, NEWTON’S LAW OF MOTION
In kinematics our concern was to describe the motion of a body without knowing its cause but in Newton’s Laws of
Motion we will study the cause of Motion.
Force:
Force is an external agent capable of changing the state of rest or motion of a
particular body.
It is a vector quantity
Force can be exerted between two bodies without in contact.
For example: Gravitational force and Electrostatic force.
Aristotle’s Fallacy:
According to aristotle, an external force is required to keep a body in motion. But later this theory of aristotle found
to be wrong.
Point Mass:
An object can be considered as a point mass if during motion in a given time, it covers distance much greater than its
own size.
Inertia:
Inertia means ‘resistance to change’. It is the property of the body due to which the body opposes the change of
itself state.
Inertia of a body is measured by mass of the body.
Inertia ∝ mass
Heavier the body, greater is the force required to change its state and hence greater is the inertia.
• Inertia is a unitless and dimensionless quantity.
• Two body of equal mass, one in motion and another is at rest, has the same inertia because it is a factor of
mass only and does not depend upon the velocity.
Newton’s First Law:
Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external
force to act otherwise.
If the net external force on a body is zero, its acceleration is zero. Acceleration can be non zero only if there is a net
external force on the body.
APNI KAKSHA 1
, For Example: A person standing in a moving bus suddenly feels a jerk in forward direction
when bus suddenly stops because when the bus is moving his whole body is moving with
the bus but when the bus suddenly stops his feet also stops due to friction but the upper
body will move forward due to inertia.
Similarly when a bus at rest suddenly accelerates person standing in the bus will feel a jerk
in backward direction because when the bus at rest his whole body is at rest but when bus
suddenly accelerates in forward direction. His feet will move with the bus in forward
direction at that instant due to friction but his upper body wants to be at rest due to property
of inertia and because of this his upper body will feel a jerk in backward direction.
Common Forces in Mechanics
Weight: Weight of a body is the force with
which earth attracts it. Its magnitude is given by
mg. It is also known as gravitational force.
Reaction or Normal Force: When a body is
placed on a rigid surface, then the body
experiences a force which is perpendicular to
the surface in contact. This force is called
reaction or normal force.
Tension: The force exerted by the end of tout
string, rope or chain against pulling (applied)
force is called the tension.
The direction of tension is away from the body.
Spring Force: Every spring resists any change
in its length. This resistive force increases with
change in length. It is given by –
𝐹 = −𝑘𝑥
Here 𝑥 is the change in length and 𝐾 is the
spring constant.
Linear Momentum:
The total quantity of motion possessed by a moving body is known as the momentum of the body. It is measured as
the product of the mass of the body and its velocity. Momentum = Mass × Velocity
𝑝⃗ = 𝑚𝑣⃗
It is a vector quantity and its direction is same as the direction of velocity of the body.
SI Unit – kg.m/sec. Dimensions — [𝑀 𝐿 𝑇 −1 ]
APNI KAKSHA 2
In kinematics our concern was to describe the motion of a body without knowing its cause but in Newton’s Laws of
Motion we will study the cause of Motion.
Force:
Force is an external agent capable of changing the state of rest or motion of a
particular body.
It is a vector quantity
Force can be exerted between two bodies without in contact.
For example: Gravitational force and Electrostatic force.
Aristotle’s Fallacy:
According to aristotle, an external force is required to keep a body in motion. But later this theory of aristotle found
to be wrong.
Point Mass:
An object can be considered as a point mass if during motion in a given time, it covers distance much greater than its
own size.
Inertia:
Inertia means ‘resistance to change’. It is the property of the body due to which the body opposes the change of
itself state.
Inertia of a body is measured by mass of the body.
Inertia ∝ mass
Heavier the body, greater is the force required to change its state and hence greater is the inertia.
• Inertia is a unitless and dimensionless quantity.
• Two body of equal mass, one in motion and another is at rest, has the same inertia because it is a factor of
mass only and does not depend upon the velocity.
Newton’s First Law:
Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external
force to act otherwise.
If the net external force on a body is zero, its acceleration is zero. Acceleration can be non zero only if there is a net
external force on the body.
APNI KAKSHA 1
, For Example: A person standing in a moving bus suddenly feels a jerk in forward direction
when bus suddenly stops because when the bus is moving his whole body is moving with
the bus but when the bus suddenly stops his feet also stops due to friction but the upper
body will move forward due to inertia.
Similarly when a bus at rest suddenly accelerates person standing in the bus will feel a jerk
in backward direction because when the bus at rest his whole body is at rest but when bus
suddenly accelerates in forward direction. His feet will move with the bus in forward
direction at that instant due to friction but his upper body wants to be at rest due to property
of inertia and because of this his upper body will feel a jerk in backward direction.
Common Forces in Mechanics
Weight: Weight of a body is the force with
which earth attracts it. Its magnitude is given by
mg. It is also known as gravitational force.
Reaction or Normal Force: When a body is
placed on a rigid surface, then the body
experiences a force which is perpendicular to
the surface in contact. This force is called
reaction or normal force.
Tension: The force exerted by the end of tout
string, rope or chain against pulling (applied)
force is called the tension.
The direction of tension is away from the body.
Spring Force: Every spring resists any change
in its length. This resistive force increases with
change in length. It is given by –
𝐹 = −𝑘𝑥
Here 𝑥 is the change in length and 𝐾 is the
spring constant.
Linear Momentum:
The total quantity of motion possessed by a moving body is known as the momentum of the body. It is measured as
the product of the mass of the body and its velocity. Momentum = Mass × Velocity
𝑝⃗ = 𝑚𝑣⃗
It is a vector quantity and its direction is same as the direction of velocity of the body.
SI Unit – kg.m/sec. Dimensions — [𝑀 𝐿 𝑇 −1 ]
APNI KAKSHA 2
