Work and Power

Work and Power

Middle School

​

Learning Objectives

• Define work and power and explain the relationship between them.

• Calculate work and power using their respective formulas and standard units.

• Identify the conditions required for work to be done on an object.

• Describe the purpose of machines and differentiate between simple and compound machines.

• Explain the concepts of efficiency and mechanical advantage in the context of machines.

Key Vocabulary

​

Work

Work is done when a force causes an object to move over a distance in that force's direction.

​

Power

Power is the measure of how quickly work is done, or the amount of work per unit time.

​

Machine

A machine is any device that makes work easier by changing the size or direction of a force.

​

Mechanical Advantage

Mechanical advantage is how much a machine multiplies input force to produce a larger output force.

​

Efficiency

Efficiency measures the percentage of work input that a machine converts into useful work output.

What is Work in Physics?

​

When Work is Done

​

When There Is No Movement

• No work is done if the object does not move at all.

• This is true no matter how much force you might apply.

• For example, no work is done when pushing on a wall.

​

Force vs. Movement

• No work is done if force and movement are in different directions.

• For instance, carrying a bag while walking does not count as work.

• The force is up, but the movement is forward.

Multiple Choice

Under which of the following conditions is work NOT done on an object?

• When the object moves in the same direction as the force.

• When a force is applied but the object does not move.

• When a force causes an object to accelerate over a distance.

• When an object is lifted against gravity.

​

​

Calculating Work

Multiple Choice

If you push a cart with a force of 60 N for a distance of 2 m, how much work have you done?

• 120 J

• 30 J

• 62 J

• 58 J

​

​

What is Power?

​

Multiple Choice

What is the definition of power?

• The rate of doing work

• The total energy transferred

• The force applied to an object

• The distance an object moves

​

​

What Are Machines?

• A machine makes work easier by changing a force's size or direction.

• Simple machines are basic devices that multiply force to make work easier.

• The six types are the lever, pulley, wheel and axle, wedge, and screw.

• A compound machine combines two or more simple machines to perform a task.

Multiple Choice

What is the primary function of a simple machine?

• To change the direction or magnitude of a force

• To create energy

• To reduce the total amount of work done

• To eliminate friction

​

​

Machine Performance: Input, Output, and Efficiency

• Work input is the work done by the person using the machine.

• Work output is the work done by the machine on an object.

• Efficiency measures how well input work becomes output work, as a percentage.

• A machine's efficiency is always less than 100% due to energy lost from friction.

Multiple Choice

Why is the efficiency of a machine always less than 100%?

• Due to energy lost to friction

• Because the output force is always smaller than the input force

• Because machines cannot multiply force

• Because the work input is always less than the work output

​

​

Mechanical Advantage

Actual Mechanical Advantage

Ideal Mechanical Advantage

• ​This is the theoretical mechanical advantage in a perfect system.

• ​​It assumes that there is no friction or any loss of energy.

• ​IMA represents the maximum possible advantage a machine can offer.

Multiple Choice

What is the key difference between Ideal Mechanical Advantage and Actual Mechanical Advantage?

• Ideal Mechanical Advantage assumes no friction, while Actual Mechanical Advantage accounts for friction.

• Ideal Mechanical Advantage is always lower than Actual Mechanical Advantage.

• Ideal Mechanical Advantage measures work, while Actual Mechanical Advantage measures force.

• Ideal Mechanical Advantage applies only to compound machines.

​

Common Misconceptions

Multiple Choice

Person A lifts a 50 N box to a height of 2 meters in 5 seconds. Person B lifts the same 50 N box to the same height in 10 seconds. How does the power generated by Person A compare to Person B?

• Person A generates twice the power of Person B.

• Person B generates twice the power of Person A.

• They both generate the same amount of power.

• They both do zero work.

Multiple Choice

If a machine has an efficiency of 75%, and the work input is 200 Joules, what is the useful work output?

• 150 Joules

• 200 Joules

• 267 Joules

• 75 Joules

Multiple Choice

A crane uses a force of 500 N to lift a steel beam 10 meters in 20 seconds. How much power did the crane generate?

• 250 Watts

• 5000 Watts

• 100000 Watts

• 100 Watts

Multiple Choice

A lever has an Ideal Mechanical Advantage of 4. If you apply an input force of 20 N, what would be the expected output force in a frictionless system?

• 80 N

• 5 N

• 20 N

• 4 N

​

Summary

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4