May the Forces be with You – Teacher Background
On this page you will find a brief description of the background knowledge required to teach the Year 4 – May the Forces be with You module.
During our lessons about forces, there are eight ideas we will need to be familiar with: force, distance, mass/inertia, time, speed, acceleration, momentum, and friction. Each of these is discussed briefly below. It is anticipated that many students may already have an intuitive idea of the first four of these concepts, but that the second four may be new words to many.
Forces can be a push or a pull. They can make objects move, cause them to spin or rotate, and bend, stretch, twist and break.
Forces act over a huge range of distances. From across the Solar System and the Universe to ordinary, everyday distances like the spaces we live and work in, down to miniscule distances such as the forces that act between atoms.
Forces have two features: their size and their direction. The size is measured in units called Newtons. One Newton is roughly the weight of a medium apple, or 100 grams. Direction is shown as an arrow pointing up or down, right or left, or using the compass directions like North or South.
Although it is too early in most children’s development to formally define a Newton it is included here for completeness. A force of 1 Newton is the force needed to accelerate a mass of 1 kilogram at 1 metre per second per second. It is based on Newton’s second law of motion which says that force is the product of mass and acceleration, or F = ma. The acceleration due to gravity at the Earth’s surface is about 10 m/s2 (9.81 m/s2 to be more precise), so 1 Newton is the weight force of about 100 grams which is 1/10 th of 1 kg.
Forces are easily visualised if they are represented as arrows: the length of the arrow shows the size of the force and the direction of the arrow shows the direction of the force. Forces can add up if they act in the same directions or cancel if they act in different directions, and so do arrows. The idea of using arrows is very important and we call it ‘the maths of arrows’.
The maths of arrows can be explored using the PhET Forces and Motion: Basics simulator:
https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion-basics_en.html
Start with the ‘Net Force’ simulation and explore by dragging and dropping the red and blue ‘people’ onto the rope, tick the values so you can see how their forces add, their values and the speed the cart travels and press Go! Note that this simulation may seem counter intuitive because it simulates a frictionless world.
Distance is a measure of how far apart two or more objects or points are, or how far an object moves. In science and everyday use, distance is measured in metres, kilometres, centimetres or millimetres.
Mass is a measure of how much force is needed to change an object’s speed – to make it move, stop, speed up or slow down. It is connected to inertia. Massive trains have huge mass, so they speed up very slowly. Mass is measured in grams and kilograms.
Inertia is another way of describing mass: it is a measure of how hard it is to start or stop an object from moving. Heavy things like cars and ships are difficult to get moving, and once moving, they are difficult to stop, so they have a lot of inertia.
Time describes when an event occurs, normally described as being in the past, present, or future. Measures of time often represent the time between two events (this is more specifically termed ‘duration’). Time is measured with clocks using the units seconds, minutes, hours, days and years.
However, Albert Einstein showed us that time is not always like this. Time is not the same everywhere and one person can say something has already happened while another person is still waiting for it to happen. Time passes a little bit faster on the top of mountains and slower when you get closer to a big mass. It even stops if you go to the ‘event horizon’ of a black hole!
Speed is a measure of how far an object travels in a certain amount of time. Speed in science is normally measured in metres per second, while kilometres per hour (km/h) are used in everyday applications, such as measuring the speed of a car.
Acceleration is a measure of a change in speed or direction. We feel a change in speed when something gets faster or slower. When things fall, they get faster and faster. We call this the acceleration due to gravity.
Momentum is the combined effect of the mass and the speed of an object. A heavy and fast object, like a basketball, has large momentum. This is why it can hurt to catch a fast-moving heavy ball. Your hands need to absorb that momentum. We often refer to momentum as ‘bulletiness’ because it has the ability to cause damage. Bullets are not very heavy but they have very high speed. In space a flake of paint can have enough momentum to make a hole in the side of a spacecraft. Whereas objects with large mass like trains have large momentum, even if they are moving slowly.
Friction is a force that acts in the opposite direction to an object’s movement and causes the object to slow down. We feel friction whenever we slide something.
Electricity is the movement of tiny particles called electrons. Each electron has a small negative electric charge. Electrons want to keep away from one another because like charges repel. Electricity flows through a wire when one end is negatively charged, causing the electrons to move away from the negative, to the other positive end. Electrons are attracted to places that have positive electric charge. We say objects that have no overall charge are neutral.
An object that is initially neutral can become positively charged by removing electrons from it. When this happens, the overall number of positive protons is more than the number of electrons. For example, if electrons are removed from one balloon and placed onto another balloon, then this caused the first balloon to become positively charged, while the second balloon becomes negatively charged. Then they will be attracted to each other. If extra electrons are put onto two balloons they will repel each other as they both are negatively charged, and like charges repel.
Electric forces are called electrostatic when the electrons are not moving, and magnetic forces are produced by moving electrons. Combined, they are called electromagnetic forces. The previous example of transferring electrons onto a balloon is one way to create electrostatic forces. Magnetic forces are produced when electrons flow along a wire, (an electromagnet), or in special materials called magnets where atoms are lined up in such a way that the magnetic fields electrons all move in circles that are lined up.
Electromagnetic forces are the reason we feel something when we use a push or pull force. What we think of as ‘touching’ is the ‘close up’ electric forces between atoms in our skin on the end of our fingers and the surface of an object.
If you are interested in running lessons based on electrical forces with your students, some of the activities from the Year 3 Atom Frenzy Module Lesson 4 may be appropriate.
Matter and atoms
Matter is made of miniscule atoms. Each atom has a cloud of electrons around an even more miniscule nucleus. The nucleus is made of protons and neutrons. Protons attract electrons with an electric force because protons have a positive electric charge (+1), while electrons have a negative electric charge (-1). This is called ‘opposite charges attract’. Positive repels positive and negative repels negative: this is called ‘like charges repel’.
When electric and magnetic forces change very fast they can create photons which are tiny vibrating lumps of electric and magnetic energy. Radio waves, microwaves, infra-red radiation, light, ultra-violet light and X-rays are all streams of photons.
Electric forces are behind all sorts of familiar chemical processes like burning, digesting, cooking, even seeing and thinking! Attraction between negatively-charged electrons and positively-charged protons allows single atoms to combine to produce different materials. When photons strike cells in our eyes called rods and cones, they exert tiny forces that cause chemical changes which make electrons move. They travel down nerve fibres to our brain. All this happens by electric forces, allowing us to sense light and see.
Most other forces we encounter, such as elastic forces, forces from wind and air pressure, collision forces, friction, and bending, twisting and breaking forces, arise because of electric forces between atoms. The force of the wind you feel on a windy day is from the atoms in the air bouncing off the atoms in your skin. They bounce because electrons repel each other. The force between magnets is when the electric forces in atoms line up to make big force patterns that we can feel with other magnets or with iron things like nails.
Atomic force microscopes are devices that move a tiny stylus just above a surface to ‘feel’ the forces of atoms. What they reveal is that up close we never ‘make contact’: the electrons in the atoms repel each other through empty space. There is no such thing as touching or ‘contact’ forces: there are only forces that act through the tiny empty spaces between atoms. It is these forces that allow us to feel, touch and push. Everything in our daily lives is about the attraction and repulsion of like and unlike charges and magnetic forces caused by the movement of electrons, all invisibly happening at atomic and molecular level much smaller.
The following 2½-minute Nature video Have you ever seen an atom? provides a concise summary of how Atomic Force Microscopes are able to produce images of atoms:
Atoms and molecules are not the focus of the Year 4 module, however this background knowledge is helpful for explaining electrical forces if not already covered in Year 3. For additional background on atoms see the Teacher Background Section of the Year 3 Atom Frenzy Module.
In the 1600s, Isaac Newton proposed that objects move in a straight line at constant speed unless acted upon by a force. He suggested that gravity is a force that pulls objects together. However in 1915, Einstein proposed a more accurate theory. In Einstein’s theory, gravity is not a force at all, it is a curving of spacetime.
In Einstein’s theory, everything still follows a straight line, but in a curved space time. Therefore gravity makes objects appear to take curved paths or come together, but in reality it has curved the space that objects move in. When an object is following its natural straight path in spacetime we call this free-fall.
Everything wants to follow free falling patterns like the orbits of planets and satellites, or a child in the air on a trampoline. But on Earth we cannot fall freely because the Earth gets in the way and stops us from falling forever. The Earth is just a huge ball of atoms held apart by electric forces. If the electric forces stopping us from passing through the Earth’s atoms.
The larger the central mass, the stronger the curving of spacetime and the more objects are causes to fall towards it. The moon is smaller than the Earth, which is why astronauts can easily bounce around. Gravity doesn’t make them fall to the Moon as quickly as it does on Earth.
If you are falling freely, you don’t feel any force. There is no Earth, or other mass to push against you. This is why the astronauts on the International Space Station float. They are in free fall around the Earth. Gravity is causing them to fall towards the Earth, but because they are fast enough, they keep ‘missing’ and going around. They are in free fall.
Gravity can grow indefinitely the more mass you have, and in the end, gravity can overwhelm everything, including light. That is when you have a black hole. There are lots of black holes out there in the universe.
These ideas originated from Albert Einstein a little over a century ago. He showed that gravity is the curving or warping of space and time. Gravity holds the Solar System, Milky Way, and all other galaxies together. However, gravity is about a billion, billion, billion times weaker than electric forces.
Both gravitational and electrical forces act over very long distances, but electric forces always get neutralised. In a lightning flash the electrons that had been carried up into the clouds on water drops suddenly flow back again in a gigantic electric spark. So, on the scale of space and the universe, gravity rules everything, but for making solids and liquids and people, electric forces rule.
Brian Greene’s 2-minute video: How Do Motion and Gravity Affect Time? Explains how the speed that and object moves and the strength of the gravitational field affect time. The video is located at: https://youtu.be/JB973lbTuCQ?
The NASA website has a straight-forward discussion about time and whether time travel is possible: https://spaceplace.nasa.gov/time-travel/en/
The Four Forces
There are just four known types of forces. In the Einstein-First program we introduce the first two of these forces, electric forces and gravity in Year 4 because these are the two types of forces that we can see and feel acting in everyday life. We leave the weak and strong nuclear forces to Year 9 when we introduce students to radioactivity and start to explore fundamental particles and processes that occur within the nucleus of atoms. We include a small section below for interest.
Strong and Weak Nuclear Forces
There are two other forces that act over miniscule distances within the nucleus of atoms. These nuclear forces hold the protons and neutrons together in the tiny nucleus. These nuclear forces will not be introduced until Year 9 when students learn about radioactivity. They are the forces that created the 94 naturally occurring elements, are associated with making of new atoms during nuclear fusion, and the breaking up of atoms called nuclear fission. They are the forces that release energy in radioactivity, nuclear power stations, nuclear weapons and stars, including our Sun.
If you wish to learn more about forces, including the forces inside atomic nuclei, then watch:
D-News 4-minute YouTube video: The Four Fundamental Forces of Physics Explained: https://youtu.be/a-6skWBuHaE
Arvin Ash: The Four Fundamental Forces of nature – Origin & Function: https://youtu.be/669QUJrF4u0?t=727