chapter 6 - physicsphysicsweb.phy.uic.edu/141/outline/06_lecture_outline.pdf · university physics,...

Copyright © 2012 Pearson Education Inc.

PowerPoint® Lectures for

University Physics, Thirteenth Edition

– Hugh D. Young and Roger A. Freedman

Lectures by Wayne Anderson

Chapter 6

Work and Kinetic

Energy


Goals for Chapter 6

• To understand and calculate the work done by

a force

• To understand the meaning of kinetic energy

• To learn how work changes the kinetic energy

of a body and how to use this principle

• To relate work and kinetic energy when the

forces are not constant or the body follows a

curved path

• To solve problems involving power


Introduction

• The simple methods we’ve learned using Newton’s

laws are inadequate when the forces are not

constant.

• In this chapter, the introduction of the new concepts

of work, energy, and the conservation of energy will

allow us to deal with such problems.


Work

• A force on a body does work if the body undergoes a displacement.

• Figures 6.1 and 6.2 illustrate forces doing work.


Work done by a constant force

• The work done by a constant force acting at an angle to the displacement is W = Fs cos . Figure 6.3 illustrates this point.

• Follow Example 6.1.


Positive, negative, and zero work

• A force can do positive, negative, or zero work depending on

the angle between the force and the displacement. Refer to

Figure 6.4.


Work done by several forces

• Example 6.2 shows two

ways to find the total

work done by several

forces.



Kinetic energy

• The kinetic energy of a particle is K = 1/2 mv2.

• The net work on a body changes its speed and therefore its kinetic

energy, as shown in Figure 6.8 below.


The work-energy theorem

• The work-energy theorem: The work done by the net

force on a particle equals the change in the particle’s

kinetic energy.

• Mathematically, the work-energy theorem is

expressed as Wtot = K2 – K1 = K.

• Follow Problem-Solving Strategy 6.1.


Using work and energy to calculate speed

• Revisit the sled from Example 6.2.

• Follow Example 6.3 using Figure 6.11 below and

Problem-Solving Strategy 6.1.


Forces on a hammerhead

• The hammerhead of a pile driver is used to drive a beam into the

ground.

• Follow Example 6.4 and see Figure 6.12 below.


Comparing kinetic energies

• In Conceptual Example 6.5, two iceboats have

different masses.

• Follow Conceptual Example 6.5.


Work and energy with varying forces—Figure 6.16

• Many forces, such as the

force to stretch a spring,

are not constant.

• In Figure 6.16, we

approximate the work by

dividing the total

displacement into many

small segments.


Stretching a spring

• The force required to stretch a

spring a distance x is

proportional to x: Fx = kx.

• k is the force constant (or

spring constant) of the spring.

• The area under the graph

represents the work done on

the spring to stretch it a

distance X: W = 1/2 kX2.


Work done on a spring scale

• A woman steps on a bathroom scale.



Motion with a varying force

• An air-track glider is attached to a spring, so the force on

the glider is varying.

• Follow Example 6.7 using Figure 6.22.


Motion on a curved path—Example 6.8

• A child on a swing moves along a curved path.

• Follow Example 6.8 using Figure 6.24.


Power

• Power is the rate at which work is done.

• Average power is Pav = W/t and instantaneous power is P = dW/dt.

• The SI unit of power is the watt (1 W = 1 J/s), but other familiar units are the horsepower and the kilowatt-hour.


Force and power

• In Example 6.9, jet engines develop power to fly

the plane.



A “power climb”

• A person runs up stairs. Refer to Figure 6.28 while

following Example 6.10.

chapter 6 - physicsphysicsweb.phy.uic.edu/141/outline/06_lecture_outline.pdf · university physics,...

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