Work and Power

The word “WORK”• What does it mean in

everyday life?

• “Work” has a specific definition in science.

Remember: Newton’s 1st Law• An object only begins

moving when an unbalanced force acts on it.

• Net force > 0

Work• Work is done when a force

acts on an object in the direction the object moves.

• Work = force x distance

• Work requires motion• If there is no movement, no

work is done.

• Force must act in SAME DIRECTION as motion

• Any part of a force that does not act in the direction of motion does no work on an object.

• Examples

Which ones require work?• Ronnie pushes on a wall until he is

exhausted.

• Timara knocks a book off the table and it falls to the ground.

• A rocket accelerates through space.

• Doneisha carries her books to class.

Question:• You carry two heavy grocery bags

upstairs to your kitchen.

• Will you do more work on the bags if you carry them up one at a time?

• Explain.

Units of Work• Remember: units of force?

– Newton

• Remember: units of distance?– meter

Work = Force x Distance

Work = N x m

N x m = 1 Joule

JOULE (J) is the unit for Work

Calculating WorkA weightlifter lifts a 1800-N barbell

over his head. He has to lift it for 2.0 m. What amount of work is done?

F = 1800 N

D = 2.0 m

W = F x D

W = 1800 N x 2.0 m

W = 3600 Nm = 3600 J

PracticeHow much work does a 25-

Newton force do to lift a potted plant from the floor to a shelf 1.5 meters high?

PracticeA factory machine uses 2600-

Newtons of force to move a robotic arm 3.5 meters. How much work does the machine perform?

Power• What does it mean in

everyday life?

• “Power” also has a specific meaning in science.

Power• The rate of doing work.

• How FAST can work get done?• To get work done faster, more

power is required.

Power = Work

Time

Which has more power:

• A person with a snow shovel?

• A snow blower?

• A truck with a snow plow?

Predicting• Two cars have the same weight, but

Car A has an engine that provides twice the power of Car B.

• Which car can make it to the top of a mountain pass first?

• Which car does more work to reach the top?

Units of Power• Remember: units of Work

– Joule• Remember: units of Time

– Seconds

Power = Work

Time

Power = Joule

second= 1 Watt

Watt (W) is theunit for Power.

Example:

• 40-Watt light bulb–Requires 40 joules of work for

every second that it is lit

• Same power as lifting your textbook a height of 1 meter in half a second.

Calculating PowerYou exert a vertical force of 72

Newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?

F = 72 N

D = 1.0 m

T = 2.0 s

Power = Work

Time=

Force x Distance

Time

72 N x 1.0 m

2.0 sPower = =

72 J

2.0 s

Power = 36 J/s = 36 Watts

Practice1. Deshawna ice skates 70 m in 60

seconds with a force of 10 N. How much work did she do? How much Power did she have?

2. Darane runs up the NLCP stairs in 3 seconds. She uses a force of 20 N and the stairs are 4 meters tall. How much work did she do? How much Power did she have?