A push or a pull on an object is called a force. A force causes motion. For example, we lift, push or pull a book lying on a table, kick a football, hit a cricket ball with a bit, throw a stone, stretch a rubber band and compress a spring. In all these actions, we make bodies move by pushing or pulling them. We can also stop a moving body or change the direction of its motion by applying a force to the body.
Definition of Force:
A push or a pull which produces or tends to produce a change in the motion of a body is called force. The direction in which the body is pushed or pulled is called the direction of force. Force has both magnitude and direction.
Effects of Force:
- Force can cause motion in a stationary object. It can cause displacement of a body. Example: Lifting a book from a table causes motion in the book. Kicking a stationary football causes motion.
- Force can change the speed of a moving object. It can make a moving object stop. Example: kicking a moving football can increase or decrease its speed. The goalkeeper uses his muscular force to stop the moving football.
- Force can change the direction of motion of a moving object. Example: Smoke from a burning agarbatti rises upwards. It changes its direction if air is blown on it. Blowing is a kind of force (push).
- Force can change the shape and size of an object. Example: shape or size of a spring changes on pulling or pushing it. Pulling increases its length while pushing decreases its length.
The shape of a sponge changes on pressing. Similarly the shape of a balloon changes on pushing (pressing) or pulling.
Types of Forces:
1. Muscular Force
We can push or pull an object by our muscles. The force applied by the muscles of our body is called muscular force. Animals exert muscular force to do work. When we kick a football, we apply our muscular force.
2. Gravitational Force
When any object is released from a height, it falls to the ground. While it is falling there is motion in the object which means that there must be force acting on the moving object. This force is due to the attraction between the Earth and the object. This force is called the force of gravity. This is a force acting at a distance. The Earth exerts the force of gravity on all objects. Just as the Earth exerts force on objects, the other objects also exert force on each other. The force of attraction exerted by an object on another object is called gravitational force (pull).
The measure of the gravitational pull of the Earth on an object is the weight of that object. Weight of a body is a force (pull) which always acts towards the center of the Earth.
Mass of an object is the quantity of matter contained in it. Therefore, a bigger object has a greater mass than a smaller one.
Measurement of weight of an object: The weight of an object is measured with a spring balance, also called a spring scale. A spring scale measures the force of gravity. A small object is suspended from the spring scale. The reading of the pointer on the scale indicates the weight of the object. Due to its weight the suspended object extends the length of the spring. If we add another equal weight, the total stretch in the length of the spring will double. In other words, we can conclude that an equal mass added to the spring causes it to stretch by equal amounts. The change in the length of the spring helps us to measure the weight of an object.
3. Magnetic Force
When a magnet is brought near iron nails, they are pulled towards it even if they are lying at a distance. The force exerted by the magnet on the nails is called magnetic force.
Magnets are generally made of iron. A magnet has a North Pole (N) and a South Pole (S). If we bring the similar poles of two magnets close to each other, they repel (or move away) from each other. Opposite poles of the two magnets attract each other. Each magnet exerts a force on the other from a distance.
4. Electrostatic Force
This force is similar to the magnetic force. Magnetic force is due to N and S poles whereas electrostatic force is due to positive and negatives changes. In other to understand electrostatic force, let us perform a simple experiment.
Take a sheet of paper and tear it into very small pieces. Place them over a table. Take your plastic comb and rub it into your dry hair several times. Bring the comb near the small pieces of paper. The pieces move from the table towards the comb even from a distance. The above experiment can also be performed with a plastic ball-point pen instead of a comb. The hair must be dry.
The movement of pieces of paper is caused due to electrostatic force.
This force is created due to the collection of electrical changes on the comb or ball-point pen. These changes remain stationary at the site of rubbing. Electro means electricity (charge); static means stationary. The force exerted by electrostatic charge is called electrostatic force. The electric charges are produced only by rubbing a particular pair of objects on against the other.
5. Frictional Force
Suppose a ball is rolling down an inclined plane. It stops after covering a distance on the ground. If the ground is smooth, the distance covered by the rolling ball would be more before it stops that if the ground is rough. We know that force is required to stop a moving body. If means that there must be force acting on the moving ball this opposes its motion. This force which opposes the motion of the moving body is called frictional force or friction. This force acts between the two surfaces which are in contact. It depends on the smoothness of the surfaces in contact. The two surfaces in contact are the surface of the ball and the smooth ground. The two surfaces are the surface of the ball and the sand. The frictional force is greater in the second case than in the first.
Methods to increase friction:
In some cases greater friction between two surfaces is required. This can be achieved by the following methods.
- If the friction between the tyres of automobiles and road is less, they will slip. To increase friction, grooves are made in the tyres.
- The friction between the sole of a shoe and the ground is increased by providing spikes. This is done to minimize chances of slipping by athletes.
Methods to reduce friction:
Sometimes there is a need to reduce the friction between two surfaces. Some ways by which friction can be reduced are:
- Polishing and smoothening of rough surfaces reduces friction.
- Due to friction there is wear and tear o the moving arts of machines. Also, greater friction decreases the speed of moving parts of machines and automobiles. It also damages them because the moving parts rub against each other and cause undesirable heat. To reduce friction, these moving parts must by lubricate with a suitable oil or grease.
- Wheel ball-bearings and roller bearings are sued in machines to reduce friction between the moving parts.
It is difficult to slide a suitcase on the floor whereas a suitcase with wheels can be moved easily. This is because wheels reduce friction, and rolling friction is must less than sliding friction.
Unit of Force:
The force acting perpendicular to a surface or thrust can be expressed in S.I. units in kilogram force (kgf) or newtons. There is a relationship between these two units, i.e.
1 kgf =9.8 newtons