Newton's Laws of Motion in Physics
Newton’s experimental observations and conclusions are divided in three parts –
Newton’s First Law of Motion: it states that every body continues to be in state of rest or of uniform motion along a straight line unless it is compelled to change that state by an applied force.
Newton’s Second Law of Motion: It states that the rate of change of momentum of a body is proportional to the impressed force and acts in the direction of the force.
Newton’s Third Law of Motion: It states that to every action, there is an equal and opposite reaction.
Salient Features of First Law:
- This law qualitatively defines the force.
- The inability of the body to change its state is called inertia. So, it is known as the law of inertia of Galilio.
- Inertia resists change in the state of motion of the body.
- Force tends to overcome inertia.
Some interesting examples of First Law
- If a body is thrown up from a uniformly moving train then it returns back to the same place.
- When a stationery vehicle suddenly moves then the passengers inside the vehicle fall backwards.
- When a carpet or a blanket is beaten with a stick then the dust particles are separated out of it.
- A coin on the cardboard placed on glass falls into the glass if it is given a sudden jerk.
- If a moving vehicle suddenly stops then the passengers inside the vehicle bend forward.
- The window glass pane is bored by a bullet from a gun.
Salient features of Newton’s Second Law:
A force F acting on a body is equal to its change in momentum per second.
F = Mass of the body × Change in velocity per second
Therefore, F = k ma (where k is a constant)
But, k =1, from experiments
F = ma
Weight – the weight of a body is equal to the force with which the body is attracted by the earth towards its centre.
Salient features of Newton’s Third Law
- Action – reaction forces always occur in pairs.
- If a body A exerts a force (action) on a body B, then B will exert an equal and opposite force (reaction) on A.
- When a force acts on a body then the reaction acts normally to the surface of the body.
- These forces are produced between objects by direct contact when they touch, or by gravitational forces, for example, when they are apart.
Some practical examples of Newton’s Third Law:
- If a ball is kicked upwards, the force on the ball by the kicker is equal and opposite of the force on the kicker by the ball. The initial upward acceleration of the ball is usually very much greater than the downward acceleration of the kicker because the mass of the ball is much less than that of the kicker.
- In the case of a rocket, the downward force on the burning gases from the exhaust is equal to the upward force on the rocket.
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