Force may be defined as an agency (a push or a pull) which changes or tends to change the state of rest or of uniform motion or the direction of motion of a body.
Effects produced by a force:
1. Force can change speed of an object.
When force is applied on a body the body starts to move. Again, when a force exerted by the brakes slows or stops moving train.
2. Force can change the direction of motion of an object.
Force exerted by a bat to a ball, changes the direction of the ball.
3. Force can change the shape of an object.
If we apply a force on a rubber ball, round shape of a rubber ball gets distorted.
Galileo's Laws of inertia:
A body moving with certain speed along a straight path will continue to move with same speed along the same straight path in the absence of external forces.
INERTIA:
The inherent property of a material body by virtue of which it cannot change, by itself, its state of rest or of uniform motion in a straight line is called inertia.
Different types of inertia:
a. Inertia of rest: The tendency of a body to remain in its position of rest is called inertia of rest.
Example: A person standing in a bus falls backward when the bus suddenly starts moving forward.
b. Inertia of motion: The tendency of a body to remain in its state of uniform motion in a straight line is called inertia of motion.
Example: When a moving bus suddenly stops, a person sitting in it falls forward.
c. Inertia of direction: The inability of a body to change by itself its direction of motion is called inertia of direction.
Example: When a bus takes a sharp turn, a person sitting in the bus experiences a force acting away from the centre of the curved path. It is due to inertia of direction.
Measurement of inertia of a body:
Mass of a body is the measure of its inertia. If a body has more mass, it has more inertia, it means it is more difficult to change its state of rest or of uniform motion.
Linear momentum (p):
Momentum of a body is the quantity of motion possessed by the body. It is equal to the product of Mass and velocity of the body.
Momentum = mass x velocity
Momentum is a vector quantity because the velocity v is a vector and mass m is a scalar. Its direction is same as the direction of the velocity of the body. Its magnitude is given by
p = mv
SI unit of momentum = kg m/s
CGS unit of momentum = g cm/s
The dimensional formula of momentum = [MLT⁻¹]
Q1. Two objects, each of mass m and velocities v₁ and v₂. If v₁> v₂, which one has more momentum?
Ans: p₁ = mv₁ and p₂ = mv₂
∴ (p₁/p₂) = (mv₁/mv₂) = (v₁/v₂)
As v₁> v₂ , so p₁> p₂
Q2. Two objects having mass m₁ and m₂ such that m₁> m₂ , and same velocity v. Which one has more momentum?
Ans: p₁ = m₁v and p₂ = m₂v
∴ (p₁/p₂) = (m₁v/m₂v) = (m₁/m₂)
As m₁> m₂ , so p₁> p₂
Q3. Two objects having same momenta (p₁ = p₂), if m₁> m₂, which one has more velocity?
Ans. p₁ = m₁v₁ and p₂ = m₂v₂
As p₁ = p₂
∴ m₁v₁ = m₂v₂
or (v₂/v₁) = (m₁/m₂)
As m₁> m₂ , so v₁< v₂
Velocities of bodies having equal linear momenta are inversely proportional to their masses.
So, when two objects have equal linear momentum, the lighter object will move faster than the heavier one.
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