# DAV Class 8 Science Chapter 4 Notes – Force and Pressure

These DAV Class 8 Science Notes and DAV Class 8 Science Chapter 4 Notes – Force and Pressure act as excellent revision resources, particularly in preparation for board exams.

## Force and Pressure Class 8 DAV Notes

→ Force: A push or a pull is called force.

→ Effects of Force:
1. Change in state of motion: Force can change the state of motion of an object. If an object is in the state of rest, it can be changed into state of motion by force. If an object is in the state of motion, it can be brought into state of rest by force. Force can also changes the direction of motion and rate of motion.

• If a force is applied on a moving body in the direction of motion, it increases the speed.
• If a force is applied on a moving body in the opposite direction of motion, it decreases the speed.
• If a force acts at an angle to the direction of motion, it changes the direction of motion.

2. Change in size/shape of object: Force can change the size or shape of an object. Example: Dough is changed into chapatti by application of force.

→Factors associated with the magnitude of force: Mass of the object and the speed acquired by it, determine the magnitude of force needed.
Force has both magnitude and direction.

→ Balanced and Unbalanced Forces: When the net of different forces acting on a body is zero; they are called balanced forces. Movement is not possible in case of balanced forces. When the net of different forces acting on a body is not zero; they are called unbalanced forces. Movement is possible in case of unbalanced forces.
Unbalanced forces acting in the same direction combine by addition.
F1 → F2 → = F1 + F2
Unbalanced forces acting in opposite directions combine by subtraction.
F1 → F2 ← = F2 – F1
Where F2 > F1

→ Type of Forces: Forces can be broadly classified into two types, viz. contact forces and non-contact forces.
1. Contact Forces: When a force can be applied only through physical contact with a body, it is called contact force. Following are some examples of contact forces:

→ Muscular Force: Force applied by the muscles of humans or animals is called muscular force. When a horse is pulling a cart, it is applying muscular force.

→ Frictional Force: When two surfaces are in motion in relation to each other a force acts between them and opposes the motion. This force is called force of friction or frictional force. The direction of force of friction is opposite to the direction of motion.

2. Non-contact Forces: When a force can be applied without physical contact with the body, it is called non-contact force. Following are some examples of non-contact forces.

→ Gravitational Force: We know that the earth pulls everything towards it. The force of attraction of earth is called gravitational force. Every object in the universe has gravitational force.

→ Magnetic Force: Magnets attract magnetic materials such as iron. The force exerted by a magnet is called magnetic force.

→ Electric Force: A charged body has the tendency to attract another lightweight body. The force by which a charged body exerts force is called electric force or electrostatic force.

→ Pressure: When a force is applied perpendicular to a given surface area, this force is called thrust. Thrust acting on a unit area of a surface is called pressure.

Pressure = $$\frac{\text { Thrust }}{\text { Surfaceoverwhichitacts }}$$

A bigger surface area means less pressure for the given thrust, compared to a smaller surface area.

→ Applications of Pressure:

• Rear wheels of buses and trucks are usually double. This helps in increasing the surface area and reducing the pressure.
• High rise buildings and dams have a wide base. This reduces the pressure on the ground.
• Tanks and bulldozers are fitted with caterpillar tracks to increase surface area and reduce pressure.
• Railway tracks are laid on large sized wooden/iron sleepers. This helps in spreading the pressure to a wider area so that the rail does not sink in the ground.
• Sharp edge of knife has smaller surface area and hence more pressure can be applied with the given thrust.
• Due to this, it is easier to cut with a sharp knife than with a blunt knife.
• Nails, pins and spikes have pointed ends so that they can be driven into surface with least effort.

→ Liquid Pressure: The pressure exerted by a stationary liquid, at any point in the liquid is called hydrostatic (liquid) pressure.

→ Properties of Liquid Pressure:

• Liquid pressure at the bottom of the container does not depend on the area of the bottom. It acts perpendicular to the base.
• Liquid pressure depends on the density of the liquid and height of the liquid column.
• Liquids exert equal pressure on all the walls of the container.
• An external pressure; applied on a liquid in a closed container; is uniformly transferred throughout the liquid.

→ Atmospheric Pressure: The pressure exerted by atmospheric air is called atmospheric pressure.

→ Variation in Air Pressure: Air pressure decreases with increase in altitude. Due to this, the atmospheric pressure is high at the sea level and low in the mountains. Atmospheric pressure decreases with increase in temperature.

→ Importance of Atmospheric Pressure:

• Drinking a liquid with a straw is possible because of air pressure.
• Droppers are able to squeeze in the liquid because of air pressure.
• Suction cups work because of air pressure.

→ Force : A push or a pull is called force.

→ SI Unit of Force : Newton

→ Gravitational force : Every object in the universe attracts every other object with some force. This force is called force of gravitational force.

→ Thrust : A force applied perpendicular to a given surface is called thrust.

→ Pressure : Thrust applied to a unit surface area is called pressure.

→ Atmospheric Pressure : Pressure of atmospheric air is called atmospheric pressure.  