Newton’s Laws of Motion

After Galileo, Sir Isaac Newton (1642 – 1727 AD) of England made a detailed and systematic study of the motion of bodies and formulated the three laws of motion.

Newton’s first law of motion

Newton’s first law states that ‘everybody continues in its state of rest or of uniform motion in a straight line unless an unbalanced force acts on it.’ Examples:

1. When we shake mango tree, the mangoes fall down.
2. When a blanket is given a sudden jerk, the dust particles in it fall off due to inertia at rest.
3. A rider on a running horse is thrown forward when the horse stops suddenly due to the inertia of motion.
4. When a person jumps out of a moving vehicle, he falls forward due to the inertia of motion.

Interpretation of first law: According to the first law, a body continues to remain at rest if no unbalanced forces act on it. Similarly, the body will remain moving with uniform velocity in a straight line unless unbalanced forces act on it.

Newton’s second law of motion

Newton’s second law of motion states that ‘acceleration produced in a body is directly proportional to the force applied to it in the direction of motion and inversely proportional to its mass.’ Example:

1. A cricket player while catching a ball moves his hands backwards.
2. A person falling on the cemented floor is injured more than a person falling on a sandy floor or mattress.
3. It is difficult to catch a cricket ball as compared to a tennis ball moving at the same velocity.
4. It is easier to drag a stone than to kick.

Interpretation of second law: If a small body is pushed gently, a small acceleration is produced. If it is pushed harder, a larger acceleration is produced. If two bodies, one lighter and another heavier, are pulled by the same amount of force in the same direction separately, it is found that the heavier body has less acceleration than the lighter body.

Newton’s third law of motion

Newton’s third law of motion states that ‘to every action there is equal and opposite reaction.’ Example:

1. While rowing a boat, a person pushes water backwards with the help of the oars. Due to the reaction offered by water, the boat moves forwards.
2. Birds, while flying, push air with their wings (action). The air exerts an equal and opposite force on the birds (reaction) and that force causes birds to move forward.
3. In order to swim, a man pushes water backwards with his hands. Due to the reaction offered by water to the man, the man is pushed forwards.
4. When the mouth of an inflated balloon is left open, the balloon flies away. This is because when the air inside escapes out (action), the balloon gets pushed (reaction) in the opposite direction by the air escaping out.

Interpretation of third law: Third law implies that forces always occur in a pair and a single force is thus impossible.