Grade 9 Work, energy, power | BCHydro Power Smart for Schools
Energy

# Work, energy, power

Explore the scientific definition of work and how it relates to power and energy.

9
Duration
20 mins
Type
Group work

## Overview

While carrying loads up a flight of stairs, your students demonstrate the concepts of ‘work’, ‘energy’ and ‘power’. The "Work, energy, power" worksheet helps students to compare, connect and understand the terms, while the teaching notes below provide a deeper exploration of the terms to support learning.

### Instructions

#### What you'll need

• Three things of different weights. They could be 1 kg, 5 kg, and 10 kg weights or bags filled with various objects.
• "Work, energy, power" worksheet

#### Introduction

1. Before you move into the hallway of your school, or wherever else you can find a flight of stairs, engage your students in a discussion of work. Ask students what requires more work: lifting a feather or pushing hard against a wall?
2. Ask students for definitions of the terms ‘work’, ‘power’, and ‘energy’. Do they have any ideas about how these three concepts are connected?

#### Demonstrating work

1. Gather your students near a stairway. Have the three weights with you.
2. Invite three student volunteers to take on the role of “workers” and give them each one of the three weights.
3. By carrying the weights in front of them using both hands, have the students carry their weights up and down the stairs while the rest of the class observes. If other students are interested in being a “worker” give them an opportunity to carry one of the weights.
4. Return to the classroom and begin a discussion of what your students observed. Re-introduce the words ‘power’, ‘energy’ and ‘work’ and ask the students to recall their demonstration:
• Who worked harder?
• Who used more energy?
• Who needed more power?
5. Divide students into groups of four or five and have them complete the "Work, energy, power" worksheet.
6. Have each group present their work on one of the three terms. You can use the worksheet answers and the Teaching Notes to support your students’ exploration of the terms.

### Modify or extend this activity

Discuss these two problems as a class:

1. Jess and Jade are in the weight room. Jess lifts the 100-pound barbell over her head 10 times in one minute; Jade lifts the 100-pound barbell over her head 10 times in 10 seconds. Who does the most work? Who delivers the most power?
•  A: They do the same amount of work because they use the same force to lift the same barbell the same distance over their heads. Jade is more powerful because she completed the same amount of work in less time.
2. Jack and Jill ran up a hill. Jack has twice the mass of Jill. Jill gets to the top in half the time it takes Jack. Who did the most work? Who delivered the most power?
• A: Jack does more work. He has twice her mass so applies twice the force to move the same distance. The power is equal, however, because Jill does one-half the work in one-half the time.

### Curriculum Fit

#### Content

• Voltage, current, and resistance

#### Curricular competencies

##### Questioning and predicting
• Demonstrate a sustained intellectual curiosity about a scientific topic or problem of personal interest
##### Processing and analyzing data and information
• Seek and analyze patterns, trends, and connections in data, including describing relationships between variables and identifying inconsistencies

### Teaching Notes

The words work, energy and power will likely have different meanings for students than the scientific meanings we associate with electricity and other forms of energy.

#### Work

In scientific terms, work is defined as the transformation or conversion of energy. Work is measured in joules (J).

#### Energy

Energy is the ability or capacity to do work and is also measured in joules (J). There are many different forms of energy (heat, light, electric, elastic, etc.) but all energy is either potential or kinetic.

• Potential energy is stored energy that is available to do work. It can take forms such as chemical, mechanical, nuclear, gravitational and electrical. Examples include a mobile phone or car battery, a ball sitting on a table or a stretched rubber band.
• Kinetic energy is energy in motion that is actually doing work. It can take forms such as light, heat, motion and sound. Examples include a car in motion, a ball falling off a table or a rubber band flying across the room.

#### Power

Power is the rate at which energy is transformed or the rate at which work is done. Power (P) is measured in watts (W) or joules per second (J/s). Power can be calculated if you know the work (or energy) and time: Power = Energy / time (or P=E/t).

#### Electrical power

Electrical power is the amount of electrical energy that is changed into other forms of energy each second. A good example of this is a 100-watt light bulb, which changes 100 joules of electrical energy into light and heat each second.

Electrical power can also be calculated if you know the voltage and the current in a circuit. Voltage is represented by ‘V’ and current is ‘I’.

Power = Current x Voltage (or P= I x V)

### Assessment

You’re introducing new terms with this activity, so focus your assessment on identifying what your students already know, and what they learn.

#### Worksheet answer key:

Work

• Examples: carrying a backpack full of books, a weightlifter lifting a barbell above their head, pushing a grocery cart filled with groceries
• Non-examples: a person pushing against a brick wall, a car sitting in a garage, sitting on the couch doing nothing, pulling on the reins of a horse that won’t move
• Characteristics/Facts: in order for work to be done, an object must be moved some distance by force

Energy

• Examples: heat, light, mechanical, chemical, electric, kinetic, potential
• Non-examples: rocks, minerals, piece of chalk, wood, metal
• Characteristics/Facts: energy takes many forms, energy is neither created nor destroyed

Power

• Examples: a car on a highway, a hydroelectric dam
• Non-examples: a chair, a table, a car with no engine, a sleeping person
• Characteristics/Facts: power relates to the time it takes to perform work; power is measured in Watts (W)