RBSE Class 9 Science Notes Chapter 9 Force and Laws of Motion

These comprehensive RBSE Class 9 Science Notes Chapter 9 Force and Laws of Motion will give a brief overview of all the concepts.

RBSE Class 9 Science Chapter 9 Notes Force and Laws of Motion

→ Force : A force is that external push or pull acting on an object which change or tries to change the state of rest or of uniform linear motion of an object or change the direction of motion. A force may also change the shape and size of an object.

→ The forces can be classified into two types :

Balanced forces	Unbalanced forces
When a number of forces acting simultaneously on an object do not produce any change in its state of rest or of uniform linear motion. Examples : (i) When a girl tries to push a heavy box and is unable to do so. (ii) When a person is unable to move a wall, even after pushing hard.	When two or more forces acting simultaneously on an object have a non-zero resultant force and change the state of rest or of uniform linear motion of the object. Examples : (i) A girl moves a box lying on floor (ii) When one team wins a tug of war.

→ Newton’s Laws of Motion : Sir Isaac Newton further developed Galileo’s ideas on force of motion, he gave three laws of motion :

→ First Law of Motion : The first law of motion states that an object maintains its state of rest or of uniform motion unless compelled by some external applied force. According to first law of motion force can be defined as an external effort in the form of a push or a pull which changes or tends to change the state of rest or of uniform linear motion of an object on which it acts. The tendency of undisturbed objects to stay at rest or to keep moving with same velocity is called inertia, due to this the first law of motion is also known as the laws of inertia.

→ Types of Inertia :
Inertia of rest :

• When a bus or a car suddenly starts moving forward, the passengers fall backwards.
• When a carpet is beaten with a stick, dust particles come out of it.
• When we vigorously shake the branch of a tree, some of the leaves, flowers and fruits fall from the tree.

Inertia of motion :

• When the driver of a bus, running at a high speed suddenly applies brakes the passengers experience a forward push.
• An athlete runs some distance before taking a jump.
• A person falls forward with his face downward while getting down from a running bus or train.
• The luggage is usually tied with a rope on the roof of the bus or car.

Inertia of direction :

(i) Let a stone tied to a string be whirling in a horizontal circle. If the string breaks, suddenly, the stone flies off tangentially.
(ii) A heavier object has to longer inertia than a lighter object. More massive the object, larger is its inertia so quantitatively, the mass of an object is a measure of its inertia.

→ Momentum : The momentum of an object is defined as the product of its mass and velocity.

If an object of mass m is in motion with a velocity υ, then its momentum is given as :
Momentum (p) = mass (m) × velocity (υ)
Momentum is a vector quantity, direction of momentum of an object is same as the direction of its velocity υ.
SI unit of momentum = SI unit of mass × SI unit of velocity = kg × ms^-1
Every moving object possesses momentum. However, an object at rest has zero momentum because its velocity is zero.

→ Second Law of Motion : The second law of motion states that the rate of change of momentum of an object is directly proportional to the applied unbalanced force and the change of momentum takes place in the direction of applied force.

According to second law of motion,

→ Unit of Force :

F = m × a
If m = 1 kg, a = 1 m/s²
then F = 1 kg × 1 ms^-2 = 1 Newton

→ Applications of Newton’s Second Law of Motion :

• A fielder while catching a fast moving ball gradually pulls his hands backwards with the moving ball.
• Automobiles are fitted with springs so as to reduce jerks while passing over a rough road.
• A karate player can break a slab of ice or a pile of tiles or bricks with a single blow.

→ Third Law of Motion : When one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitudes but opposite in direction. These two forces act on different objects and never on the same object.

→ Law of Conservation of Momentum :

For an isolated system (where there is no net external force) the total momentum of the system remains conserved.
Consider two balls A and B of masses m₁ and m₂ travelling in the same direction along a straight line with velocities u₁ and u₂ respectively. Let there is no external unbalanced force of any sort acting on the balls. Then,
Total momentum before collision = total momentum after collision

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

∴ Total momentum of balls A and B remains unchanged or conserved during their collision when no external force acts on them.