Chapter 6 Work and Kinetic Energy Power Point

Goals for Chapter 6 • To understand calculate the work done by a force

Introduction • The simple methods we’ve learned using Newton’s laws are inadequate when the

Work • A force on a body does work if the body undergoes a

Work done by a constant force • The work done by a constant force

Positive, negative, and zero work • A force can do positive, negative, or zero

Kinetic energy • The kinetic energy of a particle is K = 1/2 mv

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

Work and energy with varying forces—Figure 6. 16 • Many forces, such as the

Stretching a spring • The force required to stretch a spring a distance x

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,

Power • Power is the rate at which work is done. • Average power

Work done by several forces • Example 6. 2 shows two ways to find

Using work and energy to calculate speed • Revisit the sled from Example 6.

Forces on a hammerhead • The hammerhead of a pile driver is used to

Comparing kinetic energies • In Conceptual Example 6. 5, two iceboats have different masses.

Force and power • In Example 6. 9, jet engines develop power to fly

A “power climb” • A person runs up stairs. Refer to Figure 6. 28

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Chapter 6 Work and Kinetic Energy Power. Point® Lectures for University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson Copyright © 2012 Pearson Education Inc. Modifications by Mike Brotherton

Goals for Chapter 6 • To understand 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 Copyright © 2012 Pearson Education Inc.

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. Copyright © 2012 Pearson Education Inc.

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. Copyright © 2012 Pearson Education Inc.

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. Copyright © 2012 Pearson Education Inc.

Kinetic energy • The kinetic energy of a particle is K = 1/2 mv 2. • The net work on a body changes its speed and therefore its kinetic energy, as shown in Figure 6. 8 below. Copyright © 2012 Pearson Education Inc.

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 = K 2 – K 1 = K. • Follow Problem-Solving Strategy 6. 1. Copyright © 2012 Pearson Education Inc.

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. Copyright © 2012 Pearson Education Inc.

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 k. X 2. Copyright © 2012 Pearson Education Inc.

Power • Power is the rate at which work is done. • Average power is Pav = W/ t and instantaneous power is P = d. W/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. Copyright © 2012 Pearson Education Inc.