Energy And Work Done

In this article, the following are the objectives that will be discussed.

• Understand what energy is.
• Understand what work done is.
• State some examples of work being done and no work being done.
• Calculations involving work done.

Energy

Energy is the ability to do work. Without energy, machines and people will not be able to do work.

The SI unit of energy and work done is joule (J). One joule of energy is needed to do one joule of work.

In order for work to be done, there are two conditions that must be met:

1. A force acts on an object.
2. The object moves in the same direction as the force.

If any one of these two conditions is not met, no work is done. 

Examples of work being done

1. lifting up an object from the floor

Image Credits: unsplash - John Arano

2. pushing an object

Image Credits: unsplash - Peter Trones

3. pulling an object

Image Credits: unsplash - Anna Samoylova

4. a falling object

Image Credits: unsplash - Bithin raj

In the above scenarios, as a force is applied onto the object and the object is moving in the direction of the force, we can consider that work is being done.

Examples of no work being done

1. carrying an object

Image Credits: unsplash - René DeAnda

2. holding an object

Image Credits: unsplash - Jakob Owens

In the scenarios shown above, there is no work done as the movement of the object is perpendicular to the direction of the force.

Formula to determine work done

As mentioned earlier, in order for work to be done, a force must be applied to an object and the object must move in the same direction as the force.

Therefore, the formula to determine the work done is as follows:

work done (J) = force (N) × distance (m)

Revision questions

Here are some questions for us to look into on the objectives of this article.

Question 1: 

An object with a mass of 20 kg is being lifted up to reach a height of 10 m. Calculate the work done to lift up the object.

Solution: 

weight of object = m × g
= 20 kg × 10 N/kg
= 200 N

∵ distance = 10 m

work done = 200 N × 10 m
= 2000 J

*Note: g is the gravitational field strength to which is approximately 10 N/kg on Earth. 

Question 2:

Determine whether the statement below is true or false.

'Energy is the ability to push.'

1. true
2. false

Solution:

(2) false

Solution explanation:

Energy is the ability to do work.

Question 3:

Determine whether the statement below is true or false.

'Work is being done when the object moves perpendicular to the force applied on it.'

1. true
2. false

Solution: 

(2) false

Solution explanation:

In order for work to be done, the object must move in the same direction as the force applied on it.

Question 4: 

Which of the following situations is work being done?

1. carrying a heavy book
2. pushing a box so that it moves by 1 m
3. holding a heavy bag
4. pushing a wall 

Solution: 

(2) pushing a box so that it moves by 1 m

Solution explanation:

In order for work to be done, a force must be applied to an object and the object must move in the same direction as the force. As a force is applied onto the box and the box moves 1 m in the direction of the force, work is being done.

Question 5:

Which of the following situations is no work being done?

1. reeling in a fish
2. pushing a wheelbarrow so that it moves by 10 cm
3. holding a heavy bag
4. kicking a ball

Solution: 

(3) holding a heavy bag

Solution explanation:

In order for work to be done, a force must be applied to an object and the object must move in the same direction of the force. There is no work done as the movement of the heavy bag is perpendicular to the direction of the force.

Question 6:

Calculate the work done in the following situation.

An elevator carries a person with a mass of 70 kg to the 7th floor, which is 20 m high.

1. 490 J
2. 1400 J
3. 4900 J
4. 14000 J

Solution: 

(4) 14000 J

Explanation:

work done (J) = force (N) × distance (m)

weight of object = m × g
= 70 kg × 10 N/kg
= 700 N

∵ distance = 20 m

work done = 700 N × 20 m
= 14000 J

*Note: g is the gravitational field strength to which is approximately 10 N/kg on Earth.

Question 7:

Calculate the work done in the following situation.

A person exerts a pushing force of 500 N onto a huge box, causing it to move by 4 m.

1. 200 J
2. 2000 J
3. 100 J
4. 1000 J

Solution: 

(2) 2000 J

Solution explanation:

work done (J) = force (N) × distance (m)
= 500 N × 4 m
= 2000 J

Question 8:

Calculate the work done in the following situation.

An 80 kg bag is falling from the 10^th floor, which is 30 m high.

1. 800 J
2. 2400 J
3. 800 J 
4. 24000 J

Solution:

(4) 24000 J

Explanation:

work done (J) = force (N) × distance (m)

weight of object = m × g
= 80 kg × 10 N/kg
= 800 N

∵ distance = 30 m

work done
= 800 N × 30 m
= 24000 J

*Note: g is the gravitational field strength to which is approximately 10 N/kg on Earth. 

Question 9:

Calculate the work done in the following situation.

A person with a mass of 80 kg exerts a pulling force of 1000 N onto a trolley. The trolley, with a mass of 40 kg, moves 5 m away from its original position.

1. 800 J
2. 200 J
3. 2000 J
4. 5000 J

Solution:

(4) 5000 J

Solution Explanation:

work done (J) = force (N) × distance (m)
= 1000 N × 5 m
= 5000 J

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