Introduction to Energy

• G7 Science
• Secondary Education
• Age 12

ENERGY

An introduction

What is energy?

• Energy is the ability to do work or cause change in matter.
  
• In other words:
  
• Work is the transfer of energy to an object by a force that causes the object to move in the direction of the force.
  
• Energy is a property of all matter and all matter has energy.
  

For example

The boy is pushing the box.

The push is a force to make the box move.

The boy is using energy from his body to do this.

Energy is transferred to the box and it moves along the floor.

We can measure energy in terms of work done.

Work (j) = Force (N) x distance (m)

j = joules N = Newtons m = meters

How much work is done when you lift an apple (which weighs 2N) 3 meters off the ground?

What is the relationship between work and distance?

As the distance an object moves increases ...

... the work needed to move the object increases.

Work is directly proportional to the distance moved.

SUMMARY

• Energy is the ability to do work and work requires a transfer of energy.
  
• Energy is transferred every time work is done.
  
• Energy and work are both measured in joules (J).
  
• If an object does not move (d = 0m) then no work is done.
  

What is energy?

1. Energy is a push.
2. Energy is the ability to do work.
3. Energy is the ability to attract objects.
4. Energy is a pull.

What is the equation to calculate work?

1. W = d/F
2. W = Pt
3. W = Fd
4. W = F/d

What is the unit of work and energy?

1. Watt
2. meter
3. Newton
4. joule

A force of 25N is used to push a box along the floor a distance of 5 meters. How much work was done?

1. 125J
2. 135J
3. 145J
4. 155J

It took 100J to push a chair 4 meters across the floor. What was the force used to push the chair?

1. 15N
2. 25N
3. 15J
4. 25J

A force of 50N was used to lift a rock. A total of 160J of work was done. How far was the rock lifted?

1. 1.8m
2. 2.3m
3. 3.2m
4. 4.1m

Mr. Nicholson pushed on a box with a force of 10,000N, but he was unable to move it. How much work was done?

1. 0J
2. 5,000J
3. 10,000J
4. 20,000J

What is a force?

• You will sometimes need to calculate the force before you can calculate work (W = Fd).
  
• A force is a push or a pull on an object.
  

What is weight?

• Weight is a force that depends on the acceleration due to gravity on an object and the object's mass. The object is pulled towards the center of Earth
  
• It is easy to confuse weight and mass.
  
• Mass (kg) is the amount of matter (atoms) an object contains.
  
• Weight (N) is the pull of gravity on an object.

How do we calculate force?

Force (N) = mass (kg) x acceleration (m/s/s)

*acceleration on Earth is 9.8 m/s/s

Calculate the weight of a box that is 55 kilograms.

1. 240N
2. 385N
3. 420N
4. 539M

An object accelerates at 5.0 m/s/s and it has a mass of 4.0 kilograms when an unknown force is applied to it.
What is the amount of the force?

1. 5N
2. 15N
3. 20N
4. 25N

An object accelerates at 10 m/s/s when a force of 20N is applied to it.
What is the mass of the object?

1. 1kg
2. 2kg
3. 3kg
4. 4kg

An object with a mass of 10kg has a force of 2N applied to it.
What is the acceleration of the object?

1. 0.1 m/s/s
2. 0.2 m/s/s
3. 0.3 m/s/s
4. 0.4 m/s/s

What is power?

Power is the distribution of energy over a period of time.

In other words ...

... it is the rate of doing work or 

... how much work is done over a period of time

How is power calculated?

Power (W) = Work (J)/time (s)

W = Watt  J = joule  s = second

What is the power of a microwave oven if it can perform 4,000J of work in 10 seconds?

1. 400W
2. 500W
3. 600W
4. 700W

A box weighs 100N. It is lifted 4 meters in 20 seconds.
How much power is used?

1. 10W
2. 15W
3. 20W
4. 25W

What is the power output of a person that pushes a box 7 meters in 5 seconds with a force of 10N?

1. 8W
2. 10W
3. 12W
4. 14W

How much work is done using a 250W blender for 2 minutes?

1. 15,000J
2. 25,000J
3. 30,000J
4. 35,000J

Key Equations

1. Work (J) = Force (N) x distance (m)
   
2. Force (N) = mass (kg) x acceleration (m/s/s)
   
3. Power (W) = Work (J)/time (s)
   

A machine uses 150W of power to move a box 20m in 20s. Find the force needed and the work done by the machine and the mass of the box.

0