Mr. Borosky Physics Section 11.1 Notes Page 3 of 3

Chapter 11 Energy and Its Conservation

In this chapter you will:

Learn that energy is a property of an object that can change the object’s position, motion, or its environment.
Learn that energy changes from one form to another, and that the total amount of energy in a closed system remains constant.

Sections

Section 11.1: The Many Forms of Energy

Section 11.2: Conservation of Energy

Section 11.1 The Many Forms of Energy

Objectives

Use a model to relate work and energy.
Calculate kinetic energy.
Determine the gravitational potential energy of a system.
Identify how elastic potential energy is stored.

Read intro paragraph p. 285

Work – transfer of energy by mechanical means.

Energy – the ability of an object to produce a change in itself or in its surroundings.

Conserved Properties – properties that are the same before and after an interaction. Examples are energy and momentum.

A MODEL OF THE WORK-ENERGY THEOREM

Read Section.

Work – is the product of the force and the object’s displacement. It is equal to the constant force exerted on an object in the direction of motion times the object’s displacement. It is the transfer of energy by Mechanical means. It is denoted by W. It is measured in Joules.

W = Fd

Energy – the ability of an object to produce a change in itself or in its surroundings.

Kinetic Energy – is equal to ½ times the mass of an object times the speed of the object squared. It is denoted by KE. It is measured in Joules.

KE = ½ mv2

Work Energy Theorem – states that work is equal to the change in Kinetic energy.

W = ΔKE

KINETIC ENERGY

Read Section.

Kinetic Energy – is equal to ½ times the mass of an object times the speed of the object squared. It is denoted by KE. It is proportional to the mass and the velocity squared. It is measured in Joules.

KE = ½ mv2

Rotational Kinetic Energy – the kinetic energy of an object, proportional to the object’s moment of Inertia and the square of its angular velocity.

KERot = ½ Iω2

Do Practice Problems p. 287 # 1-3

STORED ENERGY

Read Section.

GRAVITATIONAL POTENTIAL ENERGY

Read Section.

While an object moves up gravity acts against it thus Work is Negative. When it is moving down the Force and Displacement are in the same direction and thus the Work is Positive.

Gravitational Potential Energy – energy of an object due to position or state. It is equal to the product of its mass, acceleration due to gravity, and the distance from the reference level. It is measured in Joules. It is denoted by PE.

PE = mgh (Potential Energy = mass * gravity * height)

Reference Level – location at which potential energy is chosen to be zero.

Note: PE = mgh is only valid if the gravitational force and acceleration are constant.

The sum of the Kinetic Energy and Potential Energy is constant at all times because no work is done on the system by any external forces.

Example Problem 1 p. 290

A) PE = mgh B) PE = mgh

PE = 7.3(9.8)(1.12) PE = 7.3(9.8)(.51)

PE = 80.125 Joules PE = 36.485 Joules

C) W = ΔKE = ΔPE

W = mghF – mghI

PE = 7.3(9.8)(1.12) – 7.3(9.8)(.61)

W = 80.125 – 43.6394

W = 36.4856 J (This is the work you do and SO)

Work by gravity is opposite and thus -36.485 Joules

Do Practice Problems p. 291 # 4-8

ELASTIC POTENTIAL ENERGY

Read Section.

Elastic Potential Energy – the potential energy that may be stored in an object, such as a rubber band, as a result of its change in shape.

Albert Einstein – said that Mass by itself is energy. This energy is called the Rest energy and thus the famous equation E = mc2.

Rest Energy – is equal to the object’s mass times the speed of light squared. E = mc2.

Do 11.1 Section Review p. 292 # 9-14