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11. Force and Pressure

This chapter talks about physical interaction between one or more objects and how they exert forces on each other

Force is described as a push or a pull on an object which results in change in velocity of the object, including moving from a state of rest to motion.

11.1 State of motion

For a force to come into play, at least two objects need to come into contact with each other. An interaction of one object with other result       s in a force between the two objects.

  1. Forces applied on an object in the same direction add to one another
  2. If Forces are acting in the opposite directions on an object, the Net force acting in the object is the difference between the two forces.

The strength of a force is called its Magnitude. A change in direction or magnitude of the force changes its effect. In general, more than one force may be acting on an object, but the effect on the object is due to the net force acting on it.

  1. If the force is applied on the object in direction of its motion, the speed of the object increases.
  2. If the force is applied on the object in the direction opposite to the direction of motion, then it decreases the speed of the object.

The state of motion of an object is described by its speed & the direction of motion. A change in either the speed of an object, or its direction of motion, or both is termed as a change in state of motion of the object. Application of force on an object is responsible for this change in the state of motion.

The state of rest is considered to be the state of zero speed.

The following are the examples of change in state of motion due to application of force

Type of change Examples
Change in Speed
  • Pushing a stationary object like car
  • Pulling water out of a well using bucket
  • Game of tug of war (two teams pulling a rope on opposite sides)
Change in Direction
  • A batsman hitting the ball in the game of cricket

11.2 Effects of Force

Application of force on an object can result in the following

  1. May make an object move from rest
  2. May change the speed of an object if its is moving
  3. May change the direction of motion of an object
  4. May bring about a change in the shape of an object
  5. A combination of the above effects

None of the above listed actions can take place without the action of a force.

11.3Contact Forces

The forces which are applied on an object by virtue of its contact with another object/person are called Contact Forces.

11.3.1 Muscular Force

The force resulting due to the action of human/animal muscles on an object is called Muscular Force. Since, muscular force can be applied only when with direct/indirect contact with the object; it is also called Contact Force. For example

  1. When we push any object like a table or pull a bucket of water
  2. Animals also make use of muscular force to carry out like pulling heavy loads, Ploughing the field etc.

The magnitude of muscular force depends on the muscular strength

11.3.2 Force of Friction

The force which opposes the motion of an object on the surface of another object is called Force of Friction. The force of friction always acts on a moving object & it’s direction is always opposite to the direction of motion. Since, the force of friction arises due to contact between the two surfaces; it is also called Contact Force.

For example

  1. The force of friction between the surface of a ball & the ground that brings the ball to rest
  2. Friction between water & boat brings the moving boat to rest

The magnitude of frictional forces depends on the coefficient of friction of the object

11.4 Non-Contact Force

The forces which can act from a distance, that is, when it is not in contact with the object are called Non-Contact Forces

11.4.1 Magnetic Force

A push or a pull exerted by a magnet is called a Magnetic Force. Since a magnet can exert a force on other magnetic or non-magnetic material without being in contact with it, so it is called as a Non-contact Force.

For example

  1. When a magnetic force is applied on a non magnetic material like Iron, then it results in a Pull
  2. When a magnetic force is applied on a magnetic material like another magnet
    1. If Opposite poles of the magnets are interacting, they will attract each other
    2. If Similar poles of the magnets are interacting, they will repel each other

The magnitude of magnetic forces depend upon the magnitude of the magnetic field of the objects

11.4.2 Electrostatic Force

The force exerted by a charged body on another charged or uncharged body is called electrostatic Force. Since the force comes into play even when the bodies are not in contact is called Non-contact force.

For example

  1. When an electrostatic force is applied between a charged and an uncharged object, then two objects are attracted to each other
  2. When an electrostatic force is applied between two charged objects
    1. If the two objects are oppositely charged, they would attract each other
    2. If the objects are charged similarly, they would repel each other

The magnitude of electrostatic force depends on the magnitude of charge on the object

11.4.3 Gravitational Force

Every object in the universe attracts other objects towards it with a force which is known as Gravitational Force. The Gravitational Force exerted by earth is known as gravity or Force of gravity

The magnitude of gravitational force depends on the mass on the object.

11.5 Pressure

The Force applied per unit area is called Pressure.

Pressure = Force / Area

Pressure is inversely proportional to the area on which the force is applied. Therefore, the same force applied on a smaller area will result in higher pressure. This can be utilized to our advantage. For example

  1. The nails have a pointed end, so that with even with nominal force, the carpenter will be able to push a nail into a wooden plank
  2. The knives have a sharp edge, so that we can cut fruits and vegetables by applying minimal force
  3. Porters place a round piece of cloth on their head for carrying heavy loads because by doing so, the area of contact between the head & the load increases and the pressure on the head decreases, making it more comfortable to carry the load

11.5.1 Pressure Exerted by liquids & gases

Liquids as well as Gases exert pressure on the walls of the container.

The pressure exerted by liquids varies directly with the height of the column.

  1. When the height of liquid in a column is increased the pressure exerted by the liquid also increases.
  2. When the height of liquid in the column decreases the pressure exerted by the liquid also decreases.

For example, if a water tank is filled with water and a hose pipe is connected to it

  1. When the tank is filled completely, the water comes out of the pipe with high pressure and hence covers a long distance
  2. When the tank is marginally filled, the water comes out with low pressure and covers only a short distance

A thinner pipe will face higher pressure on the cross section and will throw the water to a longer distance as compared to a wider pipe. This principle is used in hydraulic machines to achieve higher displacement by applying low force.

Similar to liquids, gases also exert pressure on the walls of the container. This force is however low as compared to the force exerted by the liquids.

11.5.2 Atmospheric Pressure

The layer of air which is present around the earth is called Atmosphere. It extends upto many kilometers above the surface of the earth. The pressure exerted by air is called Atmospheric Pressure.

We can understand the atmospheric pressure with the help of a rubber sucker. When we push the sucker against a surface, the air between the surface and the sucker is removed. The sucker is stuck to the surface as the pressure of atmosphere acts on it. If we want to pull out a sucker, we needs to apply a force which should overcome the atmospheric pressure.

The weight of air in the atmosphere on a 10cm x 10cm area is as large as 1000kg. However, we do not feel this pressure, because the pressure inside has similar magnitude and the two pressures cancel each other.