Force And Pressure Class 8
Topics To Be Covered
- Definition of force
- Forces are due to interaction
- Effects of force
- Types of force
- Contact force
- Non contact force
- What is pressure
- Pressure exerted by liquid
- Pressure exerted by gas
- Application of atmospheric pressure in everyday life
Force :-
Force is an external agent and physical quantity that can change or tends to change the state of rest or state of motion and it can change the shape and size of any objects.
OR
→ The force is a physical quantity or external agent that can change or tends to change shape, size and state (rest, motion, velocity, direction) of a body.
→ Force is a vector quantity (it has magnitude as well as direction)
→ S.I unit of force is newton (N) or kg m/s²
→ Force is denoted by F.
Define 1 N of force :-
1 N force is the force when a body having mass of 1 kg accelerate with 1m/sec²
Push Force :- A force to move an object away form our body.
Pull Force :- A force to move towards our body.
Combination of Push and Pull Force
Forces are due to Interaction
Force comes into play due to an interaction of an object with another result in force between the two object at least two object must interact for a force come into play
i) If the forces applied to an object act in the same direction then the resultant force acting on the object is equal to the sum of two forces.
In other words when two forces act in the same direction then their effective magnitude increases
ii) If two forces are applied to an object acts in the opposite direction then the net force acting on the object is equal to the difference between two forces.
In other words when two forces act in opposite directions then their effective magnitude decreases.
iii) If the two forces applied to an object are equal in magnitude and acts in opposite direction then the net force acting on the object is zero or nil.
• Since the net force acting on the object is zero or Nil in this case no object does not move at all it remains in the same position.
A force cannot be seen beat we can feel and observe the effects of force .
A force can produce the following effects :-
I ) force can make a stationary body in motion.
Example :- football can be set the move by kicking or applied force.
II ) force can stop a moving body.
Example :- By applying breakers in a moving vehicles live bus and train.
III ) force can change the direction of a moving body.
Example :- in a cricket match , when a moving cricket ball is hit by a bat , then the direction of cricket ball change and it goes in another direction.
IV ) force can change the speed of a moving body.
Example :- To accelerate the moving vehicle.
V ) force can change the shape and object.
Example :- By pushing of pulling or pressing and object like clay.
Types of forces
i) contact force
ii) non contact force
i). Contact force:- A force which can be expressed by an object on another object only through physical touching is called a contact force.
• On the basis of contact force there are two types of forces.
a) Muscular force
b) frictional force (friction)
a) Muscular Force:-
The force exerted by the muscles of the body is called muscular force
• Both, human beings and animals exert muscular force to do work.
• It is the muscular force which enables us to perform all the activity involving the movement or bending of our body parts.
• We perform various muscular forces in our daily life activities like walking, running, jumping, lifting ,pushing, pulling, kicking, stretching and twisting.
• Muscular force also acts inside our body to perform various functions required for our survival.
For example:- in the process of digestion, it is the muscular force which push the food through the elementary canal.
b) Frictional Force :-
The force which always opposes the motion of one body over another body is called frictional force ( Friction).
• The frictional force acts between two surfaces which are in contact with each other.
For example:- A ball moving on the ground slows down and stops due to frictional force between the ball and the ground.
ii) Non contact force :- A force which can be exerted by an object on another object even from distance ( without touching each other) is called non contact force.
• On the basis of non contact force there are three types of forces
i) Magnetic Force
ii) Electrostatic Force
iii) Gravitational Force
i) Magnetic Force :-
A magnetic force is a force that a magnet exerted on certain metals like Iron Steel, Nickel and Cobalt.
~ Magnetic Force acts even distance.
• Since A magnet can exert its magnetic force on materials from a distance therefore magnetic force is a non contact force.
• A magnet can exert magnetic force of attraction or repulsion.
ii) Electrostatic Force:-
Electrostatic force is a non contact force. It is exerted by a charged body on another charged or uncharged body.
For example:- A strow rubbed with paper attracts another stroke but repels if it has also been rubbed with a sheet of paper.
~ The electrostatic force between two charged objects having like charges (positive and positive) or (negative or negative) is that of repulsion on the another hand the electrostatic force between two charged objects having unlike charges (positive and negative) is of attraction.
iii) Gravitational Force:-
Newton said that every object in the universe pulls every other object with a certain force.
• The force between two body is directly proportional to the product of the masses (M1 and M2) of two object in contact with each other.
And inversely proportional to the square of the distance (r) between their centers.
From equation (i) and (ii) we get
Where,
G is the proportionality constant known as gravitational constant.
• It is gravitational force between the sun and the earth which holds the earth in its orbit around the sun and it is the gravitational force between the Earth and the Moon which holds the moon in its orbit around the earth.
• The gravitational pull or force becomes strong only if one of the object has a huge mass like the earth therefore gravitational force between the Earth and the stone is very large which can be detected easily.
Gravity = The force with which the earth pulls the objects towards it is called force of gravity or gravity.
Note:- The average gravitational force of earth is 9.8 m/s²
What is pressure?
The perpendicular force acting on the surface of an object per unit area.
The SI unit of measuring force is Newton (N) and the SI unit of measuring area is square meter (m²). Therefore, the SI unit of measuring pressure is N/m² which is also called pascal (pa).
Define 1 Pascal
When 1 Newton force is applied to the surface area of 1m² of an object then that pressure is called pascal.
Pascal is very small unit of pressure, so a bigger unit of pressure is called kilo pascal.
The symbol of kilo pascal is kpa.
1 kilo pascal = 1000 pa
Q) A force of 250 Newton is applied to an object of area of 5m². Calculate the pressure.
Given → Force (F) = 250 N
Area (A) = 5 m²
To find → Pressure (p)
Solution → As we know that
Q) The area of 1 foot of a camel is 175 cm². If the mass of the camel is 550 kg, find the pressure exerted by the camel on the ground. Take force of gravity = 10 m/s²
Given:
Area of 1 foot of a camel = 175 cm²
Mass of camel = 550 kg
gravity = 10 m/s²
To find → Pressure (p)
Solution:
Area of 1 foot of a camel = 175 cm²
Area of 4 feet of a camel = 175 × 4 = 700 cm²
Now:
Mass of camel = 550 kg
We know that
Pressure exerted by liquids
All the liquids exerted pressure on the base or bottom and walls of their container. All the liquids have weight.
When we pour liquid into vessels then the weight of liquid pushes down on the base of the vessel producing a pressure. The liquid exerts pressure on the base of the vessel.
→ Liquid exerts pressure by on the surface of the container in which they are placed.
The pressure exerted by liquid changes with depth in the liquid.
Pressure exerted by liquid increases with increasing depth inside the liquid.
The pressure exerted by liquid is small just under the surface of the liquid.
✶ The properties of liquid pressure
i) The pressure exerted by liquid depends on the height of the liquid and the density of the liquid.
ii) Pressure exerted by liquid of the same depth will be same.
iii) Liquid exerts pressure in all direction.
iv) Liquid pressure directly the acceleration due to gravity (g).
pressure = ρgh
Where,
P = liquid pressure
ρ = density
g = acceleration due to gravity
h = height
✶ How liquid exerts pressure
Liquid is made up of a large number of very small particles (molecules). These particles have rapid motion and are continuously colliding with each other and with the walls of the container. When these particles collide with the wall they exerted a force on the wall and thus exerted a pressure on it. Therefore liquid exerted pressure on the walls of the container in which they are kept.
✶ Pressure exerted by gases
The substance such as hydrogen, helium, nitrogen, carbon dioxide are gases. All the gases exert pressure on the walls of their containers. Air is the mixture of gases and exert pressure in all direction.
➢ All the gases are made up of tiny particles called molecules which move around quickly in all direction randomly due to which they collide each other and with the walls of the container that the gas is present.
➢ The gas molecules collide with the walls they exert a force perpendicular to the walls. The sum of all such forces divided by the surface area of container wall is called the gas pressure.
➢ The gas pressure is measured of the average of linear movement term of the gas molecules.
➢ The SI unit of gas pressure is pascal.
