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    • Unit 1: One Dimensional Motion: Physics Introduction
    • Unit 2: Two Dimensional Motion: Projectile and Non-Projectile
    • Unit 3: Newton’s Laws of Motion and Force
    • Unit 4: Universal Gravitation and Circular Motion
    • Unit 5: Work, Power, Mechanical Energy, and Simple Machines
    • Unit 6: Momentum Impulse and Conservation of Momentum
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    • Unit 11: Electromagnetic Waves
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Work and Power

Work and Power

Work is done any time you are transforming one form of energy into another. Power is the rate at which you do work. Learn more and see examples here.

Work

Work is done to transform energy from one form to another.

When lifting and dropping a rock:

  • You do work to store Potential Energy (PE) as you apply a force to lift a rock a distance (height).
  • Gravity does work converting potential energy (PE) to kinetic energy (KE) while the rock falls.
Work by you and gravity

Joule (J) the MKS Unit of Work and Energy

  • A joule (J) is equal to the work done by a force of one newton to move an object one meter in the direction of the force.

Work and Power Variables and Units

Name Variable Unit Unit Abbreviation
Work W Joules J
Power P watt W
Time t seconds s
Work and Power Equations

Work Equation Facts

  • Work is force applied over a distance
  • Work is only done if an object moves
    • When applying force to an object that does not move, there is no work done.  No matter how hard you push a wall, if it does not move, you do no work.
Work Equation
  • Work is only done by the component of force in the same direction of motion
  • If motion is up at a constant rate, the force is equal to the magnitude of weight of an object lifted
  • (Fw = mg) and g is the acceleration due to gravity (g = 10 m/s2).

Click here to revisit the weight and mass page if you don' remember mass and weight)

Q1: How much work is done when a force of 50 N is applied to move a box 6 meters?

Q1

Q2: A student lifts a physics book applying a force equal to is 10 N weight a distance of 3 m up. How much work did the student do on the book?

Q2

Q3: How much work is done when a force of 50 N is applied but does not move a box?

Q3

Q4: Sergio pushed a box with 50 N of force forward.  When it did not move he pushed the box even harder with 100 N of force but the box still did not move.  When did Sergio do more work?

A) When he applied 50N of force

B) When he applied 100N of force

C) Neither

C) Neither : Movement in the direction of force must occur or no work is done

Work and Power When Force is at an Angle

  • When force is at an angle, only the component in the direction of motion is used to calculate work or power (Here its seen in red)
  • We are trying to determine the adjacent side in the picture
  • This leads to cosine when we have the angle and hypotenuse
Work and Power At Angle

Q5: A sled is pulled by a boy with a force of 60 N at 35° from the horizontal.  How much work is done if the sled moved 15 m forward?

Q5

Power

  • Power is the rate at which work is done
  • The unit of power is the watt (W)
  • A watt equals a joule per second

Power is calculated as work divided by time.  If you have force, distance, and time you can use the substituted formula you see here.

Power Equation

Q6: A car does 2450-J of work in 8 seconds.  What is the power of the car?

Q6

Q7: Joe can go up a flight of stairs in 15 seconds. If the physics teacher weighs 850 N and the vertical height of the stairs is 10 m, what is Joe’s power output?

Q7

Q8: How much force is required to move an object 25 meters in 6 seconds with a power output of 50 watts?

8

Example Work and Power Problems

1. How much work is done to push a box 5 m with a force of 12 N forward?

W = (F)(d)

Since the force is in the same direction as motion you plug numbers directly in and don't have to find the parallel component first.

W = (12)(5) = 60 J

Work and Power Gif

2. How much work is required to lift a 4 kg box at a constant rate a height of 2 meters?

2

3. What is the power output of a person that pushes the box 5 meters in 3 seconds with a constant force of 12 N?

3

4. How much work do you have when 12 N of force were applied on an object at an angle of 25° above the horizon to move an object 5 meters horizontally?

A) Find the horizontal component of force:

adj = (cosӨ)(hyp)

adj = (cos(25°))(12)= 10.9 N

B) Find out how much work is done by this component:

W = (F)(d)

W = (10.9)(5) = 54.5 J

Work and Power At Angle
(Click on the picture to enlarge it)

5. What is your power output when applying 12 N of force on an object at an angle of 25° above the horizon to move it 5 meters horizontally in 3 seconds?

Use the work from the previous problem above

W = (F)(d) = (10.9)(5) = 54.5 J

and then solve for power

P = W/t

P = 54.5/3 = 18.2 W 

(Click on the picture to enlarge it)

6. A woman is pulling her luggage with a 25 N force at an angle of 55°. What is her power output if it takes her 15 seconds to pull the suitcase 20 forward?

6

Work and Power Practice Quiz

Work and Power Quiz

Do you know your work and power?

Work and Power Equations

Take out quiz to find out

1 / 9

What is the unit of energy?

Joules (J) is the unit for energy (ME,PE,KE,Heat, or work)

Work is the transformation of one form of energy to another.

2 / 9

What is the unit of power?

Watt is the unit of power which is a Joule per second  W = F/d

3 / 9

What is the unit of work?

The joule is the unit of work and all of the energies (ME, KE, PE, Heat)

4 / 9

What is the power output when it takes a force of 50N forward is applied to move a box 6 meters in the same direction in 6 seconds?

power 1

5 / 9

How much work is done when a force of 50N forward is applied to move a box 6 meters in the same direction?

W = Fd

W = (50)(6) = 300 J

 

6 / 9

How much work would you have done if you pushed on a 2100 kg car with a force of 210N that did not move?

W = Fd

You have to move the object in the direction of force to do work.  No work is done if the object does not move.

7 / 9

A student lifts a physics book applying a force equal to is 10N weight a distance of 3m up. How much work did the student do on the book?

Work equals force times distance (W = Fd) and the force is the weight of the book that is lifted up

W = Fd = (10)(3) = 20 N

 

8 / 9

How much work would a 75 kg student do to climb up a ladder 9 meters?

You must lift your weight as the force when going up

Fw = mg = (75)(9.8) = 735 N

W = Fd = (735)(9) = 6615 J

9 / 9

Sergio pushed a box with 50 Newtons of force forward.  When it did not move he pushed the box even harder with 100 Newtons of force but the box still did not move.  When did Sergio do more work?

W = Fd

If you do not move the object, you have done no work no matter how much force you apply

Your score is

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Unit 1: One Dimensional Motion
Unit 2: 2D Motion
Unit 3: Newton’s Laws and Force
Unit 4: Universal Gravitation and Circular Motion
Unit 5: Work, Power, Mechanical Advantage, and Simple Machines
Unit 6: Momentum, Impulse, and Conservation of Momentum
Unit 7: Electrostatics
Unit 8: Current and Circuits
Unit 9: Magnetism and Electromagnetism
Unit 10: Intro to Waves
Unit 11: Electromagnetic Waves
Unit 12: Nuclear Physics

AP Physics 1 Pages (Deeper Dive into Concepts)

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