unit 4 work, energy, and power. by what factor does the kinetic energy of a car change when its...

UNIT 4Work, Energy, and Power

By what factor does the

kinetic energy of a car

change when its speed

is tripled?

1) no change at all

2) factor of 3

3) factor of 6

4) factor of 9

5) factor of 12

ConcepTest 6.5a Kinetic Energy I

By what factor does the

kinetic energy of a car

change when its speed

is tripled?

1) no change at all

2) factor of 3

3) factor of 6

4) factor of 9

5) factor of 12

Since the kinetic energy is 1/2 mv2, if the speed increases

by a factor of 3, then the KE will increase by a factor of 9.

ConcepTest 6.5a Kinetic Energy I

Follow-up: How would you achieve a KE increase of a factor of 2?

Wednesday November 9th

4

POTENTIAL ENERGY

TODAY’S AGENDA

Potential EnergyHw: Practice C (All) p168

UPCOMING…

Thur: Conservation of Mechanical Energy Fri: Bowling Ball Demo Mon: Problem Quiz 1

Wednesday, November 9

Copyright © by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

Section 2 Energy

Chapter 5

Potential Energy

• Potential Energy is the energy associated with an object because of the position, shape, or condition of the object.

• Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies.

• Gravitational potential energy depends on height from a zero level.

PEg = mghgravitational PE = mass free-fall acceleration height



Section 2 Energy

Chapter 5

Potential Energy, continued• Elastic potential energy is the energy available for

use when a deformed elastic object returns to its original configuration.

2

2

1elastic PE = spring constant (distance compressed or stretched)

2

1

2elasticPE kx

• The symbol k is called the spring constant, a parameter that measures the spring’s resistance to being compressed or stretched.



Chapter 5

Elastic Potential Energy

Section 2 Energy



Section 2 Energy

Chapter 5

Sample Problem

Potential Energy

A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight of the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?



Section 2 Energy

Chapter 5

Sample Problem, continued

Potential Energy

1. Define

Given:m = 70.0 kg

k = 71.8 N/m

g = 9.81 m/s2

h = 50.0 m – 44.0 m = 6.0 m

x = 44.0 m – 15.0 m = 29.0 m

PE = 0 J at river level

Unknown: PEtot = ?



Section 2 Energy

Chapter 5

Sample Problem, continued

Potential Energy

3. Calculate

Substitute the values into the equations and solve:

2 3

2 4

3 4

4

(70.0 kg)(9.81 m/s )(6.0 m) = 4.1 10 J

1(71.8 N/m)(29.0 m) 3.02 10 J

2

4.1 10 J + 3.02 10 J

3.43 10 J

g

elastic

tot

tot

PE

PE

PE

PE

14

END

unit 4 work, energy, and power. by what factor does the kinetic energy of a car change when its...

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