NON CONTACT FORCES
A force which can be exerted by an object on object on another object even from a distance (without touching each other) is called a non contact force. There is no physical contact between the object which exerts the force and the object on which force is exerted. The examples of non contact forces are :
1. Magnetic force,
2. Electrostatic force, and
3. Gravitational force.
We will now describe the magnetic force, electrostatic force and gravitational force, one by one. Let us start with magnetic force.
A magnet attracts things made of iron (or steel). So, if we bring a magnet near iron nails or pins, the magnet pulls them towards it. Since the iron nails and pins move towards the magnet, it mean that the magnet exerts a force on them. The force exerted by a magnet is called magnetic force. The magnetic force acts even from a distance. The magnet exerts a magnetic force on objects made of copper, wood or glass. When we bring a magnet slowly near some iron pins, then at a certain small distance, the iron pins suddenly jump towards the magnet and stick to it (even when the magnet has not touched them). Since a magnet can exert its magnetic force of iron objects from a distance (even without touching them), therefore, magnetic force is a non contact force. The magnetic force between a magnet and an iron (or steel) object is always that of attraction.
A magnet also exerts a magnetic force on another magnet placed near it. The magnetic force between two magnets placed near on another can be that of ‘attraction’ or repulsion’ depending upon which poles of the two magnets are facing one another. We will now describe the magnetic force between two magnets kept near each other. Before we do that please note that a north pole of magnet and another north pole are like poles. On the other hand, a north pole and a south pole are unlike poles.
We have already studied in Class VI that the like poles of two magnets repel each other whereas unlike poles of magnets attract each other. This means that there is a magnetic force of repulsion between the like poles of two magnets. On the other hand, there is a magnetic force of attraction between the unlike poles of two magnets. We do not have to bring the two magnets in contact with each other for observing the magnetic force between them. A magnet can exert a force on another magnet even from a distance, without touching the other magnet (or without being in contact with the other magnet). So, the magnetic force exerted by one magnet on the other magnet is an example of a non contact force. We will now perform an activity to show the magnetic force of attraction as well as repulsion between two bar magnets.
Take two similar bar magnets. Place one bar magnet over three wooden rollers (such as three round pencils) kept on a table as shown. The south pole of bar magnet kept on rollers is on the left side. We hold the other bar magnet in our hand and bring its north pole gradually near the south pole of bar magnet placed on rollers (making sure that two magnets do not touch each other).