4.1 Introduction

We normally observe around us, a number of objects or bodies at rest or in motion and find that, the objects at rest do not move by themselves or the objects in motion do not come to rest by themselves but they require some external force to do so. e.g. To move a book kept on a table, we need to push or pull it, or to stop a vehicle in motion, breaks are required.

The factor which is necessary for causing motion or change in motion is termed as force. This cause of motion (force) and effects of motion are governed by Newton’s laws of motion.

Newton’s laws of motion are three fundamental principles that describe the relationship between an object’s motion and the forces acting on it. The first law states an object at rest stays at rest, and an object in motion stays in motion unless a force acts on it (the law of inertia). The second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass, expressed by the formula \(F=m a\). The third law states that for every action, there is an equal and opposite reaction.

Newton’s First Law (Law of Inertia)

• An object will not change its state of motion unless acted upon by an unbalanced force.
• This means an object at rest will stay at rest, and an object in motion will continue moving at a constant velocity (constant speed and direction).
• This tendency to resist changes in motion is called inertia, and objects with more mass have more inertia.

Newton’s Second Law (Law of Force)

• The relationship between force, mass, and acceleration is defined by the formula \(F=m a\).
• Force ( \(F\) ) is the push or pull on an object.
• Mass ( \(m\) ) is the amount of matter in the object.
• Acceleration ( \(a\) ) is the rate of change of velocity (speed or direction).
• This law explains that to accelerate a massive object, you need to apply a larger force than you would to a less massive object.

Newton’s Third Law (Law of Action-Reaction)

• When one object exerts a force on a second object, the second object simultaneously exerts a force back on the first object that is equal in magnitude and opposite in direction.
• For example, if you push on a wall, the wall pushes back on you with the same amount of force.
• This law is responsible for actions like a rocket’s propulsion, where the rocket expels gas (action) and the gas pushes the rocket forward (reaction).

In this chapter, we will discuss the motion of a body by taking into consideration the cause of motion, i.e. the external force which produces the motion or change the motion.