Class 8 Science



Force and Pressure

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

  • At least two objects need to interact for force to come into play. Mutual interaction between two objects results in a force between the two objects.
  • When two forces are applied on an object in the same direction, there is an add-on effect of both forces.
  • When two forces are applied in opposite directions, the difference between two forces gives the value of the net force.
  • Force is expressed in terms of its magnitude and the direction in which it is working. Any change in direction or magnitude of the force, there is a change in effect of the force.
  • Generally, more than one force is acting on an object at any given time.


Effect of Force on State of Motion

A force can change the state of motion. Both rest and motion are called states of motion. When an object is at rest, its speed is zero.

Effect of force on an object at rest: A force can move an object at rest. For example; if a force is applied on a book the book can move in the direction of force.

Effect of force on a moving object: A force can have following effects on a moving object.

  • When the applied force is in the same direction as direction of motion, there is an increase in the speed of object.
  • When the applied force is in opposite direction of motion, there is a decrease in the speed of object. The object may also come to rest when the applied force is in opposite direction to the direction of motion.
  • When the applied force is at some acute or obtuse angle to the direction of motion, there is a change in the direction of motion.

Effect of Force on Shape: Force can change the shape of an object. When force is applied on a ball of dough, there is a change in the shape of dough. Similarly, a potter changes a lump of clay into pots by applying force. When air is filled in a balloon; the shape of the balloon changes because of the applied force.


Types of Force:

There are two main types of forces, viz. contact force and non-contact force.

  • Contact Force: Contact force is a force which can produce its effect only by making a contact with object. Following are the contact forces:

      Muscular Force: It is the force applied by muscles of humans or animals. A bullock is able to pull a cart because of muscular force. The bullock needs to be harnessed to the cart for applying the force.

    • Friction: When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction. Friction always opposes motion because it acts in opposite direction to motion. Force of friction arises due to contact between two surfaces. If there is no contact between two surfaces there would be no friction.
  • Non-contact Force: The non-contact force is a force which can produce its effect without making a contact with object. Following are the non-contact forces:
    • Magnetic Force: The force applied by a magnet is called magnetic force. A magnet can exert its force on an object even from a distance and does not need to touch that object.
    • Electrostatic Force: It is the force applied by a charged body on another charged or non-charged body. When you brush a comb in your hairs and then bring the comb near small bits of paper, the comb attracts the bits of paper. This happens because of electrostatic force.
    • Gravitational Force: Every object in the universe exerts a force on another object. This force is called gravitational force. It is the gravitational force of various celestial bodies which keeps them in their position in the space.

Pressure

Pressure is the force acting on a unit area on an object.

Pressure = Force/Area on which it acts

In this equation, force is the numerator and area is the denominator. This means that pressure is directly proportional to the applied force but inversely proportional to the area on which it is acting.


Following examples illustrate this effect:

  • It is easier to push a nail into a wooden door through its pointed end than through its blunt end, because small area of the pointed end helps in creating more pressure and it becomes easier to insert the nail.
  • Shoulder straps of your school bag are wide. A larger area helps in reducing the pressure on the shoulder; and makes it less painful to carry the bag.
  • Camel’s feet are wide; making for larger area. Due to this, a camel can easily walk on sand.
  • Eskimos wear ski-like footwear. Wider area of the footwear reduces pressure and thus prevents the Eskimo’s feet from sinking in ice.

Pressure Exerted by Liquids and Gases

Liquids and gases exert pressure as follows:

  • Pressure on the bottom of container depends on the height of column of gas or liquid. Due to this, divers have to withstand a large pressure at the bottom of the sea.
  • A fluid exerts pressure on walls of the container. Containers for gases and liquids are usually cylindrical ion shape to equally distribute the pressure on all portions of the wall.
  • A fluid exerts equal pressure at same depth.

Atmospheric Pressure: The pressure exerted by atmospheric air is called atmospheric pressure. The weight of air in a column of height of atmosphere and area 10 x 10 cm is 1000 kg. This is roughly same as the area of our head. Can you believe that you have 1000 kg of air on your head?