physics work and energy

Work and Energy Section 3

© Houghton Mifflin Harcourt Publishing Company

What do you think?• Imagine two students standing side by side at the top of

a water slide. One steps off of the platform, falling directly into the water below. The other student goes down the slide. Assuming the slide is frictionless, which student strikes the water with a greater speed?– Explain your reasoning.

• Would your answer change if the slide were not frictionless? If so, how?

Mr. Thompson's Physics

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What do you think?

• What is meant when scientists say a quantity is conserved?

• Describe examples of quantities that are conserved.– Are they always conserved? If not, why?


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Mechanical Energy (ME)

• ME = KE + PEg + PEelastic

– Does not include the many other types of energy, such as thermal energy, chemical potential energy, and others

• ME is not a new form of energy.– Just a combination of KE and PE


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Classroom Practice Problems• Suppose a 1.00 kg book is dropped from a height of 2.00 m. Assume

no air resistance.

– Calculate the PE and the KE at the instant the book is released.• Answer: PE = 19.6 J, KE = 0 J

– Calculate the KE and PE when the book has fallen 1.0 m. (Hint: you will need an equation from Chapter 2.)

• Answer: PE = 9.81 J, KE = 9.81 J

– Calculate the PE and the KE just as the book reaches the floor.• Answer: PE = 0 J, KE = 19.6 J


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Table of Values for the Falling Book

h (m) PE(J) KE(J) ME(J)

0 19.6 0 19.6

0.5 14.7 4.9 19.6

1.0 9.8 9.8 19.6

1.5 4.9 14.7 19.6

2.0 0 19.6 19.6


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Conservation of Mechanical Energy

• The sum of KE and PE remains constant.

• One type of energy changes into another type.– For the falling book, the PE of the book changed into KE as it

fell.– As a ball rolls up a hill, KE is changed into PE.


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Conservation of Energy• Acceleration does not have to be constant.

• ME is not conserved if friction is present.– If friction is negligible, conservation of ME is reasonably accurate.

• A pendulum as it swings back and forth a few times

• Consider a child going down a slide with friction.– What happens to the ME as he slides down?

• Answer: It is not conserved but, instead, becomes less and less.– What happens to the “lost” energy?

• Answer: It is converted into nonmechanical energy (thermal energy).


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Classroom Practice Problems• A small 10.0 g ball is held to a slingshot that is stretched

6.0 cm. The spring constant is 2.0 102 N/m.– What is the elastic potential energy of the slingshot

before release?– What is the kinetic energy of the ball right after the

slingshot is released?– What is the ball’s speed at the instant it leaves the

slingshot?– How high does the ball rise if it is shot directly

upward?


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Now what do you think?• Imagine two students standing side by side at the top of

a water slide. One steps off of the platform, falling directly into the water below. The other student goes down the slide. Assuming the slide is frictionless, which student strikes the water with a greater speed?– Explain your reasoning.

• Would your answer change if the slide were not frictionless? If so, how?


Class



Now what do you think?

• What is meant when scientists say a quantity is “conserved”?

• Describe examples of quantities that are conserved.– Are they always conserved? If not, why?


Class

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