Different kinds of forces

Your teaching is likely to concentrate on frictional forces, gravity, and magnetic forces. But you'd not want to confuse these with other forces, so there is a complete list here, to help you keep them separate.

Some forces represent how the environment can support a thing:

• Compression force (a warp force).
• Tension force (a warp force).
• Buoyancy.

Frictional forces represent how motion through an environment can be reduced, or prevented:

• Grip.
• Slip.
• Drag.

Non-contact forces – allow remote parts of the environment to affect a thing:

• Gravity force.
• Magnetic force.
• Electric force.

Forces and acrobats

The Tombolas are a circus act. The bottom row support the rest. They are supporting the three extra bodies. They must push upwards more than normal.

Pile up three large cushions. The shape of the cushions changes. What is happening to the one at the bottom?

Both situations involve things in equilibrium. The forces acting down on the cushions and on the Tombola brothers are each being balanced by a force acting upwards. We might call this upward force a support force.

But what about the ground under Papa Tombola's feet? It is also being squeezed and is supporting him (and his sons). The floor under the cushions is doing the same job. How can the ground exert a force upwards?

Compression forces

If you look closely at the ground it will be slightly distorted, dipping a little due to the force acting on it. We can explain these surface support forces by thinking of the particles which make up the surface. This takes some imagination.

These sketches show how we might imagine the particles in the floor. A simple model is to consider the atoms in the floor being connected by spring-like bonds.

It is rather like a tiny mattress structure. When a force acts downwards on the floor, the floor distorts slightly and the floor structure becomes compressed. The reaction to being compressed provides an upward force to support the gravity force and this upward force is often referred to as the reaction force from the surface (the floor in this case). It is an example of a compression force: something is compressed and exerts a force as a result of this compression.

This is how we can think of a solid inanimate object providing an upward supporting force. This upward supporting force is our first example of a contact force, where one object exerts a force on another object through being in direct contact with it (the floor pushes up directly on the Tombolas' feet).

Since the distortion does not have to be made by pushing downwards (you could push sideways by learning on a wall) a better general name for such forces is compression forces. Expect such forces wherever one object is in contact with its surrounding in such a way as to compresses the solids in the surroundings. Compression forces are drawn from the surface of the object, to remind you of the warp or distortion that caused the push.

Tension forces

When you lie on a bed the gravity force acting on you squashes the springs and they react, providing an upward force to support you. The mattress is a person-size model of what is happening at a microscopic level under the bottom row Tombola's feet. Forces that tend to squash objects are called compression forces. Forces that tend to extend objects are called tension forces.

If you extend a metal wire, perhaps a guitar string, you can feel the effect of the tension forces between the particles in the wire. As the material extends, the particles separate and the attraction between them pulls them back together. We say the wire is in tension.

Expect tension forces wherever one object is in contact with its surrounding in such a way as to stretch the solids in the surroundings (for example, a climber hanging on a rope). Tension forces are drawn from the surface of the object, to remind you of the warp or distortion that caused the pull.

Both tension and compression forces arise when solids in the surroundings are distorted or warped. We might therefore group tension and compression forces as warp forces.

But what about objects surrounded by fluids (liquids and gases)? Read on.

Floating and sinking

Floating and sinking are often the subject of study in science lessons. An object that is afloat is an object in equilibrium. To keep an object afloat there must be an upward force to balance the pull of gravity on the object. You can feel this upward force whenever you swim or try to submerge yourself in the bath. (This buoyancy force is sometimes called the upthrust.)

This buoyancy force is shown by a vertical arrow from the bottom of the object, since the origin of the force depends on interactions between the object and the particles bombarding its surface.

To explain the origin of this buoyancy force (which provides an upthrust) you need to think about the pressure in the surrounding fluid (liquid or gas), and the resultant forces acting on the surfaces of the object by the fluid environment. You'll also see why we chose to draw the force arrow from the bottom surface.

Expect buoyancy forces wherever one object is in contact with its surroundings in such a way that there will be particles of the fluid bouncing off the object's surface.

Describing friction

When two surfaces are in contact there is a force acting on each surface that acts in a direction to stop them moving past one another. This is often said to be due to friction, but there are two possibilities: movement, or no movement.

Rough surfaces have more friction than smooth surfaces and liquids such as oil or water are sometimes used as lubricants to reduce the effect of friction. There is no mystery behind friction. It acts on objects at the surfaces so as to prevent or reduce movement between the surfaces. When friction prevents sliding there is grip, when sliding is reduced there is slip. There is enough of a difference between these two that we suggest that you distinguish between grip forces and slip forces.

Grip forces are often a good thing. Without grip we couldn't walk anywhere. When we walk the grip force between our shoes and the ground enables us to push against the ground. On an icy surface walking is much harder because there is less almost no grip force.

Slip forces are useful in reducing existing motion. Without them the options for stopping a bicycle would be limited.

For a simple explanation of how grip or slip forces happen, imagine two surfaces at a microscopic level. All surfaces are full of imperfections. Nothing is totally smooth. When these imperfections catch on each other they act to prevent or reduce movement. Just think about rubbing two sandpaper surfaces together.

Friction through forces spectacles

Through forces spectacles a grip force or slip force is shown by drawing in a force arrow, parallel to the surface resting on the rough solid surroundings, to remind you of the origins of the force on the object. In the example below, the force acting on the box is shown by an arrow along the bottom surface of the box in the opposite direction to the intended motion.

A summary of contact forces

There are three kinds of contact forces that can support an object.

Warp forces can be found wherever a solid is distorted by an object:

• Add a compression force exerted by a neighbouring solid acting on the object if that solid is compressed by the object.
• Add a tension force exerted by a neighbouring solid acting on the object if that solid is compressed by the object.

You might, for teaching purposes, combine these two and call them warp forces – with the forensic clue that if an solid in contact with the object is stretched or squeezed then you can add an arrow labelled warp force.

• Add a buoyancy force if the object is partially or wholly immersed in a fluid.

Frictional forces of three kinds can be found at the surfaces of the object when it moves, or makes to move, past other particles its environment.

• If the environmental particles are a solid and no movement occurs, add an arrow at the contacting surface and label it grip force.
• If the environmental particles are a solid and movement occurs, add an arrow at the contacting surface and label it slip force.
• If the environmental particles are a liquid and movement occurs, add an arrow at the most significant surface and label it drag force.

You might, for teaching purposes, combine these three and call them frictional forces – but we'd not recommend that as it obscures the very different reasons for adding the arrows.

Some new forces

In this episode we look at a group of forces – magnetic, electric and gravitational – which are different in kind from the pushes and pulls of episode 02. So, in what way are they different? The key point is that these three forces allow remote parts of the environment to exert a force on an object without being in contact with it. Thus, a magnet attracts or repels another magnet; a rubbed (or electrically charged) rubber balloon attracts other things that are charged; the Earth attracts anything with mass. Each of these is an action-at-a-distance or non-contact force.

These days the concept of gravity is relatively common-place. If you ask people why things fall, more often than not they will tell you that it is because of the pull of gravity. Despite this familiarity, we should not lose sight of the fact that this is a very strange idea indeed and has been the subject of puzzlement throughout the history of science.

For example, if you drop a melon from the top of a building, how can the Earth (whose surface is some 10 metre away) exert a force on that melon?

What we do know is that such action-at-a-distance forces are very real and we experience them every day. All three non-contact forces decrease in strength as separation between objects increases. Each force is given a brief introduction in this narrative. This is extended in the expansion sections.

Magnetism

Magnetism is a non-contact force. For example, a magnetic force can be acting on a paper clip when a magnet is nearby but not actually touching. The magnetic force can be either attractive or repulsive and is not blocked by materials such as paper.

Although there might appear to be nothing in the space between two magnets, scientists describe this space as containing a magnetic field. The magnetic field of a magnet marks the space throughout which it can exert a force on another magnet or a piece of iron. If a magnet (or piece of iron) is placed in the magnetic field of another magnet, it will experience a magnetic force; if it is placed outside the magnetic field of the magnet it will experience no force. This is another example of how scientists have created a theoretical model to account for a phenomenon that cannot be directly observed. You can see the paper clip moving but you cannot see the magnetic force which is acting. Sprinkling iron filings near a magnet enables the form of this magnetic field to be displayed in a more concrete fashion. The iron filings, influenced by the magnetic field, line up to show the directions in which the magnetic forces are acting. A magnetic field pattern is the result.

Weighing machines

There isn't a place on the face of the Earth where there is no gravity acting. This means that every object we ever come across is located in the Earth's gravitational field and is therefore acted upon by at least one force, the force due to gravity.

To keep the physical basis of the interaction in mind we suggest you call this the gravity force on the object (purists might prefer the gravitational force – but that's just much harder to spell). Gravity acts towards the centre of the Earth or, more simply, downwards. The force arrow representing the gravity force is best drawn from the centre of an object in a direction straight downwards.

The gravity story, of course, goes way beyond the Earth. Gravity is a universal force which acts between any two masses wherever they might happen to be in the universe. It's also what holds stars and planets together: that it pulls from the centre of one mass to the centre of another provides the explanations for why these objects are nearly perfectly spherical. (There is more on gravitational force in the Gravity and Space episode in the SPT: Earth in space topic.)

It is within our everyday experience that some things weigh more than others. Just try lifting them. Weighing machines measure how much force you need to hold an object up steadily. So it seems simple to call this supporting force the weight.

For example, in supermarkets you'll find top pan scales and also hanging basket scales. Both instruments use the pull of gravity to measure the weight of groceries. They work on the principle of finding the upward force required to stop the groceries from falling to the ground. When a measurement is taken, the upward force from the weighing machine, or scales, balances the downward pull of gravity. This is an example of two forces in equilibrium. Weight then is an everyday name for this supporting force, which is measured in newtons. Weighing machines show the magnitude of this force.

Electric forces acting on a pair of charged objects

You can demonstrate the effect of an electric force by first rubbing a pair of party balloons on your jumper, then separating them from the jumper: the balloons will then repel each other.

This force is often called an electrostatic force or a force due to static electricity. First you charge the balloons, then you isolate them.

The force arises because electrically charged particles called electrons are transferred through the rubbing action – from the balloon to the jumper or vice versa. These are the same charged particles (or charge carriers) that drift around the bench-top and domestic circuits and so constitute the electric current in a metal wire.

However, because the balloon or sweater do not conduct electricity, the electrons are unable to move around and are therefore stationary on the balloon or the sweater (hence the term static electricity). The balloon is isolated if there is no route for the charged particles to move onto or off of the balloon.

As with magnetic and gravitational forces, scientists describe the space around the balloon as containing an electric field. The electric field is set up by the charge on the balloon and marks out the space throughout which the charged balloon is able to exert an electrical force.

Electrical forces acting on uncharged objects, by charged objects

You can also demonstrate the effect of an electric force by rubbing a single party balloon on your jumper and then sticking it to a wall. The balloon is pulled towards the wall before the charged balloon comes into contact with the uncharged wall. If the balloon is rubbed and held over some small pieces of paper (without touching them) the uncharged pieces of paper will be attracted to the charged balloon.

In the first case you can re-describe the pulling as an electrical force acting on the balloon, and in the second case as an electrical force acting on the paper. (If you've chosen to isolate the balloon from its environment in the first case, and the paper in the second.)

This is somewhat different from the first example of the force between two charged objects, as one of the objects is not charged. The force on either object is always attractive. A different mechanism is in play, and it is rather subtle. As the objects (balloon, wall, paper) are isolated, after the initial charging, electrons cannot flow onto or off of the objects, so their charge cannot change: the wall and paper remain neutral, yet an electrical force is acting on them.

The charged balloon still has an electrical field around it, and is able to exert an electrical force on things in that volume. But there are no charged things: the paper and the wall are neutral: they do not have an excess or deficit of electrons.