Question

force and laws of motion class 9 notes ​

Answer 1

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Introduction to Force

A force is an effort that changes the state of an object at rest or at motion. It can change an object’s direction and velocity. Force can also change the shape of an object.

Balanced and Unbalanced Forces

When balanced forces are applied to an object, there will be no net effective force acting on the object. Balanced forces do not cause a change in motion.

Unbalanced forces acting on an object change its speed and/or direction of motion. It moves in the direction of the force with the highest magnitude.

Net force

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

First Law of Motion

First Law of MotionFrictional force

The force that opposes relative motion is called friction. It arises between the surfaces in contact.

Example: When we try to push a table and it does not move is because it is balanced by the frictional force.

First Law of Motion

A body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. The First Law is also called the Law of Inertia.

Inertia

Basically, all objects have a tendency to resist the change in the state of motion or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends on the mass of a body. Mass of an object is the measure of its inertia.

More the mass → more inertia and vice versa.

Second Law of Motion

Second Law of MotionMomentum

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. Vector quantity, has direction and magnitude.

Second Law of Motion

The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force.

Δptαm(v−u)t

⇒Δptαma

⇒Fαma

⇒F = kma

For 1 unit of force on 1 kg mass with the acceleration of 1m/s2, the value of k = 1.

Therefore, F = ma.

Conservation of Momentum

Conservation of MomentumConcept of system

The part of the universe chosen for analysis is called a system.

Everything outside the system is called an environment.

For example, a car moving with constant velocity can be considered a system. All the forces within the car are internal forces and all forces acting on the car from the environment are external forces like friction.

Conservation of momentum

The total momentum of an isolated system is conserved.

Isolated system → net external force on the system is zero.

Example: Collision of 2 balls A and B.

From Newtons 3rd law F_{AB} = -F_{BA}

⇒mAVa−Uat=mBVb−Ubt

⇒mAUA+mBUB=mAVA+mBVB

Third Law of Motion

Newton’s 3rd law states that every action has an equal and opposite reaction. Action and reaction forces are equal, opposite and acting on different bodies.

Inertial and Non-inertial frames

A non-inertial frame of reference is a frame of reference in which Newton’s laws of motion do not hold. A non-inertial reference frame is a frame of reference that is undergoing acceleration with respect to an inertial frame. An accelerometer at rest in a non-inertial frame will, in general, detect a non-zero acceleration.

A frame of reference where Newton’s Laws hold is known as an inertial frame of reference.

Answer 2

Force is an influence that changes the motion of any object. There are three laws of motion.

Explanation:

Force

• Force is any push or pull exerted on a body which can alter its state.

• Force can cause an object with mass to change its velocity i.e. to accelerate.

• Force can be expressed as:

$F=ma$, here m is mass of the object, a is the acceleration with which the body is in motion.

• Unit of force is Newton or $kg.m/s^{2}$.

• Force is a vector quantity as it has magnitude as well as direction.

Laws of motion

• Newton's first law of motion

A body at rest remains at rest and a body in motion remains in motion with constant speed and in a straight line until it is acted upon by an unbalanced force. It introduces the concept of inertia.

• Newton's Second law of motion

The acceleration of an object depends on the mass of the object and the amount of force applied. It gives the expression for force.

• Newton's third law of motion

Whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first.