Revision: Laws of Motion Physics (English Medium) ICSE Class 9 CISCE

The weight of the body is the force with which the earth attracts it towards the centre. It depends on acceleration due to gravity.

The gravitational force acting on an object is called the weight of the object.

A force is seen to act through direct contact of the objects or via one more object. Such a force is called 'Contact force.'

A force is applied between two objects even if the two objects are not in contact; such a force is called a 'Non-contact force.'

"If no force is acting on a body, its velocity does not change, i.e., the body does not accelerate. In other words, if a body is stationary, it will remain stationary. If it is in motion, it will continue moving with the same velocity and in the same direction."

or

"An object continues to remain at rest or in a state of uniform motion along a straight line unless an external unbalanced force acts on it."

or

"Every inanimate object continues to be in its state of rest or of uniform unaccelerated motion unless and until it is acted upon by an external, unbalanced force."

Inertia: The inherent property of a body to resist any change in its state of rest or the state of uniform motion, unless it is influenced upon by an external unbalanced force, is known as ‘inertia’.
Types of Inertia

• Inertia of rest
• Inertia of motion
• Inertia of direction

"The rate of change of momentum is proportional to the applied force, and the change of momentum occurs in the direction of the force."

Newton’s second law of motion states that the rate of change of momentum is directly proportional to force applied and takes place in the direction of the force.

"Every action force has an equal and opposite reaction force which acts simultaneously."

The concise law statement is: "To every action (force), there is an equal and opposite reaction (force)."

The gravitational force due to the earth on a body results in its acceleration. This is called acceleration due to gravity and is denoted by ‘g’.

The acceleration produced in a body under the influence of the force of gravity alone is called acceleration due to gravity.

The weight of an object is defined as the force with which the earth attracts the object.

Mass is the amount of matter present in the object. The SI unit of mass is kg.

Absolute unit of force in the CGS system is dyne and in SI system is Newton (N).
One dyne: When the body of mass 1 gram moves with an acceleration of 1 cms⁻², then the force acting on the body is called one dyne.
1 dyne = 1 g cms⁻²
One Newton: When a body of mass 1 kg moves with an acceleration of 1 ms⁻², then force acting on the body is said to be one newton.

OR

That force is said to be one newton, which producers an acceleration of 1 ms⁻² in a body of mass 1 kg.
1 N = 1 kg ms⁻²

"Every particle of matter attracts every other particle of matter with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them."

\[\vec F\] = m \[\frac{d\vec{\mathrm{v}}}{dt}\] = m\[\vec a\] ... (for constant mass)

Thus, if \[\vec F\] = 0, \[\vec v\] is constant. Hence, if there is no force, velocity will not change. This is nothing but Newton's first law of motion.

General Form: \[\vec F\] =\[\frac{d\vec{p}}{dt}\]

For Constant Mass: \[\vec F\] = m\[\vec a\]

Momentum: \[\vec p\] = m\[\vec v\]

\[\vec{F}=\frac{d\vec{p}}{dt}=\frac{d\left(m\vec{\mathrm{v}}\right)}{dt}\]

The value of the acceleration due to gravity (g) on the surface of the Earth is given by the formula:

Where:

• g = Acceleration due to gravity (in m/s²).
• G = Newton's Universal Gravitational Constant (≈ 6.67 × 10⁻¹¹ N · m² / kg²).
• M = Mass of the Earth (≈ 6 × 10²⁴ kg).
• R = Radius of the Earth (≈ 6.4 × 10⁶ m).

The gravitational force of attraction (F) between two bodies of mass m₁ and m₂ separated by a distance r is:

• F: Gravitational Force of attraction (in Newtons, N).

• \[m_1, m_2\]: Masses of the two objects (in kilograms, kg).

• r (or d in the first part): Distance between the two objects (in meters, m).

• G: The constant of proportionality, called the Universal gravitational constant.

  • Value in SI units: \[G=6.67\times10^{-11}\mathrm{N}\cdot\mathrm{m}^2/\mathrm{kg}^2\]

  • Dimensions: \[[G]=[\mathrm{L}^3\mathrm{M}^{-1}\mathrm{T}^{-2}]\]