## 10. Force and Types of Force

1. Will a boat move ahead unless pushed with the help of an oar ?
2. Who exerts force to move a bullock-cart forward?
3. What makes the ball move during a game of football ? How does its direction change?
4. Will the wheel go forward if it is not pushed with a stick?

When we kick the football coming in our direction, towards the goal, we apply force. In day-to-day life, we do many actions such as lifting, pulling, riding a bicycle and stopping it at times, pushing a load, squeezing or bending something and driving vehicles. It is necessary to apply force for doing all these actions. Force is applied to an object to pull it or push it in any manner.

Types of force

1. Muscular force

In all the actions shown in the picture 10.1, the movements take place with the help of the bones and muscles in the body. The person in the picture alongside is lifting the weight by using muscular force.

1. Mechanical force

We use different machines for doing many tasks. Muscular force is used for running some machines. Some machines are run by using electricity or fuel. Machines like the latter are called ‘automatic machines’, because a mechanical force is used here. For example, sewing machine, electric pump, washing machine, mixer, etc. Make a list of other such machines.

1. Gravitational force

If an object is thrown upward by applying force, it reaches a certain height and comes down again. Why is this so? Why do fruits on trees fall to the ground? The earth pulls all the objects towards itself.

In the past …

Sir Isaac Newton discovered gravitation in the 17th century. The earth’s gravitational force acts in a direction opposite to that of an object moving upwards. Hence, the speed of that object goes on decreasing till in the end it becomes zero. Then the object starts falling down instead of going up any further. While falling, its speed goes on increasing all the time due to gravitational force.

1. Take a small stone and a bucketful of water. Drop the stone into the water from a height of about 20 cm. You will hear the sound of the stone dropping in the water. Now drop the same stone into the water from a height of about 100 cm. You will hear its sound again. What is the difference in the sounds in the two actions above ? What can we conclude from this ?
2. The actions of lifting a sack are shown in the picture. One sack looks empty, the other one is full. What difference do you notice in the way the two sacks are lifted? The gravitational force on the empty sack is less, that is to say, it weighs less. The gravitational force on the bigger sack is greater. In other words the bag weighs more.

The gravitational force operates between the sun and the planets in the solar system. Because of this, the planets revolve around the sun. At the same time, gravity also operates between planets and their satellites. Then, why don’t all the planets and satellites fall towards the sun ?

1. Magnetic force

Place a magnet on a table. Take a big iron nail towards the magnet. It sticks to the magnet. Now hold the magnet upright in the air, away from the nail. What happens ?

1. Frictional force

When a carrom piece is flicked lightly, it slides over the carrom board, but stops at a certain distance. A ball rolling over a flat ground stops at a certain distance.

Why does this happen ? When two surfaces rub against each other, the force of friction comes into play. It always acts against the direction of motion.

When a brake is applied while riding a bicycle, it stops after running a short distance. How does the brake work? Between which parts is friction produced?

Take two pieces of smooth paper and two of sandpaper and rub them against each other. What do you notice ?

The smooth surfaces can be easily rubbed against each other because the force of friction between them is less, while rough surfaces cannot be easily rubbed against each other because the force of friction between them is much greater. It is possible for us to walk on the ground only because of the force of friction. If there is no friction, we would slip and fall. For example, we are very likely to slip while walking on an oily or wet floor. Why is a wooden plank laid down for pulling out a car which is stuck in mud?

A force of friction is generated between the wheel and the wooden plank laid down under the car. Hence, it can be pulled out from the mud. In short, the force of friction can either be decreased or increased as desired.

1. Electrostatic force
1.Spread small pieces of paper on a table. Rub a piece of thermocol or an inflated balloon against silk cloth and bring it near these pieces.
2. Rub a plastic comb against dry hair and repeat the above activity.
3. Rub a peacock feather between two pages of a notebook and bring it near your fingers. What do you see ? In the above activities, the pieces of paper, hair, and the peacock feather are seen to move. Why does this happen?

Combined forces : While an action is taking place, various types of forces act on the object in question. You might have seen a roller coaster or the juggling of a sail board on the seashore. A variety of forces act together in these cases. To obtain more information about these, type the words ‘Trick Science’ in Google search on the Internet.