Credit: By Roger Price from Hong Kong, Hong Kong [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

## What is a force?

A force is a push or a pull. A force cannot be seen but its effects can be seen or felt. A push is a force applied in a direction away from our body and a pull is a force applied in a direction towards our body.

### Effects of force and motion

When a force is applied to a object, it can:

• start the motion of a stationary (not moving) object
• stop the motion of a moving object
• increase the speed of the moving object
• decrease the speed of the moving object
• change the direction of the moving object
• change the size of an object
• change the shape of an object

•  a larger force will cause an object to move a greater distance
• a larger force will also cause an object to move faster
• if equal forces are applied from opposite direction, they will cancel each other out
• a cup placed on a desk is being pulled down by gravity but does not move because it is also being pushed up by the table.
• if forces are applied from opposite directions, the object will move in the direction of the force which is greater
• if opposite sides exert a pull on an object, the object will be pulled towards the side with a greater force
• if opposite sides exert a push on an object, the object will be pushed away from the side with the greater force

### Example of  forces in action: Tug-of-war

A tug-of-war shows the effects of push and pull in action. When both sides of a tug of war pull with equal amounts of forces, both sides will not move. However, when one side pulls with a greater force, the other side will be pulled towards them.

## Types of forces

In our daily lives, there are many examples of forces at work. Some of them are friction, magnetic force, gravitational force (gravity) or elastic spring force.

### Friction

Friction is a force that results from the surface of two objects rubbing against each other. Objects that have a rougher surface produce more friction than objects with a smoother surface.

Examples of friction:

• When you write, there is friction between the pen or pencil and your paper
• When you go for a walk outside, there is friction between the soles of your shoe and the ground
• When you rub your hands together, there is friction between the palms of of your hands
• When you hold a cup, friction exists between the cup and your hand
• When a car is moving on road, friction exists between its tyres and the road

What does friction do?

• Friction produces heat
• Friction causes a moving object to slow down
• Friction acts in the direction opposite to the direction of the motion
• Friction causes wear and tear

How can friction be reduced?

• Friction can be reduced by lubrication. For example using oil and grease between objects
• It can also be reduced by using ball bearings and rollers
• Friction can also be reduced by using streamlined shapes that reduce air or water resistance (friction in air or water)

### Credit: By Oguraclutch (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia CommonsMagnetic force

Magnetic force is the pulling force of a magnet on a object made from magnetic materials such as an iron nail or a paper clip. It can also refer to the pushing force of like poles of magnets on each other.

• All magnets have a north pole and a south pole
• The like poles of magnets repel and unlike poles attract
• Magnetic force acts on magnetic materials such as iron, steel, nickel, and cobalt.
• Magnetic force does not act on non-magnetic materials such as copper, gold, plastic, wood, and rubber.

### Gravitational force

Gravitational force or gravity refers to the pull that exists between all matter. A greater gravitational force exists between objects that have a larger mass. A greater gravitational force also exists between objects that are closer.

All objects on Earth are pulled towards the Earth by its gravitational force. Without gravity, we would not fall back down when we jump and instead float into space.

Weight and gravitational force

• The weight of an object is the amount of gravitational force that is acting on it
• Objects that have a larger mass have a greater weight than objects with a smaller mass
• Objects on Earth have weight because gravity is acting on them
• Mass and weight are not the same thing -- mass is the amount of matter in an object

Gravity, the moon, and other plants

The amount of gravity or gravitational force on the moon and on different planets are different than that on the Earth. Because of this, if we go to the moon (or any other planet), our weight will become different. Here's what you'll weigh on the moon and different planets if you were 110 pounds or 50 kg on Earth:

• 18.2 pounds or 8.3kg on The Moon
• 41.5 pounds or 18.9kg on Mercury
• 99.7 pounds or 45.3kg on Venus
• 41.4 pounds or 18.8kg on Mars
• 260 pounds or 118.2kg on Jupiter
• 117 pounds or 53.2kg on Saturn
• 97.7 pounds or 44.4kg on Uranus
• 123.7 pounds or 56.2kg on Neptune

These weights were calculated using the page Your Weight on Other Worlds[1].

From the weights above, can you guess which planet has the greatest mass?

### Elastic spring force

Elastic spring force is the pushing or pulling force created when a spring is compressed or stretched.

Springs are elastic and can change shape when a force is acted on them. When the force is removed, they return to their original shape.

• A spring is stretched (becomes longer) when a pulling force is acting on it. For example, when a weight is hung on a spring
• A spring is compressed (becomes shorter) when a pushing force is acting on it. For example, when a weight is pressing down on a spring

Uses of elastic spring force

• They can be used in spring balances to measure the weights of objects
• Springs are used in beds and mattresses to help keep their shapes