Types of Forces - Force & Pressure | Science Class 8 PDF Download

KINDS OF FORCE

Contact Force:

Force that comes into action after the interaction between objects is called contact force. Contact force acts on the point of contact. For contact force, interaction between objects is necessary. Example: Pushing a car, opening a drawer, kicking a ball, etc. In these examples, interaction between objects is necessary.

Pushing a CarMuscular force, force of friction, etc. are types of contact force.

Muscular Force:

Force caused by the action of muscles is called muscular force. In other words, force resulting because of action of muscle is called muscular force. Muscular force is applied only after interaction with the object. Hence it is a type of contact force.

Example:

While kicking a ball, the player applies force over the ball using his leg muscles.

While opening a drawer, you pull the drawer holding it by hand. In this action, muscles of your hand apply the force.

While pulling a cart, the horse or the ox applies the force of its muscles.

Friction:

Force acting between the surfaces of two objects is called the force of friction. Force of friction always acts in the opposite direction of the movement of object. Force of friction is acting over all the moving objects.

Example:

A moving football stops after going to a certain distance. This happens because of force of friction between the surface of ground and the surface of football.

A moving boat stops after some distance because of friction between the surface of water and the surface of boat.

Since force of friction comes into action only after interaction between two objects, thus, it is a type of contact force.

Non-contact Force:

Force that comes into action without interaction between two objects is called non-contact force. For example, A magnet can pull an iron nail from a distance.

Magnetic force, Electrostatic force and Gravitational force are examples of non-contact force.

Magnetic Force:

Force exerted by a magnet on another magnet or on magnetic substances is called magnetic force. A magnet can exert force even without coming in contact, thus it is a non-contact force.

Example: A magnet attracts the opposite pole of another magnet and repels the similar pole of another magnet.

Electrostatic Force:

Force exerted by a charged body is called electrostatic force. A charged body attracts an uncharged body. A positively charged body attracts a negatively charged body and repels a positively charged body without coming in contact, thus it is a non-contact force.

Gravitational Force:

Force exerted by earth, moon, sun and other planets is called gravitational force. Earth attracts all objects towards it. Similarly, all other planets along with moon attract all objects towards them. Since, earth attracts all objects even without coming in contact, thus gravitational force is a non-contact force.

Example:

When anything is released from a height, it falls over the ground because of gravitational pull of earth.

Pressure:

A force applied over the unit area of a surface is called pressure. In other words, force per unit area is called pressure.

Pressure = Force / Area

In this case, area is indirectly proportional to the pressure, thus pressure decreases with increase in area and increases with decrease in area.

Example:

1. While cutting an apple, we need to use the sharp edge of the knife. Using the blunt edge of knife shall not serve the purpose. The blunt edge of knife has larger surface area than the sharp edge. Because of smaller surface area, more pressure can be applied through the sharp edge of the knife and something can be easily cut.

2. While putting a nail into a wooden board, the pointed end of the nail is kept at the front. The pointed end of the nail has very small surface area and this enables us to apply a greater pressure with the applied force.

3. School bags have broad shoulder straps. Because of broad shoulder straps, the pressure because of the weight of the school bag is distributed over a larger area and it becomes easier to carry the bag.

Pressure exerted by Liquid and Gas

Similar to solid, liquid and gas also exert pressure. Liquid and gas exert pressure on inner walls of the container in which they are kept.

Example:

Water starts leaking if there is a pore in the bottle. This happens because water exerts pressure over the walls of the bottle.

Water flows from higher level to lower level of the ground. This happens because of pressure exerted by water.

When air is filled, a rubber balloon gets inflated from all sides. This happens because air, which is a gas, exerts pressure over inner walls of the balloon.

Atmospheric Pressure:

Our atmosphere is made of air. Since air is mixture of gases and gas exerts pressure, thus air exerts pressure. Pressure exerted by air (present in atmosphere) is called atmospheric pressure.

Atmosphere of 10cm X 10cm X 10cm exerts about 1000 kg of pressure.

We do not feel the atmospheric pressure over us because the pressure inside our body nullifies the atmospheric pressure.

Example:

1. A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure.

2. When you go to hills and mountains, it is difficult to breathe. This happens because of low atmospheric pressure.

3. The ink of a fountain pen spills out in aeroplane, because of low pressure at higher altitude.

4. At higher altitude, lentils (pulses) take longer to get cooked. This happens because of low atmospheric pressure at higher altitude. Because of low atmospheric pressure, water boils at a lower temperature and that temperature is not enough for cooking the pulses.

Important terms

Force: A pull or push over an object is called force.

Magnitude of Force: Strength or amount of force is expressed in terms of magnitude.

Contact Force: Force that comes into action after the interaction between objects is called contact force.

Muscular Force: Force caused by the action of muscles is called muscular force.

Friction: Force acting between the surfaces of two objects is called force of friction.

Non-contact Force: Force that comes into action without interaction between two objects is called non-contact force.

Magnetic Force: Force exerted by a magnet on other magnet or on magnetic substances is called magnetic force.

Electrostatic Force: Force exerted by a charged body is called electrostatic force.

Gravitational Force: Force exerted by earth, moon, sun and other planets to pull an object towards them, is called gravitational force.

Pressure: A force applied over the unit area of a surface is called pressure.

Atmospheric Pressure: Pressure exerted by air present in atmosphere is called atmospheric pressure.