Laws of Motion

First Law or Law of Inertia: - Each body continues to move in its state of rest or of continued motion in a straight line, except in so far as it is compelled by the impressed force to change that direction.

Second Law: - The Rate of change of momentum is proportional to impressed force and takes place in the direction of the force.

Third Law: - To each action there is an equal and opposite reaction.

Applications of Newton’s Laws

• If a cantering horse suddenly stops, the rider, if he is not firmly seated, is thrown forward (Inertia of Motion). 

•  An athlete runs for some distance before taking a jump so that his forward momentum may help him to achieve a longer jump (Inertia of Motion).

• Dust is removed from a hanging carpet by beating it with a stick because the dust particles tend to remain at rest and fall off as the carpet suddenly moves forward (Inertia of Rest)

•  When a man jumps out of a running bus, he falls forward on the road. Reason: While in the bus, the man shares the motion of the bus. As he jumps out, his lower portion comes to rest on touching the ground but the upper portion keeps on moving in the same direction of bus. Hence he falls, forward. In order to save himself falling down, the passenger runs for some distance on the road in the same direction (Inertia of Motion)

• When a bus at rest suddenly starts, a passenger falls or leans backward. Reason: With the bus at rest, passenger’s whole body was also at rest. As the bus starts moving, the lower part of passenger’s body, which is in contact with the bus, begins to move with the bus while the upper part remains at rest and is hence thrown backward (Inertia of Rest)

• When a men jumps from a boat, the boat moves back due to reaction as man jumps from a boat to ground (Third Law)

• When a shot is fired from the gun, the gun recoils backward due to reaction of shot on the gun. Many police recruits thus hurt their collar bone during initial firing practice with .303 rifles (Third Law)

•  While walking on the ground, we press backward on the ground with our feet, and the reaction on the ground gives us an equal and opposite force which makes us move (Action and reaction – Third Law)

•    If we try to open a door by kicking it, the foot gets hurt. Reason: The force exerted by us on door accelerates the door and it opens. But at the same time, the door exerts an equal and opposite force on us which reduces forward velocity of foot. The opposite reaction is responsible for the feeling of being hurt.

• While sitting on chair, we cannot lift the chair up by applying upward force on its arms. As we exert an upward force on chair, the chair exerts an equal and opposite reaction on us (Third Law).