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.

Action-at-a-distance and fields

You can read more about the way in which physicists think about a force acting without obvious contact.

Magnetism is an example of a non-contact or action-at-a-distance force. These are forces which can act on an object without being in physical contact with it. The force of gravity is another example. Thus, gravity will pull a raindrop down to Earth without any tangible physical link between the Earth and the drop. There are no strings attached (it would probably be easier for us to understand if there were some strings that made it all work).

A magnet will attract a paper clip from a distance of a few centimetres. The attractive force of the magnet on the clip acts at-a-distance. Pupils meeting this idea for the first time will be embarking on one of the great journeys within science – a journey to understand the concept of fields. The field is the means by which these mysterious action-at-a-distance forces act. If the paper clip is located in a magnetic field, a magnetic force will act on it.

The earth is surrounded by an invisible gravitational field. A magnet is surrounded by an invisible magnetic field.

Some words of caution. The attractive force due to gravity and the attractive force due to magnets are two phenomena which share the same paragraphs in this storyline. However, it must be noted that they are not the same thing. It would be potentially very misleading to start explaining the Earth's gravity by suggesting that the Earth acts like a giant magnet, attracting everything to its surface. Avoid such comparisons. Gravitational and magnetic forces are two different phenomena that appear together here because they both act at a distance. Beyond this point the similarity ends.

Weight and 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 objects with mass wherever they might happen to be in the universe. (There is more on gravitational force in the Gravity and Space episode in the SPT: Earth in space topic.)

For most everyday purposes, there really is no need to distinguish between mass and weight. People understand equally well if you say that the mass of the bag of potatoes is 5 kilogram or if you call this the weight of the potatoes. However, in science, and particularly in physics, there is a clear distinction between the mass of an object and the pull of gravity acting on an object. If learners are to understand this area of science, they need to appreciate the difference between mass and force.

Over to you. Give it your best shot. Just what is the difference between mass and gravity force? Give yourself a minute to gather your thoughts. Try explaining this to a friend.

It may be that your thoughts involve ideas about forces, particles (or stuff), perhaps even the Moon – which makes a regular appearance in such explanations. It is a good idea to start with the force of gravity. 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, and helpful, 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. In school a newtonmeter will do the same job. The weight then is a supporting force, which is measured in newtons. Weighing machines show the magnitude of this force, which is often a tension force or a compression force.

Mass and weighing

What then of mass? The best place to start is to realise that you can't show mass with an arrow in a sketch. Mass doesn't have a direction. It is not about pushes or pulls. It is about how hard it is to change the motion.

Things with more mass are harder to get going and harder to stop once they are going. The mass is an inertial property. A 3 kilogram bag of potatoes will be harder to throw than a 5 kilogram bag. Mass is measured in units of kilograms. The number of particles in side something is measured in moles, and is the correct unit for quantity of matter.

There is a clear link between the mass of a bag of potatoes and the pull of gravity on the same bag. A 5 kilogram bag will weigh more than a 3 kilogram bag (the 5 kilogram bag has a force acting on it of about 50 newton at the Earth's surface and the 3 kilogram bag a a force acting on it of about 30 newton). The more the mass of something, the greater the a force acting on that thing. There is a deep connection between an object's reluctance to being accelerated and the gravity force acting on it.

Let's suppose you take a 5 kilogram bag of potatoes to the Moon. Don't ask why! If the bag felt heavy on the Earth it will be much easier to lift on the Moon. Can you explain why?

Everything weighs less on the Moon because the pull of gravity at the surface of the Moon is weaker than that on Earth. It is about 15 ^th that on the Earth. Thus the 5 kilogram bag of potatoes has a force of about 50 newton acting on it at the surface of the Earth and about 10 newton on the Moon. Everything feels lighter. This is simply because the Moon has a smaller mass than the Earth.

However there is still exactly the same number of potatoes in the bag, so it's just as hard to accelerate. The 5 kilogram mass has not changed but the gravity force (and so the weight) has. Therein lies the difference. Force depends on gravity; mass just depends on the object. Consider the force needed to bring an Earth-bound running rugby player to rest inside a metre. The same force would be needed to stop the same player, moving at the same speed, within the same distance, on the Moon. You are still faced with stopping the same mass moving at the same speed.

The essential point is that mass does not vary. If you measure the mass of an object here on Earth and on the Moon, you would find it is exactly the same. This is in line with common-sense. If you take an object to the Moon, it is the same object: Some properties should remain the same and mass is one of those intrinsic properties.

The 5 kilogram bag of potatoes would weigh about 120 newton on the surface of Jupiter (the strength of Jupiter's surface gravity is about 24 newton on every kilogram). Planets more massive than the Earth have stronger surface gravity. Stars, millions of times more massive than the Earth, have enormous surface gravity. Black holes, so massive it is almost impossible to imagine, have such strong surface gravity that even light rays are pulled inwards. This is why we can't see them. They appear black.

Finally, just to confuse us all, most everyday weighing machines don't give you a reading in newtons. For example, any set of bathroom scales that you are likely to use at home will be calibrated in kilograms (and stones and pounds!). In day-to-day life we find our weight in kilograms. In scientific contexts we measure force in newtons. This is a good example of a situation where everyday and scientific ways of talking and thinking differ from one another.

The supermarket weighing machine that tells you that a bag of bananas weighs 3 kilogram really measures the support force to be about 30 newton and then divides by ten to give you the mass of the bananas as 3 kilogram. It can be programmed to do this because on Earth gravity pulls every 1 kilogram down with a force of about 10 newton (actually about 9.8 newton, but 10 newton is close enough at this level). So something weighing(needing a support force – compression or tension) about 30 newton is going to have a mass of about 3 kilogram.

The idea of mass is subtle: read more about it.

The definition of mass in terms of the amount of stuff is the one which is commonly used in school science. We suggest you do not follow this practice, as the amount of stuff or matter should be measured in moles.

For a physicist:

The mass of an object is a measure of its resistance to being accelerated by a given force.

Thus a 1 kilogram mass has a certain resistance to being accelerated by a given force, and a 2 kilogram mass has double that resistance.

We sometimes express this in terms of inertia, stating that a bigger mass has a greater inertia or reluctance to being accelerated. You might consider the inertia of a juggernaut lorry or a super-tanker ship. Both of these require huge forces to set them moving (to accelerate them from rest). Equally, both need huge forces to bring them to a halt once they are moving (to decelerate them). Both have a large mass and a large inertia.

The relationship between mass and resistance to acceleration can be seen in Newton's Second Law of motion:

acceleration = forcemass

You could also write, less helpfully;

force = mass × acceleration

This equation tells us that an object with a big mass will undergo a small acceleration if a given force is applied to it and vice versa.

So, although the physics definition of mass is in terms of resistance to acceleration, we can see that this measure is directly linked to the amount of stuff idea, in that the more matter or stuff there is in an object, the harder it is to set into motion, or to stop.

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.

Electric forces between electrically charged objects

Most pupils will have experienced, and be aware of, some of the everyday effects of static electricity. These might include getting a shock from walking on a synthetic fibre carpet and then touching a metal door handle, or hair sticking to a comb or a sweater. However, that's very different from making a connection between these experiences and the explanations involving the transferring of charged particles from one place to another.

The origin of this force is not obvious – the mobile particles involved (electrons) are much too small to see, as are the immobile charged particles, the protons. At the same time, it is the case that electric forces are responsible for holding most of the perceived world together. Atoms and molecules stick close together in solids, liquids and gases because of electric forces. When you tear a piece of paper, you are overcoming the electrical interactions that hold the fibres together. When you chop a piece of wood you are doing the same.

You can explain the electric forces using a simple model: everything is made of atoms and each atom has a number of smaller parts, each carrying an electric charge, which can be either positive or negative. Atoms are neutral – their positive and negative charges add to give no overall charge. It's the negative parts of atoms that can readily depart the atoms, to leave a positive residue. So every time you charge and object by rubbing, you are transferring electrons from one thing to another, leaving one positive and the other negative.

When the negatively charged part (the electrons) is separated from the positively charged part the parts attract each other, since oppositely charged objects attract. Two positively charged objects repel each other, as do two negatively charged objects. Rubbing a plastic material is one way to separate electrons from the atoms of the plastic.

The electric force of attraction reaches out through space without the need for contact. Again we can use the idea of a field to moderate this action – connecting the two interacting charges.

This idea of neutral as a balance of positive and negative charge turns out be really important: when you charge objects, you're upsetting the balance, by shifting electrons to (resulting in negatively charged objects) or from (resulting in positively charged objects) one of a pair of objects.