1. FORCES AND MOTION

introduction

Forces and motion are surrounding our world. Forces are either pushing or pulling forces. Types of forces can be gravity, friction, upthrust, magnetic force, surface tension, electric force etc.

What are forces and motion?

Thanks to motion the world goes around the Sun. So do the other planets in the solar system. Animals and vehicles move form one place to another because of motion. We play, run, dance, jump, do sports everyday. All these activities require motion. We cannot think about a world without motion. In fact, motion is the changing of position or location. However, to change the position or location, motion needs another thing. It is a force. A force should be there to cause a change in a position and we call it ‘motion’. Thus, forces and motion go hand in hand around us.

what is force?

A force is a push or pull. A force makes something move.

Look at the following forces and motion diagrams which show pushing and pulling forces:

Examples of forces – push and pull

You often use a trolley when you do shopping. You can use a pushing force to move a trolley forward.

The following forces and motion diagram shows how a pushing force applies.

how do you play football?

You have to kick the ball when you play football. Here, you use the pushing force to move the ball forward with your foot.

The harder you kick the ball, the faster it will move. How fast the ball goes depends on the strength of the kick.

The following forces and motion diagram shows the harder you kick the ball, the faster it will move:

How do forces change direction of things?

A force can change the direction of something that is moving.

A tennis player puts a force on the moving ball, which is travelling towards him, by hitting it with the racquet. The ball changes its direction by moving away from the racquet in a different direction.

Similarly, a cricketer or baseball player hits a ball that is moving towards them. The ball then changes direction and moves away from them. Which way the ball goes depends on the direction of the hit.

Using an arrow you can show which way a force is moving and how strong it is.

The arrows in the following forces and motion diagram shows how the ball changes direction and moves away from the batter:

What are the different types of forces?

Following is a list of examples of forces which act on or around us:

Force of friction
Gravity
Upthrust
Magnetic force
Surface tension
Electric force

Force of friction

Things which are moving will slow down because of the force of friction. Whenever two surfaces rub on each other a force of friction is produced. Friction always works in the direction opposite from the direction the object is moving or trying to move.

The following forces and motion diagram shows how friction works in the opposite direction:

2. Gravity

Gravity pulls everything downwards. This means gravity pulls everything towards the centre of the earth. Everything on Earth, and on other planets, is affected by the force of gravity.

3. Upthrust or Buoyant

A rubber or plastic ball floats on water. A force called upthrust keeps the ball up. This force is produced by water pushing on the ball from underneath.

Gases and liquids can both supply upthrust. When you release a hydrogen balloon it rises, because of the force upthrust.

4. Magnetic force

Things that have iron, steel, cobalt, or nickel in them are magnetic, because they are attracted to the magnet by a magnetic force.

5. Surface tension

A raindrop keeps its shape when it lands on a waterproof surface. That’s because of surface tension. The water molecules attract each other strongly and form a kind of skin on the water surface. It is also the high surface tension of water which allows insects to walk over it.

6. Electric force

Have you ever tried picking up pieces of paper with a plastic pen?

To do this first you have to rub the plastic pen on a duster. Rubbing gives the plastic an electric charge. This electric force pulls the paper to the pen. We also call this electric force static electricity.