Do NowMaterialsNotebookHomeworkPen/Pencil1) Take Out Your Homework, Put on Corner of Desks2) In Notebooks: What is Power?

-Write a sentence in response.3) In Notebooks: When Mr. Van Houten bench

presses large amounts of weight he often will do 500 J of work in just 4 seconds. How much power do I develop?

Objectives

• SWBAT solve simple power problems.• SWBAT solve work-energy theorem problems.

Agenda

• Introduction to New Material: Work – Energy Theorem

• Practice• More Practice• Exit Ticket

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Work – Energy Theorem

• So far our exploration of energy has been so exciting.– A box sits on a cliff, how much energy does it have?– A spring shoots a dart straight up into the air, what’s

the change in energy?– Your moving in a car how much kinetic energy do you

have?

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Work – Energy Theorem

• We’ve been talking about the energy of objects that are in closed systems.

• For example, one system might be a box in a warehouse.

• We haven’t considered the external world that may affect the box.

• The box has no energy if it sits there, but if I pick it up and put it on a shelf…

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Work – Energy Theorem

• Through the process of doing work, energy can move between the external world and the system.

• The direction of this energy transfer can go both ways.

• External world system– I lift a weight off the ground and hold it above my

head.• Or, system external world– A moving golf club hits a still golf ball.

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Work – Energy Theorem

Weight is the

system

Work – Energy Theorem

• External world system– I lift a weight off the ground and hold it above my

head.– W is positive

– Mr. Van Houten picks a weight up off the ground by doing 50 J of work on the weight. How much energy does the weight now have?

fi EWE

J50500 fi EWE

Work – Energy Theorem

• Or, system external world– A moving golf club hits a still golf ball.– W is negative

– A 2 kg golf club is being swung at 4 m/s, when it collides with a golf ball. During the collision, the club does 5 kg of work on the ball. What is the final energy of the golf club equal to?

fi EWE

fi EWE fEKE 5

Work – Energy Theorem

• The work-energy theorem states that when work is done on an object, the result is a change in kinetic energy.

• The work-energy theorem can be represented by:

• Think back to Mr. Van Houten lifting the weight.

EW if EEW

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Work – Energy Theorem

Weight is the

system

Work – Energy Theorem

• The relationship between work done and the change in energy that results was established by James Prescott Joule.– To honor him, a unit of energy is called a joule (J).

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Example Problem #1• A 105-g hockey puck is sliding across the ice.

A player exerts a constant 4.50-N force over a distance of 0.150 m. How much work does the player do on the puck? What is the change in puck’s energy?

dF

Known

• m=105 g• F=4.50 N• d=0.15 m

Unknown

• W= ?• ∆E= ?

Example Problem #1• A 105-g hockey puck is sliding across the ice.

A player exerts a constant 4.50-N force over a distance of 0.150 m. How much work does the player do on the puck? What is the change in puck’s energy?

Known

• m=105 g• F=4.50 N• d=0.15 m

Unknown

• W= ?• ∆E= ?

JWW

FdW

675.0)0cos()150.0)(50.4(

)cos(

Example Problem #1• A 105-g hockey puck is sliding across the ice.

A player exerts a constant 4.50-N force over a distance of 0.150 m. How much work does the player do on the puck? What is the change in puck’s energy?

Known

• m=105 g• F=4.50 N• d=0.15 m

Unknown

• W= 0.675J• ∆E= ?

JKEEW675.0

Does the sign make sense?

Example Problem #2

• Juan lifts a potted plant into the air. If he gives the plant 90 J of potential energy in 0.65 seconds, how much power did Juan develop?

Known

• t=0.65 s• ∆E=90 J• W=∆E• W=90 J

Unknown

• P= ?

Example Problem #2

• Juan lifts a potted plant into the air. If he gives the plant 90 J of potential energy in 0.65 seconds, how much power did Juan develop?

Known

• t=0.65 s• ∆E=90 J• W=∆E• W=90 J

Unknown

• P= ?

WP

P

tWP

46.13865.090

Example Problem #3• A skater with a mass of 52 kg moving at 2.5

m/s glides to a stop over a distance of 24.0 m. How much work did the friction of the ice do to bring the skater to a stop?

Known

• m=52 kg• Vi=2.5 m/s• Vf=0 m/s• d=24 m• W=∆E

Unknown

• W=?

Example Problem #3• A skater with a mass of 52 kg moving at 2.5

m/s glides to a stop over a distance of 24.0 m. How much work did the friction of the ice do to bring the skater to a stop?

Known

• m=52 kg• Vi=2.5 m/s• Vf=0 m/s• d=24 m• W=∆E

Unknown

• W=?

if EEWEW

2

2

)5.2)(52(21210

0

W

mvW

KEW

JW 5.162

Rest of Class

• Power Practice Worksheet– Work quietly with 1 other person

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.

Homework: Due Thursday

• Finish Power Practice• Physics Update #3– Based off notes from today.

SWBAT solve simple power problems.SWBAT solve work-energy theorem problems.