What do we mean by energy?
Energy is the capacity of a physical system to do work or cause a change. We will examine what this means in detail below, but to help establish students’ baseline understanding of energy get students to do Activity 1. What is energy?
Why is understanding energy important?
When we design and build stuff important to society such as bridges, cars, electronic technologies, or even developing the optimum diet for humans, we need to understand energy and how it works, otherwise the bridges will collapse, the car’s brakes won’t stop the car in time and risk human lives, our electronic devices will use too much energy (contributing to larger energy bills and climate change) or keep blowing every fuse in the house, and human health would be compromised. In other words, understanding energy helps us understand how the universe and everything in it works.
What is energy?
There are lots of different forms of energy that we will examine further on, but we first need to understand what energy is. Energy is not something you can see, taste or smell, but we can measure its effect, that is, it is quantifiable. It is essentially a mathematical concept that, most importantly, helps us predict how stuff in the universe will behave, whether we are considering something the size of atoms or as large as the universe itself.
Remember energy is the capacity for something to do work or cause change in a system. A key part of this definition is the concept of work, but work is distinct from energy.
For example, work is apparent when parents tell children to clean up their room and move that box of toys or sport gear from the floor. When they (eventually) push that box across the floor to under the bed, work has been done. Their muscles have done work by exerting a force (pushing) on a box to move it across the floor. The box has done work when it moved across the floor. There is mechanical movement over a distance – muscles moved your arms a distance to push (apply a force to) the box, and the box moved from the floor to under the bed. Energy enabled this movement (work) to happen.
But where does the energy come from? Your muscles get their energy from the food you eat; the box gets its energy from your muscles pushing on it. How much work something can do depends on how much energy it has. But the food itself is not the energy; it is just the source of energy. For the box, its source of energy is the force applied on it by your muscles.
To help students think more deeply about energy and work and how we quantify it, see Activity 2 Energy and work. See also the Powerpoint presentation.
How do we measure energy and work?
The standard unit used to measure energy is the joule. The Calorie is another unit of energy you will come across and you often see on the back of food packets for consumers to use as a guide to the amount of energy available in the food.
How many joules or calories something has, tells us how much energy that something has. The more joules or calories something has, the more work can be done.
[There are a variety of units used to measure energy, for example, as well as the Joule and Calorie there is the British Thermal Unit (BTU) and horsepower.]And now for some math (years 8-10)
Math can help explain the relationship between energy and work and we can calculate the amount of energy needed or used and how much work is done. We will keep it simple here.
One joule is equal to the work done by a one-newton force acting over a one-metre distance.
A Newton (N) is the force necessary to accelerate a mass of one kilogram at one metre per second per second. Think about the force it took to move that box of toys/sport gear. The heavier the box, the more force would have been required to move it. The further the box was pushed, the more work was done and the more energy was needed to do the work.
While we will stick to Joules here, as a comparison of units, one Calorie (C) is the amount of energy required to raise the temperature of one gram of water by 1˚ Celsius.
Work is done when a force (N) is applied to an object to make it move a certain distance. Thus, work is related to the force applied to something and the distance it moves or is displaced. This relationship can be expressed mathematically as –
Work = Force × distance (or displacement)
As noted above, force is measured in Newtons and a force is essentially a push or pull applied to an object such as the box of toys or sport gear to push it under the bed. Distance or displacement is measured in metres. How far did the that box get pushed?
Therefore, work is measured in Newton metres (Nm). And 1 Nm (or one newton of force causing a displacement or movement in one direction of 1 metre) = 1 joule.
Or 1 joule (1 unit of energy) = 1 Newton of force moving an object 1 metre.
For example, let us say your you had to use 50 Newtons of force to push your box 3 metres across the room.
Work (W) = 50 × 3M.
W = 150Nm
References: https://www.britannica.com/science/energy and https://www.physics.uci.edu/~silverma/units.html
Brain teaser: Does a stationary object – such as your box in the middle of the floor – have any forces acting on it?
Answer: Yes. Gravitational force pulls the box toward the floor and there is an opposite and equal force of the floor pushing back on the box. We know that gravitational force exists because things fall if you drop them from a height. But without the force from the floor opposing the gravitational force, your toy box would keep falling toward the centre of the Earth. In this case, the forces are balanced and the box does not move. When one force (a push or pull) is greater, you will get movement over a certain distance. When something exerts a force (an action) in one direction there will be an equal action in the opposite direction. This is Newton’s 3rd law of motion. See Activity 3 Balloon Rocket activity
The fun home science version is here
And for further insight into energy, check the Science Asylum’s take on energy below
Next up: Kinetic, potential, conservation and transformation
Back to FLEET School: Forces and energy
Or head to the next section in this resource, Kinetic, potential, conservation and transformation