Contents
Studying Physics Topics can lead to exciting new discoveries and technological advancements.
Types of forces with examples and Picture
There are five common types of forces which we notice in our everyday life. These are :
- Muscular force,
- Frictional force (or Friction),
- Magnetic force,
- Electrostatic force, and
- Gravitational force.
All type of forces can be divided into two main groups : Contact forces and Non-contact forces. Please note that the state of one object ‘physically touching’ another object is known as ‘contact’. So, when one object physically touches another object, they are said to be ‘in contact’. A force can act on an object with or without being in contact with it. We will now describe contact forces and non-contact forces in detail, one by one.
Contact Forces
A force which can be exerted by an object on another object only through ‘physical touching’ is called a contact force. The examples of contact forces are :
- Muscular force, and
- Frictional force (or Friction).
Let us now discuss muscular force and frictional force in detail, one by one.
1. Muscular Force
Suppose a book is lying on a table. To lift this book from the table, some force is required. When we lift this book from the table by hand, the force is exerted by the muscles of our arm. Similarly, when we kick a football, the force is exerted by the muscles of our leg. The force exerted by the muscles of the body is called muscular force.
Both, human beings and animals exert muscular force to do work. The human beings exert muscular force for performing the various day to day activities like walking, running, jumping, climbing, lifting, pushing, pulling, kicking, stretching, squeezing, twisting and pressing the objects. A boy pushing a cart on a road, a porter carrying a load on a wheel-barrow, a child riding a bicycle and a person drawing water from a well are all examples of the use of muscular force. It is the muscular force which enables us to perform all the activities involving the movement or bending of our body parts.
The muscular force also acts inside our body to perform various functions required for our survival. For example, in the process of digestion, it is the muscular force which pushes the food through the alimentary canal. The expansion and contraction of lungs when we inhale and exhale air during breathing, also involves muscular force. And the beating of heart also takes place by the muscular force produced by cardiac muscles.
The animals like an ox, camel and horse provide the force of their muscles and help the man in doing harder jobs. For example, ox pulls the plough by using its muscular force. Similarly, a horse and camel exert their muscular force in pulling the carts. An elephant pulls heavy logs of wood by using the muscular force. And in arctic regions (snow-bound regions), reindeers pull the sledges (wheel less vehicles) by using their muscular force.
Since muscular force can be applied to an object only when our body (or body of an animal) is in contact with the object, therefore, muscular force is a contact force.
For example, when we lift a book from a table by applying muscular force, then our hand is in contact with the book. We cannot lift the book from table without touching it. Our contact (or that of an animal) with an object for applying muscular force can also be through a stick or a piece of rope, etc. For example, when we hit a hockey ball, we apply muscular force to ball through the hockey stick. And when we draw a bucket of water from a well, then we apply the muscular force through a rope (tied to the bucket).
2. Frictional Force (or Friction)
A ball moving on the ground slows down gradually and stops after covering some distance. We know that a force is required to stop a moving body. This means that a force is exerted by the ground on the moving ball which opposes its motion and brings it to a stop. This force which opposes the motion of ball on the ground is known as frictional force. We can now define the frictional force as follows: The force which always opposes the motion of one body over another body is called frictional force (or friction). The frictional force acts between the two surfaces which are in contact with each other. A ball moving on the ground slows down and then stops due to frictional force between the ball and the ground. In this case the two surfaces in contact are the surface of the ground and the surface of the ball. Frictional force is also known as ‘force of friction or just ‘friction’. We will now give some more examples from everyday life where frictional force is involved.
(i) If we stop pedalling a running bicycle, it slows down gradually and stops after covering some distance. The bicycle moving on the road slows down and finally comes to a stop due to the frictional force between the tyres of the bicycle and the road. This frictional force opposes the motion of bicycle and brings it to a stop. In this case, the two surfaces in contact are the surface of the road and the surface of tyres of bicycle. Similarly, a car or scooter moving on the road comes to rest (or stops) due to frictional force when its engine is switched off.
(ii) When we stop rowing a boat which is moving in water, it slows down and then stops. The boat moving in water slows down and finally stops due to the frictional force between the boat and water. The frictional force exerted by water opposes the motion of boat and makes it stop. In this case the two surfaces in contact are the surface of boat and the surface of water.
The frictional force (or force of friction) always acts on all the moving objects, and its direction is always opposite to the direction of motion. Since frictional force arises only when the surfaces of two objects are in touch with each other, therefore, frictional force is an example of a contact force.
Please note that it is not necessary that the agent applying a force on an object should always be in contact with it. There are some forces which act on an object even from a distance. We will now discuss such non-contact forces.
Non-Contact Forces
A force which can be exerted by an 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 :
- Magnetic force,
- Electrostatic force, and
- Gravitational force.
We will now describe the magnetic force, electrostatic force and gravitational force, one by one. Let us start with the magnetic force.
1. 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 (see Figure). Since the iron nails and pins move towards the magnet, it means 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 iron, steel, nickel and cobalt. It does not exert a 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 on 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 one 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. Similarly, a south pole and another south 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.
Activity 1
Take two similar bar magnets. Place one bar magnet over three wooden rollers (such as three round
pencils) kept on a table as shown in Figure. The south pole of bar magnet kept on rollers is on the
left side (see Figure 16). 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 the two magnets do not touch
each other).
We will observe that when the north pole of hand held magnet is brought near the south pole of other magnet, the magnet placed on rollers begins to move towards the hand held magnet) (see Figure). This shows that the north pole of hand held magnet is exerting a magnetic force of attraction on the south pole of magnet kept on rollers which makes it move towards the hand held magnet. Thus, unlike poles (N-pole and S-pole) of magnets exert a magnetic force of attraction.
Now bring the south pole of hand held magnet gradually near the south pole of magnet placed on rollers as shown in Figure. We will observe that when the south pole of hand held magnet is brought near the south pole of other magnet, the magnet placed on rollers begins to move
away from the hand held magnet (see Figure). This shows that the south pole of hand held magnet is exerting a magnetic force of repulsion on the south pole of magnet placed on rollers which makes :
it move away from the hand held magnet. Thus, like poles (S-pole and S-pole) of magnets exert a
magnetic force of repulsion.
The magnetic force is widely used in our everyday life. For example, many toys work due to the magnetic force exerted by magnets fixed inside them. The magnetic strips help to keep the refrigerator door closed tightly. Certain stickers stick to the iron objects like steel almirahs, refrigerators, etc., without any glue. Such stickers contain tiny magnets and hence stick to the iron objects due to magnetic force. The magnetic force exerted by powerful electromagnets is used to separate scrap iron objects from among a heap of waste materials.
Before we go further and study electrostatic force, we should know the meaning of the terms ‘electric charge’ electrostatic’ and ‘electrically charged object’. The electric charges are of two types : positive electric charges and negative electric charges. Electric charges can be produced by friction when one object is rubbed over another object. The term ‘electrostatic’ refers to ‘stationary electric charges’. An object having stationary electric charges (or static electric charges) on it is called electrically charged object.
A comb is said to have acquired electrostatic charge after it has been rubbed with dry hair. Such a comb is an example of a charged object. An electrically charged object exerts an electrostatic force. An electrostatic force is actually an electrical force. It is called electrostatic force because the electric charges involved in it remain static (or stationary). The electric charges remain confined to the charged object and do not move.
2. Electrostatic Force
The force exerted by an electrically charged object is called electrostatic force. An electrically charged object can exert an electrostatic force on an uncharged object (or another charged object). For example, a plastic comb (or plastic pen) which is electrically charged by rubbing in dry hair, exerts an electrostatic force on uncharged tiny pieces of paper and attracts them. Let us discuss this in detail.
We take a plastic comb (Or plastic pen) and rub it in our dry hair for about one minute [see Figure] Then bring this comb near the tiny pieces of paper. We will find that this comb attracts the tiny pieces of paper. The tiny pieces of paper move towards the comb and stick to it [see Figure]. This can be explained as follows: When the plastic comb is rubbed in dry hair, the comb gets electric charges by friction. And when this electrically charged comb is brought near the tiny pieces of paper, it exerts an electrostatic force of attraction on them. Due to this electrostatic force, the tiny pieces of paper move towards the comb and stick to it. Thus, the movement of tiny pieces of paper is due to the electrostatic force exerted by electric charges acquired by the comb on rubbing in hair. It is the electrostatic force which is responsible for raising our body hair when we try to take off a terylene or polyester shirt in dry weather. If an inflated balloon is rubbed with a piece of synthetic cloth (like polyester or nylon cloth) and then pressed against a wall, the
balloon gets electrostatic charges and hence gets electrically charged. The electrically charged balloon attracts the uncharged wall and sticks to it.
The electrostatic force can be exerted by a charged object on another object from a distance (even when they are not in touch with each other). So, electrostatic force is an example of a non-contact force. For example, an electrically charged plastic comb exerts electrostatic force of attraction on tiny pieces of paper even from a distance (without touching them), and attracts them. Just like magnetic force, an electrostatic force can be of attraction or repulsion depending on the situation. The electrostatic force between any charged object (positively charged object or negatively charged object) and an uncharged object is always of attraction. The electrostatic force between two charged objects having like charges (positive and positive or negative and negative) is that of repulsion. On the other hand, the electrostatic force between two charged objects having unlike charges (positive and negative) is of attraction. We will study this in detail later on. Electrostatic force is utilised in reducing air pollution by removing dust, soot and fly-ash particles from the smoke coming out of chimneys of factories.
3. Gravitational Force
Newton said that every object in this universe pulls every other object with a certain force. The pull exerted by objects possessing mass is called gravitational force. The gravitational force between two objects is a force of attraction and it acts even if the two objects are not connected by any means.
It is the gravitational force between the sun and the earth which holds the earth in its orbit around the sun. And it is the gravitational force between earth and the moon which holds the moon in its orbit around the earth.
The gravitational force between two ordinary objects having small masses (say, two stones) is very weak and cannot be detected easily. The gravitational pull (or force) becomes strong only if one of the objects has a huge mass, like the earth. Thus, the gravitational pull (or force) between the earth and a stone is very large which can be detected easily. This point will become clear from the following discussion.
If we drop a stone from some height, it falls down towards the earth. This is because the earth exerts a gravitational force of attraction on the stone and pulls it down. In fact, the earth pulls all the objects towards its centre.
The force with which the earth pulls the objects towards it, is called the force of gravity (or just gravity). Gravity is a natural force of attraction (or pull). We are all pulled to the earth by the force of gravity. For example, when a child falls from a tree, it is the force of gravity which brings the child down to earth. Similarly, when a diver dives into a swimming pool, it is the force of gravity which brings him down.
The force of gravity acts on all the objects. The force of gravity causes all the objects to fall towards the earth. For example, a leaf falls from the tree due to gravity. A fruit also falls down from a tree due to the force of gravity (see Figure). Rain falls down to earth due to the force of gravity. When we open a tap, water begins to flow down towards the ground due to the force of gravity. And water in rivers flows downwards also due to the force of gravity.
When a coin slips from our hand, it falls to the ground due to the force of gravity acting on it. Actually, when a coin is held in our hand, it is at rest (not in motion). As soon as the coin is released from our hand, it begins to move downwards and ultimately falls on the ground. This downward movement of coin is brought about by the gravitational force of earth (or force of gravity) which acts in the downward direction, towards the centre of earth. Similarly, when a pen slips from our hand, it falls to the ground because of the force of gravity acting on it (the pen does not go upwards!). And a ball thrown upwards also falls back to the earth due to the force of gravity (or gravitational force of earth). Please note that the gravitational force of earth (or gravity) acts on objects from a distance (without there being a physical contact), therefore, gravitational force or gravity is an example of non-contact forces.