Exploring Forces
Class 08 ScienceIn everyday life, we come across many situations where a force is applied, for example, opening a drawer, stretching a rubber band, a fielder stopping a ball, kicking a football, applying brakes on a moving bicycle, rolling a chapati, or turning the steering handle of an auto rickshaw.
The force applied on an object may:
- make an object move from rest.
- change the speed of an object if it is moving.
- change the direction of motion of an object.
- bring about a change in the shape of an object.
- cause some or all of these effects.
At least two objects must interact for a force to come into play.
Force
A force is a push or pull on an object resulting from the object’s interaction with another object. The SI unit of force is newton and its symbol is N.
Different Types of Forces
Contact Forces
Forces which act only when there is physical contact between the objects are called contact forces.
An example of contact force is muscular force. When we perform any physical activity, such as walking, running, lifting, pushing, jumping, or stretching, the force is caused by the action of muscles in our body. The force resulting due to the action of muscles is known as muscular force. Muscular force occurs when muscles contract and elongate while doing any activity.
Animals, birds, fish, and insects use muscular forces for movement and survival. Humans used the muscular force of some animals to carry out many tasks for a long time.
Friction
A ball rolling on a flat ground stops on its own after some time. If we stop pedalling our bicycle on a flat road, it slows down and comes to a stop. If the road is rough, it stops sooner than on a smoother road.
The force that comes into play when an object moves or tries to move over another surface is called the force of friction. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move. The force of friction is a contact force since it arises due to two surfaces in contact.
Friction arises due to the irregularities in the two surfaces in contact. Even surfaces which appear smooth, have a large number of minute irregularities. When placed in contact, the irregularities of two surfaces lock into each other and oppose any effort to move one surface over the other.
Non-contact Forces
There are forces whose effect can be experienced even if the objects are not in contact. These forces are called non-contact forces.
Magnetic Force
A magnet attracts objects made of magnetic materials. When two magnets are brought close to each other, like poles (North–North, South–South) repel each other while unlike poles (North–South) attract each other.
Attraction and repulsion between objects are also a form of push and pull, that is, a force. The force exerted by a magnet on another magnet or a magnetic material is called magnetic force. Since a magnet can exert a force from a distance without being in contact it is called a non-contact force.
Electrostatic Force
When two objects of certain materials are rubbed together, electrical charges build up on their surfaces. These charges are called static charges as they do not move by themselves. The object that acquires static charges is said to be a charged object. A charged object attracts, that is, exerts a force on uncharged objects made of certain materials, such as small pieces of paper. This force comes into play even when the bodies are not in contact.
The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force. It is a non-contact force.
Gravitational Force
All the objects fall towards the Earth. It means the Earth attracts (pulls) them. The force with which the Earth attracts objects towards itself is called the gravitational force. The gravitational force exerted by the Earth is also called force of gravity.
Since the gravitational force acts without contact with the object it attracts, it is a non-contact force. Gravitational force is always an attractive force, unlike magnetic force or electrostatic force, which can either be attractive or repulsive.
When an object is dropped from a height, it takes a straight vertical path downwards before touching the ground. When an object is thrown vertically upwards, the object moves up straight, slows down, stops momentarily at the top, and then takes a straight vertical path downwards.
While going up, the speed of the object goes on decreasing till the object comes to a stop, its direction of motion changes and while coming down the speed goes on increasing. The object undergoes a vertical motion when it moves in a vertical direction under the influence of the gravitational force.
Weight and Its Measurement
The force with which the Earth pulls an object towards itself is called the weight of the object. The weight measures how strongly an object is pulled by the Earth. Since the weight is a force, it is measured in the same unit as that of force. Therefore, SI unit of weight is also newton (N).
When an object is hung from a spring, the spring stretches due to the force applied on the object by the Earth. The stretch caused in the spring is different for different objects. This indicates that the Earth pulls different objects with different forces, that is, the weight of different objects is different.
Floating and Sinking
Water applies a force in the upward direction. All liquids apply a similar force. The force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.
When an object is placed in a liquid, the gravitational force due to the Earth acts on it downwards. But a buoyant force is applied on it by the liquid in the upward direction. If the gravitational force is more than the buoyant force, the object sinks, but if the two forces are equal, the object floats. One of the factors on which the buoyant force depends upon, is the density of the liquid.