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Forces and Motion


Forces

Our world is surrounded by forces. All these are either pushing or pulling forces. Not only the force of gravity pulls everything on earth towards the centre of the earth, but also it keeps our planet Earth in orbit around the Sun.

Forces and Motion

 
Thanks to friction we can walk and drive without slipping or skidding. Things which are light in weight can float on water, because of upthrust. Magnetic force, surface tension and  electric force are some other types of forces in our environment. So now, let’s learn about these forces in detail.
 

What is a force?

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

 

examples of pushing force

Examples of pushing force

 

Examples of pulling force

Examples of pulling force

 

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.
Examples of pushing force

Examples of pushing force

 

  • If you want to pluck some flowers off a high branch, you can use a long stick with a hook at one end to lower the branch and pluck the flowers. In this case you use a pulling force to lower the branch with the help of the stick.

 

  • In the game of tug of war, you use a pulling force to show the power of two teams by pulling the weak team towards the strong team. If both teams pull with the same force there is no movement.
Examples of pulling force

Examples of pulling force

 

  • If you want to move the cursor on the computer screen, you should move the mouse of the computer by pushing or pulling it. The cursor moves on the screen in the same direction as the force given to the computer mouse.

 

Forces can make things change shape, change direction, change speed.

 

How do forces change shape of things?

 

To stretch a chest expander or squash a can out of shape, a big enough force produced by a strong man is needed.

A strong force can change shape

A strong force can change shape of something

 

How do forces change speed of things? (Speeding up and slowing down)

 

A car begins to move using the force produced by the car engine and gradually it speeds up. When you apply the brakes you can slow it down, and then stop it. This means the force produced by the brakes has slowed down the car’s wheels.

Examples of forces - Applying brakes

 

You have to pedal a bicycle harder by using more force, if you want to go faster.

Examples of forces - riding a bicycle

 

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.

Examples of forces - kicking a ball

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.

 

Examples of forces - Changing 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.

 

Examples of force - Changing direction

 

A twist is a force that makes something move in a circle. Twisting the lid of a jar or bottle opens the jar. Twisting it the opposite way closes it.

 

Examples of forces - Twisting

Twisting the lid of a bottle opens the bottle

 

Forces in balance

 

Sometimes more than one force acts on an object at the same time. When people push or pull in the same direction, their forces add together to give a much bigger force.

 

For example;

Two forces can help each other. It’s difficult to move a car by pushing it yourself. It’s easier if a friend helps you to push in the same direction.

 

Examples of forces - Pushing a car

It’s easier to push a car in the same direction when two or more people push it.

 

But if two forces are acting against each other, the largest force will win.

 

For example;

 

Two forces can work against each other. That’s what makes a tug of war so tiring. Your team pulls one way. The other team pulls in the opposite direction. If one team’s force exceeds the other team’s force, the team with the highest force can pull the other team towards them and win the game.

 

There is no movement in either direction as long as both teams pull with the same force. This is where two forces are equal. When this happens, the two forces balance each other.

Examples of forces - Pulling force

There is no movement in either direction as long as both teams pull with the same force.

 

The floating toy duck

When you put a toy duck made of rubber or plastic into water, it will float on water.

 

So, how this toy duck floats on water?

 

The force of the water pushing the toy duck up is equal to the force of gravity pulling it down.

Examples of forces - upthrust

A toy duck made of rubber or plastic float in water

 

Types of forces around us

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.

 

Examples of forces - Friction

Friction always works in the opposite direction

 

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.

 

Examples of forces - Gravity

 

Upthrust

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.

 

Examples of forces - Upthrust

 

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

 

Examples of forces - Upthrust

 

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.

 

Examples of forces - Magnetic force

Magnetic things attract to the magnet by a magnetic force

 

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.

 

You can even float a pin on water. If you lay a pin carefully on the surface of water; surface tension keeps it up.

 

Examples of forces - Surface Tension

A raindrop keeps its shape when it lands on a waterproof surface.

 

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.

 

Examples of forces - Electric force

Rubbing gives the plastic an electric charge. This electric force pulls the paper to the pen.