physics work and energy

Work and Energy Section 2

© Houghton Mifflin Harcourt Publishing Company

What do you think?• You have no doubt heard the term kinetic energy.

– What is it? – What factors affect the kinetic energy of an object

and in what way?• You have no doubt heard the term potential energy.

– What is it? – What factors affect the potential energy of an

object and in what way?

Mr. Thompson's Physics

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netW F x ma x Kinetic Energy

Since

then

or

2 2 2f iv v a x 2 2

( )2

f inet

v vW m

2 21 1

2 2net f iW mv mv


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Kinetic Energy

• What are the SI units for KE?– kg•m2/s2 or N•m or J


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Work and Kinetic Energy

• KE is the work an object can do if the speed changes.

• Wnet is positive if the speed increases.


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Classroom Practice Problems• A 6.00 kg cat runs after a mouse at 10.0 m/s.

What is the cat’s kinetic energy?– Answer: 3.00 x 102 J or 300 J

• Suppose the above cat accelerated to a speed of 12.0 m/s while chasing the mouse. How much work was done on the cat to produce this change in speed?– Answer: 1.32 x 102 J or 132 J


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Potential Energy

• Energy associated with an object’s potential to move due to an interaction with its environment– A book held above the desk– An arrow ready to be released from the bow

• Some types of PE are listed below.– Gravitational– Elastic– Electromagnetic


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Gravitational Potential Energy

• What are the SI units?– kg•m2/s2 or N•m or J

• The height (h) depends on the “zero level” chosen where PEg = 0.


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Elastic Potential Energy

• The energy available for use in deformed elastic objects– Rubber bands, springs in trampolines, pole-vault poles, muscles

• For springs, the distance compressed or stretched = x


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Elastic Potential Energy

• The spring constant (k) depends on the stiffness of the spring.– Stiffer springs have higher k values.– Measured in N/m

• Force in newtons needed to stretch a spring 1.0 meters

• What are the SI Units for PEelastic?


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Classroom Practice Problems• When a 2.00 kg mass is attached to a vertical

spring, the spring is stretched 10.0 cm such that the mass is 50.0 cm above the table.– What is the gravitational potential energy associated

with the mass relative to the table?• Answer: 9.81 J

– What is the spring’s elastic potential energy if the spring constant is 400.0 N/m?

• Answer: 2.00 J


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Now what do you think?• What is kinetic energy?

– What factors affect the kinetic energy of an object and in what way?

– How are work and kinetic energy related?• What is potential energy?

– What factors affect the gravitational potential energy of an object and in what way?

– What factors affect the elastic potential energy of an object and in what way?


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physics work and energy

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