# Physics - Force and Laws of Motion

## Introduction

• If we apply a force on an object, it may change its position or/and shape as well (as shown in the image given below). • Galileo Galilei and Isaac Newton explained a different approach to understand motion and applied force.

## First Law of Motion

• According to Galileo an object moves with a constant speed when no force acts on them.

• According to Newton’s First Law of Motion, “an object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.”

• The tendency of uninterrupted objects to stay at rest or to keep moving (if in motion) with the same velocity is known as inertia.

• Newton’s first law of motion is also popular as the law of inertia. • As shown in the image given above, when the playing card is flicked by the finger, the coin placed on it falls in the glass; it explains the law of inertia.

• Therefore, inertia is a natural tendency of any object to resist a change in its state of motion or of rest. • Quantitatively, the inertia of an object is measured by its mass, as the heavier or bigger objects have greater inertia and lighter or smaller objects have lesser inertia.

## Second Law of Motion

• The second law of motion states that “the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.”

• The momentum (represented as p) of an object is defined as the product of its mass (represented as m) and velocity (represented as v).

• Likewise, Momentum (m) = Mass (m) × Velocity (v).

• Momentum possesses both the direction as well as magnitude.

• The SI unit of momentum is represented as kilogram-meter per second (kg m s-1).

• The second law of motion illustrates a method to measure the force, which is acting on an object as a product of its mass and acceleration.

## Third Law of Motion

• The third law of motion states that – “to every action there is an equal and opposite reaction.” • It is important to remembered that the action and reaction always act on two different objects.

• It is important to remember that action and reaction forces are always equal in magnitude, but these forces may not produce accelerations of equal magnitudes because each force acts on a different object, which may have a different mass.

## Conservation of Momentum

• The conservation of momentum states that, in a given area, the amount of momentum remains constant.

• The momentum is neither created nor destroyed; however, it can be changed through the action of forces (described by Newton's laws of motion).

• The mass of an object multiplied by the velocity of the object is known as momentum.