What is a force?

Think of the following situation: You are all helping your teacher to rearrange the classroom and she asks you to move her desk from one side of the classroom to the other. How would you do that? The desk is too heavy for you to lift, so how do you get it across the classroom?

That is right, you are going to either push it or pull it across the room. In doing so, you have exerted a force on the desk to get it to move.

A force is a push or a pull on an object. The unit in which we measure force is a newton (N). It is named after Sir Isaac Newton, an English physicist and mathematician. Sir Isaac Newton is recognised as one of the most influential scientists of all times. The unit of force is named after him in recognition of his work in mechanics and his three laws of motion.

We use forces every day of our lives. Our own bodies rely on forces. Our muscles pull on our bones to allow us to move. Our feet push on the ground when we walk. To open doors, to pick up our food - everything we do involves some kind of force.

What can forces do? Let's experiment with forces and see what we can do.

Effects of forces

From the last activity, you should have seen that forces can have the following effects:

• Forces can change the shape of an object. This is called deformation.

• Forces can change the motion of an object. If an object is stationary, a force can cause the object to start moving. Or, if an object is already moving, a force can cause an object to speed up or slow down.

• Forces can change the direction in which an object is moving.

How do we describe the motion of an object? When an object is moving, we say it has a velocity. Velocity is the rate of change of the position of an object. Velocity is the speed of an object and the direction in which it is moving. Speed describes only how fast an object is moving, whereas velocity gives both how fast and in what direction the object is moving.

An object can move at constant velocity. This means it travels at the same speed in the same direction. For example a car travelling along the highway at 100 km/h in a straight line has a constant velocity. However, what happens when the car moves faster or slows down?

We saw in the last activity that we could change the motion of an object by applying a force to make it speed up or slow down. The velocity of the object is changing over time due to a force acting on it. This is called acceleration. Acceleration is the rate of change of a body's velocity with time. In other words, it is a measure of how an object's speed changes every second.

Pairs of forces

We are now going to do another practical activity to investigate another concept about forces.

What we saw in the last activity is that whenever one object exerts a force on a second object, the second object exerts a force back on the first object. You saw this when you pushed against a wall. We say that forces act in pairs. Newton called the one force the action, and the other force the reaction, as shown in the following diagram.

We also saw that when you exerted a force on the wall, you experienced the wall exerting a force back on you. Forces act in pairs on different objects. The force exerted by the second object is equal in strength and opposite in direction to the first force.

In the last activity, we also saw that more than one force can act on an object at the same time. For example, when two of you were pushing or pulling on your friend in the middle. The effect of the different forces acting together depends on how big each force is and what direction each force is acting in. When two or more forces act on an object, then the forces combine to make a net (overall) force.

What happened when both of you pushed or pulled with an equal force?

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When the forces are equal to each other and opposite in direction, they balance each other and we say that the net force is 0 N.

What happened when one of you pushed or pulled harder than the other person?

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When the forces are acting in opposite directions, but are not equal, we say that the net force is greater than 0 N. There is a resultant force. If the forces are equal and acting in the same direction there will also be a resultant force.

Imagine a tug-of-war. People on either side pull on the rope. If they exert forces of equal size then the rope remains stationary. If one group is able to exert a larger force than the other group, then the rope will move in the direction of the larger force. This is because the forces are unbalanced and there is a net (resultant) force acting in the direction of the larger force.

You also saw that one of you could pull and the other could push in the same direction. In this case, there was a much larger net force as both forces were acting in the same direction and so they add together to produce a bigger overall net force.

We can work out the net forces acting on a body. To do this, we first need to speak about how we represent the forces acting on a body.

Representing forces

How do we show a force? When we want to draw a diagram to show what forces are acting, we use arrows to represent the forces. We always show forces acting from the centre of the object on which it acts. If we were to draw the force on a ball when it is pushed, it could look like this:

The direction of the arrow shows the direction in which the force is acting, and the length of the arrow is an indication of the size of the force. A small force would be shown with a short arrow. A large force would be shown with a long arrow.

More than one force can act on an object at the same time. The effect of the different forces acting together depends on how big each force is and what direction each force is acting in.

Let's look at how we can represent the following situation: Jabu pulls Rod's arm with a force of 10 N, while Viantha pulls Rod's other arm with a force of 6 N.

We can represent the forces acting on Rod in the following way: we use a circle to represent Rod and different length arrows to represent the forces acting on him. This is called a free-body diagram.

What is the net force acting on Rod? We can calculate it as follows:

net force = 10 N + (- 6 N) = 4 N to the left.

If the forces all act in the same direction then the net force is the sum of the different forces.

Imagine you are pushing someone in a go-cart, and your friend comes to help you push harder. There are now two forces acting on the person in the go-cart. These forces are acting in the same direction so they are added together to produce a net force which is the sum of the two smaller forces.

How do we measure a force? We use an instrument called a spring balance. A spring balance is a simple device consisting of a spring which, when stretched, gives a reading of the force used to stretch the spring.

Types of forces

So far, we have looked at forces acting on an object when the object causing the force is in contact with object experiencing the force. Do we always have to be in contact with an object in order to exert a force?

There were forces exerted by the magnets but they did not have to touch each other. That means that you do not have to be in contact with something in order to exert a force on it.

There are two types of forces:

• Contact forces: objects are in contact with each other and exert forces on each other.

• Non-contact (field) forces: objects are not in contact with each other and exert forces on each other.

We are now going to look at these two broad groups of forces in more detail.

Friction

What happens when you kick a ball across the grass? The ball moves quickly at first but then slows down again. Something has caused the ball to slow and stop moving. If the motion of the ball has changed then a force must have been exerted on it. The force which opposes motion is called friction. Friction forces always act in the opposite direction to the motion of the object. Friction resists movement when the object and surface are in contact. What does that mean? It means that if the ball is moving forward then friction acts backwards on the ball.

The following image shows a ball which has just been kicked. Draw an arrow to show in which direction friction would be acting.

What factors will affect the amount of force required to move objects? We need to look at all the forces acting on an object. Do you remember the following diagram from the beginning of the chapter?

We only showed the forces of the two learners pushing on the go-cart. What other forces are acting on this go-cart as it moves along?

Friction is acting on the go-cart in the opposite direction to the motion of the go-cart. The go-cart also has a weight. As we learnt in Gr. 8 Planet Earth and Beyond, an object on earth has weight due to the gravitational force of attraction of the Earth on the object. This is a force acting on an object.

We can now draw the friction and weight in the free-body diagram of the forces acting on the go-cart as follows.

There is another force acting on the go-cart. Think of when you stand on the ground: you feel the ground beneath your feet. This contact force is preventing you from penetrating the ground. This is called the normal force. The normal force always acts perpendicularly to the surface that the object is resting on.

In simple situations such as when you are standing on the ground or the go-cart is travelling along a level surface, then the normal force is equal to the weight of the object, but in the opposite direction. Think back to what we learnt about forces acting in pairs. On a flat, level surface, the normal force is the reaction force to the weight of the object. This is shown in the diagram for a box resting on the floor.

We can now complete the free-body diagram of the forces acting on the go-cart as follows,

Do you think there is a relationship between the friction that a body experiences and the normal force? Let's investigate.

The force of friction depends on the type of surface on which an object is moving and the normal force. In order to get an object to move, a force greater than the frictional force needs to be applied in order to overcome the friction between the object and the surface.

We can now look at the example of pushing a friend in a go-cart again. There is friction between the go-cart and the ground. The friction acts in the opposite direction to the forces pushing the go-cart forward. Therefore, if there are two forces of 7N and 10N pushing the go-cart forward, and the friction is 5 N, we can show only these forces as follows:

What is the net force acting on the go-cart?

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Friction is advantageous for a number of reasons. For example, the friction between our feet and the ground enables us to move forward and prevents us from slipping. Friction is also involved in keeping cars from skidding as the tyres experience friction between the tread and the roadway.

Tension and compression

There are other forces which are contact forces. Look at the following drawing of a boy pulling on a block with a rope.

The person is pulling the rope which is pulling the block. The person is not touching the block directly. The person is pulling the rope and the block is pulling back on the rope in the opposite direction. This causes a tension force to exist in the rope. The rope is tight and so there is tension in the rope. Tension is a contact force. The tension in the rope pulls the block across the floor.

Another example of a contact force is compression. A compression force is a force which acts to deform or squash an object. Let's think of some examples.

If you take a ball of dough and crush it with your fingers, you are exerting a compression force on the dough. The dough changes shape. It deforms. Another example is crushing a tennis ball or a cooldrink can between the palms of your hands.

The following diagram summarises the difference between tension and compression.

There are many other examples of compression forces in everyday life. A bridge experiences both compression and tension due to the weight of the cars and other vehicles which pass over it as shown in the following diagram.

We have been looking at contact forces, which include friction, normal forces, compression and tension. We are now going to look at the forces that act between bodies which do not touch.

Gravitational forces

Have you ever wondered why things fall down and not up?

The force which causes things to fall down towards the Earth and prevents us from falling off the planet is the gravitational force. Gravitational forces exist between any two objects with mass and they are forces of attraction (pull).

The gravitational force is a force that attracts objects with mass towards each other. Any object with mass exerts a gravitational force on any other object with mass. The Earth exerts a gravitational pull on you, the desks in your classroom and the chairs in your classroom, holding you on the surface and stopping you from drifting off into space.

The Earth's gravitational force pulls everything down towards the centre of the Earth which is why when you drop an object like a book or an apple, it falls to the ground. However, do you know that you, your desk, your chair, and the falling apple and book exert an equal but opposite pull on the Earth? Why do you think these forces on the Earth do not cause the Earth to move noticeably?

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The Earth attracts us because it has such a large mass and so we are attracted downwards towards the centre of the Earth all the time.

The bigger the mass of the objects, the greater the force between them. This means that two small objects would have a very weak gravitational attraction and so it has no noticeable effect. However, bigger objects such as the Moon and the Earth have a much greater gravitational force.

As we know from Planet Earth and Beyond, all the planets in our solar system are held in orbit around the Sun by the gravitational force of attraction between the Sun and planets.

The second factor which affects the gravitational force of attraction between objects is the distance between them. The further objects are away from each other, the smaller the gravitational force.

All the components in our Universe are held together by a gravitational force. In summary we can say:

• The greater the mass of the objects, the stronger the gravitational force of attraction between them.

• The closer objects are to each other, the stronger the gravitational force between them.

You have probably heard the term 'weight' used many times before, either in your Natural Sciences classroom, or in conversation with others. Many people use the term weight incorrectly in everyday language. For example, a relative may say to you "My weight increased by 2 kgs over the holiday period as I ate too much food." What is wrong with this statement? Discuss this with your class and teacher.

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The mass of an object is the amount of matter in the object. It tells you how many particles you have. Do you remember learning about atoms in Matter and Materials? So, for example, the mass of a wooden block tells us how many atoms there are. Mass is measured in kilograms (kg) and is independent of where you measure it. A wooden block with a mass of 10 kg on Earth also has a mass of 10 kg on the Moon.

However, an object's weight can change as it depends on the mass of the object and also the strength of the gravitational force acting on it. Weight is measured in newtons (N) as it is the gravitational force of attraction exerted on an object by the Earth (or Moon or any other planet). Therefore, the weight of an object will change when weighed in different places. The weight of a 10 kg block on Earth will be different to that on the Moon. Why do you think this is? Will the weight be more or less than on the Moon?

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Weight is the force of gravity pulling you towards the centre of the Earth. It is measured in newtons. On Earth the gravitational force causes us all to accelerate towards the centre of the Earth. The acceleration is called gravitational acceleration. On Earth it is 9,8 m/s². The gradient that we calculated in the last investigation should have given you a number close to 9,8 m/s² which is gravitational acceleration.

Objects are in free-fall when the only force acting on them is the gravitational force.

Weight (W) is calculated by multiplying an object's mass (m) by the gravitational acceleration (g):

W = m × g

But what if you went to the Moon?

The Moon also has its own gravity. The strength of gravity on the surface of the Moon is one-sixth that on the surface of the Earth, and so you would weigh one-sixth of what you do on Earth on the Moon. On Jupiter you would weigh 2.5 times more than you do on Earth as Jupiter's gravity is 2.5 times that of the Earth's. Even though you would weigh different amounts (and feel lighter on the Moon and heavier on Jupiter) your actual mass would stay the same in both cases.

So how much would you weigh on the Moon? Imagine you have a mass of 60 kg. Your weight on Earth would be 60 x 9,8 = 588 N. The gravitational acceleration on the Moon is 1,6 m/s², so your weight would be 60 x 1,6 = 96 N on the Moon.

Weight and mass calculations

QUESTIONS:

Ever wondered what it would be like to walk around on other planets? Find out how much you would weigh on other planets in the next activity.

The weight of a person is the force of gravitational attraction to the Earth that person experiences. Someone in free-fall feels weightless but they have not lost their weight. They are still experiencing the Earth's gravitational attraction.

The only reason the astronauts float is because they are in free-fall and their moving spacecraft is also in free-fall with them, falling at the same rate. Therefore, the astronauts appear to float when compared with the spacecraft because they are both falling at the same rate.

Magnetic forces

Certain materials have strong magnetic fields around them. These are called magnets. All magnets have two poles, a north and a south pole.

Other materials are strongly attracted to magnets. These materials are said to be magnetic. Magnets exert forces on other magnets and magnetic materials. Which materials are magnetic? Let's investigate.

Not all metals are attracted to magnets. Those that are attracted to magnets are known as magnetic materials. There are very few magnetic materials. They are iron, nickel and cobalt. Alloys which include any of the magnetic materials can also be attracted to magnets. Steel is an alloy which contains iron so steel can be attracted to a magnet.

So now we know that magnetic forces can act over a distance, but can they still act if there is something in the way? Let's find out.

In the last activity we saw that like poles repel each other but opposite poles attract each other. We have also seen that the magnetic force acts over a distance. The magnet does not need to touch something in order to exert a force on it. So a magnetic force is a non-contact or a field force.

What is a force field? Can we see it? Let's investigate if it is possible to see a magnetic field.

As we have seen, it is possible to visualise the magnetic force field around a magnet. We know from our previous activities that the magnetic force acts over a distance. The field is the space around a magnet in which it can attract or repel another magnet.

How do we draw a force field? The pattern you saw with your magnets can be represented by field lines. Field lines are used to show something we can't actually see. The closer the field lines are drawn together, the stronger the field being described. The more field lines that are drawn, the stronger the field. The field lines go from the north pole to the south pole. The following diagram shows the field lines around a bar magnet.

The next diagrams show the field lines between bar magnets which are attracting and those which are repelling.

Opposite poles attract.

Like poles repel.

A field is strongest next to the magnet and gets weaker further away from the magnet.

Did you know that the Earth is like a bar magnet with a North and a South Pole? The Earth has a magnetic field. You can imagine Earth's magnetic field as though there is a bar magnet running through the core with the magnet's south pole under Earth's North Pole. No one knows for sure, but the theory is that the superhot liquid iron in the Earth's core moves in a rotational pattern, and these rotational forces lead to the weak magnetic forces around the Earth's rotational axis.

This is why we can use compasses to tell direction. A plotting compass has a needle with a small magnet. The needle points to magnetic north because the small magnet is attracted to the opposite magnetic field and can be used to determine direction.

Have you heard of the Southern or Northern Lights before? Do you know how this phenomenon occurs?

Charged particles escape from the surface of the Sun and move outwards in all directions. When the charged particles reach Earth, some are trapped by Earth's magnetic field in areas in space around Earth's atmosphere, called belts. Sometimes the charged particles escape the belts and spiral along the magnetic field lines towards the magnetic poles where they enter Earth's atmosphere. They then interact with atmospheric gas particles, causing beautiful light shows.

Electrostatic forces

Do you remember learning about static electricity in Gr. 8? Let's do a quick activity to revise some of the concepts we already know.

Let's look at the example of brushing your hair in more detail to understand what is happening. You have dragged the surface of the plastic comb against the surfaces of your hair. When two surfaces are rubbed together there is friction between them. The friction between two surfaces can cause electrons to be transferred from one surface to the other.

In order to understand how electrons can be transferred, we need to remember what we learnt about the structure of an atom.

Where are the electrons positioned in the atom?

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What is the type of charge on a proton?

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What is the type charge on an electron?

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What is the charge on a neutron?

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The atom is held together by the electrostatic attraction between the positively charged nucleus and the negatively charged electrons. Within an atom, the electrons closest to the nucleus are the most strongly held, whilst those further away experience a weaker attraction.

Normally, atoms contain the same number of protons and electrons. This means that atoms are normally neutral because they have the same number of positive and negative charges, so the charges balance each other out. All objects are made up of atoms and since atoms are normally neutral, objects are also usually neutral.

However, when we rub two surfaces together, like when you comb your hair or rub a balloon against your hair, the friction can cause electrons to be transferred from one object to another. Remember, the protons are fixed in place in the nucleus and so they cannot be transferred between atoms. Only electrons can be transferred between atoms. Some objects give up electrons more easily than other objects. Look at the following diagram which explains how this happens.

Which object gave up some of its electrons in the diagram?

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Does this object now have more positive or more negative charges?

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Which object gained electrons in the diagram?

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Does this object now have more positive or more negative charges?

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When an object has more electrons than protons, we say that the object is negatively charged.

When an object has fewer electrons than protons, we say that the object is positively charged.

Have a look at the following diagrams which illustrate this.

We now understand the transfer of electrons that takes place as a result of friction between objects. But how did that result in your hair rising when you brought the charged balloon close to your hair in the last activity? Let's look at what happens when oppositely charged objects are brought together.

When we rubbed the perspex rods with the cloth, electrons were transferred from the perspex to the cloth. What charge do the perspex rods now have?

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Both the perspex rods now have the same charge. Did you notice that objects with the same charge tend to push each other away? We say that they are repelling each other. It is an electrostatic force of repulsion.

When we rubbed the plastic rod with the cloth, electrons were transferred from the cloth to the plastic rod. What charge does the plastic rod now have?

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The perspex rod and the plastic rod now have opposite charges. Did you notice that objects with different charges tend to pull each other together? We say that they are attracting each other. It is an electrostatic force of attraction.

As with gravitational and magnetic force, the distance between charged objects affects the strength of the electrostatic force. The closer the charged objects are, the stronger the force. The more charged the objects are, the stronger the electrostatic force between them.

We have now observed the fundamental behaviour of charges. In summary, we can say:

• If two negatively charged objects are brought close together, then they will repel each other.
• If two positively charged objects are brought close together, then they will repel each other.
• If a positively charged object is brought closer to a negatively charged object, they will attract each other.

Have you ever wondered where lightning comes from? Let's demonstrate an electrostatic spark.

Did you see sparks? The Van de Graaff generator can be used to demonstrate the effects of an electrostatic charge. The big metal dome at the top becomes positively charged when the generator is turned on. When the dome is charged it can be discharged by bringing another insulated metal sphere close to the dome. The electrons will jump to the dome from the metal sphere and cause a spark.

How does this little spark relate to a massive lightning strike?

During a lightning storm, clouds become charged. Friction between the clouds and the moisture in the clouds cause the clouds to become charged. The bottom of the clouds (closest to the ground) become negatively charged and the top of the cloud becomes positively charged. When the build-up of charge becomes too large, the electrons move from the bottom of the cloud to the ground where they are "earthed". The energy transfer is massive and results in extremely bright light, heat and sound. A lightning flash is a massive discharge between charged regions within clouds, or between clouds and the Earth. The thunder-clap, which we hear, is the air moving as a result of electrons moving.

Lightning is extremely dangerous. If the electrons move through a person on their way to the ground, then the large amounts of energy cause significant damage. That person can suffer serious injury, even death.

What precautions should we take during a lightning storm? Lightning can strike far from the rain shadow of the storm. This means that even if the storm seems to be far away, it is better to take precautions anyway. The safest place to be in a lightning storm is indoors. Stay away from windows and metal objects. If you cannot get inside, avoid standing next to tall objects or metal objects because if lightning strikes it will usually hit the tallest object in the area. If you are travelling in a car during a storm, stay inside the car until the storm subsides.