Lesson 13: Work and Energy

Download here: Ontario Curriculum Expectations

Consult not your fears but your hopes and dreams. Think not about your frustrations, but about your unfulfilled potential. Concern yourself not with what you tried and failed in, but with what is still possible for you to do.

Pope John XXIII

When a FORCE is applied to an object and causes it to move, ENERGY is transferred and WORK is done.

Richard Parsons

When a bulldozer applies a force to a load of dirt, it is able to move the dirt around a construction site and the bulldozer is said to be doing work. Work is done whenever a force causes an object to move. Energy is needed in order to do work. The more energy there is, the more work can be done.

Energy comes in many different forms including chemical, gravitational, thermal, and electrical, and from many different sources including the sun, food, and petrochemicals. That energy can then be used to do work. Energy and work are two different forms of the same thing and, consequently, are both measured in a unit called the Joule (J). If you have 100 J of energy, you can do 100 J of work.

One joule (J) is the work done by a force of one Newton (N) applied over a distance of one metre in the same direction as the force.

1 J = 1 Nm.

Work is only done when…

In order for work to be done, the following conditions must be met:

1. A force must be exerted on an object.
2. The force must cause the object to move.
3. At least part of the force must be in the same direction in which the object moved.

Examples of Work Being Done

Work is done in each of the following situations because the applied force causes the motion and is in the same direction as the motion.

	A weightlifter applies an upward force to the barbell causing the barbell to be moved upwards.
	Pushing the lawnmower to the left causes it to move left.
	The man pulling the toboggan is applying both an upward force and a force to the right. Since the toboggan is moving to the right, only part of the applied force is doing work on the toboggan.

Examples Where Work is NOT Done

	In order to hold this plant, the person must apply an upward force. However, since there is no displacement, no work is being done.
	This skateboarder is moving to the right so there must be a displacement. However, at the moment, he is not applying a force to cause his motion. Therefore, he is not doing any work.

Calculating the Work Done

The equation for work is:

where  the force applied, measured in Newtons (N)

the displacement over which the object is moved by the force, measured in metres (m)

W = the work done, measured in Joules (J)

Force and displacement are both vectors and have direction. Work is a scalar quantity and does not have a direction.

Practice Questions

1. Salim belongs to the recycling club at school. He pushes a box of paper 20.0 m down a hallway with a force of 90.0 N.
   1. How much work has Salim done?
   2. How much energy did Salim need to do this amount of work?

Answer

Given:

20.0 m

 90.0 N

Required:

W = ?

Solution:

W = (90.0 N)(20.0 m)

W = 1800 J

Salim has done 1800 J of work.

Salim will need 1800 J of energy to do 1800 J of work.

2. The recycling box has a mass of 15.0 kg. Salim does 220 J of work on the recycling box when he lifts it into the recycling truck. How high did the Salim lift the box of paper?

Answer

Given:

m =15.0 kg

W = 220 J

g = 9.8 N/kg

Required:

F_g = ? (in order to determine the force Salim must overcome to lift the box)

? (the height of the box)

Solution:

Salim lifted the recycling box 1.5 m.