Work / Power

Work and Power

WORK!!!
Now Hiring:

$ 20.00 per hour

PLACE HANDS ON WALL

AND EXERT FORCE.

You will be paid $20.00 per hour of 1 Joule of mechanical work that you do.

No experience required

Work - Have you done any work today?

• Work: (W) the transfer of energy

that occurs when a force makes an
object move.

– Measured in Joules

• Two conditions must be met:
• Object must move
• Force must be applied in the direction of the movement.

Multiple Select

Work must include

• a force causing motion in the same direction

• a force acting perpendicular to the motion

• a transfer of energy

• a force acting against the direction of motion

Multiple Select

The units for work are

• m/s2

• N

• Nm

• J

Work - Can be summarized as an equation.

W = Fd

W = work
Measured in J

F = force

Measured inN

d = distance
Measured in m

Example #1: You move a 75 kg refrigerator 35 m.
This requires a force of 90 N. How much work, in
joules, was done while moving the refrigerator?
m = 75 kg
d = 35 m
F = 90 N
W = ?

W = Fd

Multiple Choice

Which is the unknown in the previous problem?

• distance

• force

• work

• power

Multiple Choice

For the previous problem, to calculate work use:

• 75kgx90N

• 75kgx35m

• 35mx90N

• none of the choices are correct

Multiple Choice

How much work is being done with 200 N of force over 426 meters?

• 85,200 J
• 45,620 J
• 90,480 J
• 62,329 J

Multiple Choice

In which of these cases is work being done? 

• A student sleeps
• A student pushes a locked door that does not move
• a student carries his chair into another room
• A student sits on the carpet

Multiple Choice

True or false: when the distance an object moves is zero the work done on the object is also zero.  

• True
• False

Multiple Choice

Work is done when a weightlifter holds a barbell still above his head.

• True
• False

Multiple Choice

You push against the back of your friends car that is stuck. You push and be become very tired, but the car does not move. Did you do work?

• Yes, work was done

• No, work wasn't done

Practice

1.

A book weighing 1 N is lifted 2 m. How much work
was done?

2.

A 1 kg book is lifted 2 m. How much work was
done?

Multiple Choice

What would 54N be an example of?

• work

• power

• distance

• force

Multiple Choice

what would 14J be an example of?

• work

• distance

• time

• force

Multiple Choice

What is an example of a force?

• Energy

• Power

• Pressure

• Weight

Power

P = power

Measured in J/s

Or

Watts = W

P =W
t

W = work
Measured in J

t = time

Measured in s

Power: (P) the amount of work done in a
certain time; the rate at which work is done.

Multiple Choice

The amount of work done in a certain amount of time is known as 

• work
• energy

• force 

• power

Multiple Choice

If you increase your work and decrease your time, you will have

• more power

• less power

• same amount of power

• power rating can not be determined

Multiple Choice

If you have the same work as someone, but in less time

• You are more powerful

• You are less powerful

• You have the same power

Multiple Choice

Person A and Person B are both using the same amount of work to lift something. Person A takes half as much time to do it. Who has greater power.

• Person A

• Person B

Example #2

It took 150 seconds to move a refrigerator.
You did 3,150 joules of work in the process.
How much power was required to move the
refrigerator?

t = 150 s
W = 3,150 J
P = ?

P = W
t

Multiple Choice

How would you calculate the power of the previous problem?

• 150s/3150J

• 3150J/150s

• 150sx3150J

• none of these choices are correct

Multiple Select

What are the correct units for Power

• J

• J/s

• Nm

• W