/s and has 8200 Joules of kinetic energy. What is the zombie’s mass?

• 56.9 kg

• 683 kg

• 85.4 kg

• 113.88 J

Conservation of Energy

•Energy can never be created or destroyed, just converted from one form to another

•Conservation of energy can be stated in the following way

•E_l = E_f

•KE_l+PE_l= KE_f + PE_f

Work

Work

•Result of a force moving an object in a direction

•work results when the force and the object are moving in the same direction

•The object must move for work to occur

Multiple Choice

If an object is not moving, is work still being done?

• Yes

• No

Work

•Work is done by forces, but work is done on
objects

•Unit= Joule (J) or N x m


•Work out is NEVER greater than the work in
Displacement INSTEAD of distance

Work and Energy

• Work requires energy

• Work can be thought of as energy transfer into or out of a system

• In theory, work input should be the same as work output

  • But in reality some energy (work) is transformed to thermal energy

Work and Direction

Positive work

Negative work is performed when the direction of the force and the direction of motion are the opposite

descent phase of the bench press

catching

landing phase of a jump

Work Example: Climbing Stairs

Work = Fd

Force
Subject weight
From mass, ie 65 kg

Displacement
Height of each step
Typical 8 inches (20cm)

Work per step
637N x 0.2 m = 127.4 Nm
Multiply by the number of steps

Practice Problems #1

Calculate the work done by a woman lifting a 50 N baby 0.5 meters. What is the work done on the baby?

Practice Problem #2

Multiple Choice

Who is doing more WORK?

• The man

• The woman

• They are doing the same amount of work

• Cannot answer

Work and Energy

• Energy does not always yield work, but work
  requires energy

• Energy= the ability to do work, also considered stored work

• Work-energy theorem= total work done = amount of available energy

Multiple Choice

Work is done by ...

• energy

• forces

• joules

• mass

Match

Match the following

• Direction of the force and motion are opposite

• Direction of force and the motion are the same

• Result of a force moving an object in a direction

• Stored work relative to the position of the object to Earth

• Energy of motion

Negative work

Positive work

Work

Potential Energy

Kinetic Energy

• Negative work

• Positive work

• Work

• Potential Energy

• Kinetic Energy

Efficiency and Power

Multiple Select

What is the unit for Work?

• N

• Nm

• J

• m/s

Power

Power - the rate at which work is done is
called POWER
Power = work/time

•Units for power are watts
– It makes a difference how fast you do your work!
–The faster you work the greater the power

Power and Work

Ex. 2 people drag an object with a force of 100
Newtons for a distance of 10 meters.

•1 person took 10 seconds and the 2nd person took 60 seconds
• Calculate the Power for each person
•Work = force x distance
•Power = work/time

Calculate and Compare Power

• During the ascent phase of a rep of the bench press, two lifters each exert an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m upward

Lifter A: 0.50 seconds
Lifter B: 0.75 seconds

Multiple Choice

Who had more power?

• Lifter A

• Lifter B