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CLASS 9 - Force and laws of motion - Chapter notes

Force and Laws of motion

Force

A force is an action (efforts) that changes the state of an object at rest or at motion or try to change. It can change direction and velocity of an object. Force can also change the shape and size of any object.

Balanced and Unbalanced force

Balanced Force:- If balanced forces is applied to an object, there will be no net effective force acting on the object. Balanced forces is not creating any change in motion.


F1 = F2 = 5N actting on the body in opposite direction. If F1 force is treated positive, then F2 will be negative.
Therefore, total force acting on the body F1 + (-F2) = 5N + (-5N) = 0.



Unbalanced Force:- Unbalanced forces acting on any object changes its speed and direction of motion. It moves in the direction of the force with the higher magnitude.

F1 = 10N, F2 = 5N actting on the body in opposite direction. If F1 force is treated positive, then F2 will be negative.
Therefore, total force acting on the body F1 + (-F2) = 10N + (-5N) = 5N (in the direction of F1).



Net force

When a number of forces act on an object, they can be resolved into one component known as the net force acting on the object.


Laws of Motion

First Law of Motion:

Newton’s first law of motion says that a body at rest will remain at rest position only and a body which is in motion continues to be in motion unless a unbalance external force applied on it.
In simple words, we can say that the objects cannot start or stop or change its direction by themselves and they require some external force to act on them.

Inertia

- Every object has a tendency to resist the change in the state of motion or rest. This tendency is called inertia.

- All bodies don't possess the same inertia. Inertia is dependent on the mass of an object. Mass of an object is the measure of its inertia.

- Higher the mass higher the inertia and vice versa.


Second Law of Motion:

Newton’s second law says that the acceleration of any object depends upon two variables – the net force acting on the object and the mass of the object. The acceleration of the body is directly proportional to the net force acting on the body and inversely proportional to the mass of the body.

Let a force (F) applied on an object of mass (m) which creates an acceleration (a) in the object.
According to second law of motion,
a ∝ F -------------------(Equation 1)
a ∝ 1/m ----------------(Equation 2)

Combining Equation 1 and 2, we get
a ∝ F/m
a = K x F/m (Value of K = 1)
a = F/m
F = m.a => Force = mass x acceleration



Let an object of mass m, moving along a straight line with an initial velocity u, after time t, with a constant acceleration, the final velocity becomes v.
Initial momentum (p1) = m × u
Final momentum (p2) = m × v

Change in momentum
p2 – p1 = (m × v) – (m × u)
p2 – p1 = m (v – u)

(p2 – p1) / t = m (v – u) / t
(p2 – p1) / t = m x a
(p2 – p1) / t = F

Rate change of momentum = Force applied
The rate change of momentum with respect to time is proportional to the applied force.

Momentum

- Momentum of an object gives the idea of impacts produced by objects depend on their mass and velocity.
- The momentum of an object is defined as the product of its mass and velocity. p = mv.
- SI unit of momentum is kg.m/s.
- Vector quantity, has direction and magnitude.


Third Law of Motion

Newton’s third law of motion says that every action have equal and opposite reaction.


Law of Conservation of Momentum

The law of conservation of momentum states, 'When two or more bodies collide with each other in the absence of an external force, then the total final momentum of the bodies is equal to their total initial momentum. Means momentum remains conserve before and after collision.


Let two object of masses m1 and m2 is moving with initial velocity u1 and u2 respectively, after the collision final velocity become v1 and v2.

Therefore, initial momentum before collision = m1u1 + m2u2
final momentum after collision = m1v1 + m2v2

As per the law of conservation of momentum,
m1u1 + m2u2 = m1v1 + m2v2