Energy

The ability to do work

Objectives

• Explain the relationship between energy and work.

• Define potential energy and kinetic energy.

• Calculate Kinetic energy and potential energy.

• Distinguish between mechanical and nonmechanical energy

• Identify nonmechanical forms of energy

Energy Types

• Potential Energy• Kinetic Energy• Mechanical Energy

• All are measured in joules, J

• Work is also measured in J

Potential Energy

• Sometimes called gravitational potential energy.

• PE = mass x gravity x height

• PE = mghg = 9.8 m/s2

PEm g h

Add this to your circle sheet.

Potential Energy problem samples

A. Calculate the PE of a car with mass of 1200 kg at the top of a 42 m high hill.

Ans.: PE=mgh=1200kg(9.8m/s2)(42m)

=490 000 = 4.9 x 105 J, for the car

B. What is the PE of a 65 kg climber on top of Mount Everest (8800m high)?

Ans.: PE=mgh=65kg(9.8 m/s2)(8800m)

=5 600 000 = 5.6 x 106 J, climber

Kinetic Energy

The energy an object has as a result of its motion.

In the picture at right,

which block has the

greatest kinetic

energy?

Which one has the greatest

potential energy?

Formula for Kinetic Energy

KE = ½ mass x velocity2

KE = ½ mv2

Put these other forms

of the formula at the bottom

of your circle sheet.

• Calculate the kinetic energy of 1500 kg car moving at 29 m/s and at 42 km/h

KE = ½ mv2 = ½ (1500kg)(29m/s)2

= 6.3 x 105J

42km/h: V = 42km 1000m 1h = 12m/s h 1km 3600sKE = ½ mv2

KE = ½ (1500kg)(12m/s)2 = 1.1 x 105J

• A 39 kg child has 190 joule of kinetic energy after sledding down a hill. What is the child’s speed at the bottom of the hill in m/s?

• V = 2KE

• m

• V = 2(190 j)

• 39kg

• V = 3.3 m/s

Your assignment is to finish 1, 2, and 3

• But first, add these formulas to the bottom of your circle sheet for other forms of the kinetic energy formula:

• V = 2KE m = 2KE

• m v2

Answers to1bc,2,3

1b. KE=1/2mv2 = ½(1500kg)(18m/s)2

= 240000 =2.4 x 105 J

1c. V = 42km 1000m 1h = 12m/s

h 1km 3600s

KE = ½ (1500kg)(12m/s)2 = 1.1 x 105J

• #2

• V = 2KE = 2(190J) = 3.3m/s

• m 35kg

• #3. m = 2KE = 2(16J) = 8.0 kg

• v2 (2.0 m/s)2

Mechanical Energy

• The sum of potential energy and the kinetic energy in a system.

• Like a pendulum. Potential energy changes to kinetic energy and vice versa as the pendulum swings back and forth.

• The total mechanical energy for “the system” remains constant.

Other Forms of Energy - Nonmechanical

• Energy at the atomic level that does not generally affect the motion of an object.

Thermal EnergyThe temperature of an object

affects how fast the atoms of the object move. (Remember the

atomic theory?)Faster motion greater Kinetic

Energy!!

Chemical Reactions

• When chemical reactions occur, energy can be released

• Examples: heat from a burning log in the fireplace or fireworks!

Solar Energy

• Light from the sun sustains life here on earth.

• The sun gets its energy from nuclear reactions.

• Light from the sun travels through space in the form of electromagnetic waves.

Electrical Energy

• Moving electrons through conducting materials allow machines to do a tremendous amount of work for us.

• Can you think of any?

Assignment

• 1-9ab on the next slide

• For EXTRA CREDIT:

• Solve 9d on next slide. Hand this in separately with formula, work, and answer…(you know the routine…) for full credit. Worth 3 test points

Answers to 7-9ab

• 7.PE=mgh=93.0kg(9.8m/s2)(550m)=• =500 000 J• 8.KE=1/2mv2 = ½(0.02kg)(300m/s)2 • =900 J• 9.a. PE=mgh=2.5kg)(9.8m/s2)(2.0m)• = 49 J• 9.b. KE=1/2mv2=1/2(0.015kg)(3.5m/s)2

• = 0.092 = 9.2 x 10-2 J

Book assignments

• Read chapter 15• Write and answer Key Concepts for each

section• Define all terms• Add new formulas to list (see notes)• 469-70/1-22,26• 471/FCAT practice