Work & Power NOTES. Define / Describe WORK Work is the transfer of Energy that occurs when a...

Work & Power

NOTES

Define / Describe WORK

•Work is the transfer of Energy that occurs when a force makes an object move.

•Work happens when a force moves an object over a distance.

•If the object does not move, NO work is done!

IMPORTANT• Work is the transfer of energy.

– If work is done on an object its energy will INCREASE

– If work is done by an object its energy will DECREASE

• If you are given an amount of work, you also know the amount of energy that has moved

Two things must happen for work to be done.

What are they?•A force must be applied to

an object•The object must move in the

same direction as the force.

Wrap –UP #1:Is work being done?

1. Lifting a spoonful of soup to your mouth.

2. Holding a stack of books motionless over your head

3. Letting a pencil fall to the ground.4. A person shovels snow off a

sidewalk.5. A person carries a bundle of

bricks across a construction site.

1. Lifting a spoonful of soup to your mouth.Yes

2. Holding a stack of books motionless over your head

NO- no distance3. Letting a pencil fall to the ground.

YES- by gravity4. A person shovels snow off a sidewalk.

YES5. A person carries a bundle of bricks across a

construction site. NO- arms don’t move

Wrap –UP #1:Is work being done?

Work Formula & SI UnitsWork = Force x Distance

W = F x dForce: Newton (N)Distance: meter (m)Work: Joule (J)

1 J = 1 N ·m

W

F d

1. W=20J force=? d=2m

2. work=? F= 10N d= 500m

3. W=5000J F= 500N distance=?

1. W=20J force=? d=2m F = W/d = 20J/2m = 10 N

2. work=? F= 10N d= 500m

3. W=5000J F= 500N distance=?

1. W=20J force=? d=2m F = W/d = 20J/2m = 10 N

2. work=? F= 10N d= 500m W = F x d = 10 N x 500 m = 5000 J

3. W=5000J F= 500N distance=?

1. W=20J force=? d=2m F = W/d = 20J/2m = 10 N

2. work=? F= 10N d= 500m W = F x d = 10 N x 500 m = 5000 J

3. W=5000J F= 500N distance=? d = W/F

= 5000J/ 500N= 10 m

Wrap-Up #2: Sample Calculation

• A father lifts his daughter 2.0 m and exerts a force of 190 N. How much work does he do?

• A father lifts his daughter 2.0 m and exerts a force of 190 N. How much work does he do?

• F = 190 N• D = 2.0 m W = F x d

= 190 N x 2.0 m W = 380 J

Wrap-Up #2: Sample Calculation

Power•Power is the rate of doing work.– How much work is done in a

certain amount of time

Power = Work time

P = Wt

Work: Joule (J)Time : seconds (s)Power: Watt (W)

1 Watt = 1 J/s

W

P t

1. P = 200 W W=2000 J t=?

2. P =15 W Work= ? t= 10 sec

3. P= ? W= 56 J t= 2 sec

1. P = 200 W W=2000 J t=? t = W/P = 2000J/200W = 10 sec

2. P =15 W Work= ? t= 10 sec

3. P= ? W= 56 J t= 2 sec

1. P = 200 W W=2000 J t=? t = W/P = 2000J/200W = 10 sec

2. P =15 W Work= ? t= 10 sec W = P x t = 15 W x 10 sec

= 150 J3. P= ? W= 56 J t= 2 sec

1. P = 200 W W=2000 J t=? t = W/P = 2000J/200W = 10 sec

2. P =15 W Work= ? t= 10 sec W = P x t = 15 W x 10 sec

= 150 J3. P= ? W= 56 J t= 2 sec

P = W/t = 56 J/ 2 sec = 28 W

Wrap-Up #3: Power Calculation

• It takes 3, 150 Joules of work to move a refrigerator. If this is done in 5 minutes, how much power was used to move the refrigerator?

• It takes 3, 150 Joules of work to move a refrigerator. If this is done in 5 minutes, how much power was used to move the refrigerator?

• W= 3, 150 J• t= 5 min x 60s = 300 s• P = W/ t

= 3150 J/ 300 s = 10.5 Watts

Wrap-Up #3: Power Calculation

• Suppose two students are asked to unpack identical cartons of books. One student completes the job in ten minutes, while the other takes twenty minutes.

– Which student did more work?

– Which student used more power?

• Suppose two students are asked to unpack identical cartons of books. One student completes the job in ten minutes, while the other takes twenty minutes.

– Which student did more work?•Both students did the same amount of work

– Which student used more power?

• Suppose two students are asked to unpack identical cartons of books. One student completes the job in ten minutes, while the other takes twenty minutes.

– Which student did more work?•Both students did the same amount of work

– Which student used more power?•The one who finished in ten minutes