non-contact

4.3

Forces can be contact or non-contact

Learning intentions

By the end of this topic, you will be able to: • define and provide examples of contact forces and non-contact forces.

contact force a force acting between two bodies in direct contact

non-contact force a force acting between two bodies that are not in direct contact

magnetic poles (here) the north and south ends of a magnet

attraction force the force that attracts one object to another

unlike poles the north and south poles of a magnet

like poles two north poles or two south poles of a magnet

repulsion force a force that pushes one object away from another

Key ideas

• Contact forces involve two objects touching each other. • Non-contact forces occur when one object is able to push or pull another without touching the other. • Magnetism is an example of a non-contact force.

All forces occur between two or more objects. This is called an attraction force (Figure 2). These objects can be living or non-living. The two unlike poles (a north and a south) attract each other. Magicians use this attraction Contact forces force to slide something along a table. You can do this too. Place one magnet on top of a thin Some forces make objects move because of a table and a second magnet under the table. Can direct push or pull. For example, if you push a you make the top magnet move? Can you see pencil with your fi nger the pencil moves. Your the pull force? Are the two magnets contacting fi nger has to touch the pencil or be in contact each other? before the pencil will move. This is called a contact force . When two like poles (two north poles or two south poles) are placed together, they push Non-contact forces each other apart. You can use one magnet to push another magnet along a table. This is Some forces cause movement without touching. called a repulsion force (Figure 3). The two These are called non-contact forces . An magnets do not need to touch to be affected by example of this is the force of attraction the repulsion force. It is a non-contact force. between a magnet and a metal paperclip (Figure 1). When a magnet is held near a metal paperclip, the paperclip is pulled towards the magnet. There is no touching, or contact. How magnets push and pull Magnets are made of an alloy (a mixture of metals) that is mostly iron. The bar magnets used most commonly in schools are usually made of the alloy alnico, which is iron mixed with aluminium, nickel and cobalt. New, strong magnets are made from metals known as ‘rare earth’ metals. These are much stronger than normal magnets and do not lose their magnetism. One end of a magnet is labelled ‘N’ for north and the other end ‘S’ for south. If you hang a bar magnet from its centre by a piece of string, the north end will swing to point north. The magnet is said to have two magnetic poles – north and south. When the north pole of one magnet is placed near the south pole of another magnet, the two magnets are pulled to each other.

Figure 1 The attraction between the paperclip and the magnet is a non-contact force. DRAFT Figure 2 The north pole of one magnet pole of one magnet is pulled by the south pole of another magnet. Figure 3 Like Like poles push poles push away from/repel each other.

What causes a magnetic force?

An iron needle can be made into a magnet by sliding a strong magnet along one side of it (in one direction only). The strong magnet pulls tiny groups of particles so that they all line up in one direction. Each time you stroke the needle, these particles line up. This causes larger sections of the magnet called domains to point in the same direction. When most of the domains are pointing the same way, they can pull or attract a metal pin. Dropping the needle can cause the domains to become mixed up again.

Figure 5 When a magnet is broken, it forms two magnets.

Strongly magnetic

Non-magnetic

Weakly magnetic

One magnetic domain S N Figure 4 The magnetic domain theory 4.3 Check your learning Retrieve 1 Name three places where you might fi nd a magnet.

Some magnets never lose their magnetic force. These magnets are called permanent magnets. The domains in these magnets are often arranged while the metal is still buried deep under the ground. Breaking these magnets in half does not change the arrangement of the domains. The two halves become smaller magnets with the same pull or push forces as the larger magnet (Figure 5). The push forces of magnets are used in the design of Maglev (magnetic levitation) trains (Figure 6). A series of electronic magnets on the train and track suspend the train above the tracks. The magnets on the train and the track have like poles, causing them to push away from each other and for the train to sit above the track. There is no contact between the train and the metal track. To make the train move, the driver changes the pole of the train magnet, and the track magnet pushes the train magnet forward. DRAFT

domain a small section of a magnet where the magnetic fi eld of all the atoms is aligned in the same direction

Figure 6 Repulsive magnetic forces cause this Maglev train to move.

Comprehend

2 Identify a magnetic force as either a contact or a noncontact force. Explain your answer. 3 Explain why one part of a magnet is called the north. 4 Describe what will happen when the following poles of two magnets are pushed close together. a N and S c S and S b N and N d S and N

Apply

5 Draw how you might arrange bar magnets to create the letters of your name. Label the north and south poles of the magnets. 6 Propose how you might levitate a magnetic skateboard above a large magnet on the ground. Mention the arrangement of the poles of the magnet in your description.

Figure 7 Spelling out your name using magnets

Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level.