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Newton's Laws of Motion

Isaac Newton was the greatest scientist that had ever lived. The illustrious natural philosopher was born at Woolsthrope Manor in Lincolnshire. He was one of the greatest thinkers of modern times.
Isaac Newton
In 1666, the fall of an apple led to his greatest discovery of all - the law of gravitation. He was knighted in 1705. His new ideas had given the great concepts and laws regarding motion and gravitation. His laws of motion could show the clear picture of motion of object when force is applied on the body. Further the concept of gravitation and universal law of gravitation had given the new dimensions to Physics. The queries like why the things fall downwards? & how the universal objects are capable of moving without any forces being applied, have been answered after the development of concept of gravitation. Thus it is called Isaac Newton Gravity theory which is a world wide well known theory which answered so many queries and helped a lot in the many scientific calculations.

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Newtons Three Laws of Motion

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First Law of Motion:
The first law of motion is related to the law of inertia, it says that the body will remain in rest or in continue motion, unless some external force is applied on it.
The First Law States that
Every body continues its state of rest or of uniform motion in a straight line, unless compelled by some external force to act otherwise.Inertia: Its the property of body by the virtue of which the body remains in rest or in continue motion, unless some external force is applied OR any body has tendency to oppose the external force applied due to the Inertia. This is also called as Law of inertia.

Second Law of Motion:

Newton's second law of motion states that rate of change of momentum is directly proportional to applied force and takes place in the same direction as the applied force.
Second Law of Motion
Momentum, p = mass x velocity
Momentum is a property of the body possessed by virtue of its mass and velocity. It is the product of mass of the body and its velocity.

According to the second law of motion, F $\propto$ $\frac {dP} {dt}$
where dP is change in momentum and
dt is time taken.

F = $\frac{m(v_{f} - v_{i})}{t} = \frac{m(v_{f} - v_{i})}{t}$

F = $\frac {m(v_{f} - v_{i})} {t}$ = ma

Hence we conclude from second law of motion that when the force is applied on the mass m, then the acceleration is produced in it.
Therefore, a = $\frac {F} {m}$.
Thus, for same force larger acceleration is produced in smaller mass or
we can say that we need larger force to produce the same amount of acceleration in larger mass.
One Newton is the force applied on body of mass 1kg to produce an acceleration of 1m/s2.
Third Law of Motion
According to the third law of motion
To every action there is equal and opposite reaction.

Examples of Newtons Law of Motion

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Newton's First Law Example

1) Why do the dry leaves and fruits fall when we shake the tree?
When we shake the tree, the fruits and dry leaves remain at their position due to first law of motion, the inertia resist the motion, so, as the branches shake, they get detached from the tree.

2) Passengers fall forward when bus suddenly stops. Why?
The passengers are in motion due to first law and as the bus suddenly stops due to inertia their upper body opposes the force and continues to remain in motion.

3) How do you say that Mass is measure of inertia?
When we push the empty box, it easily moves from its position but when we apply the same force to the filled box, we are unable to move it, this is because the box is filled now and its mass has increased, so, the capacity of box to oppose the external force increases.
Thus, we conclude that the mass is measure of inertia, larger is the mass, larger is the capacity of the body to oppose external force and hence larger is inertia.

Newton's Second Law Example

1) In case of an apple falling from a tree, the apple is accelerated. So, its velocity changes as it is hanging on the tree and moves towards the ground.
Thus, by Newton's 2nd law there must be a force that acts on the apple to cause this acceleration. If we call this force "gravity", then the associated acceleration is "acceleration due to gravity".
Newton's Second Law Example

Newtons Third Law Example

1) In case of inflated balloon, the air rushing outward is action while the balloon going upward is reaction.
Newtons Third Law Example
2) Same thing happens in the rocket propulsion.
The rocket leaves or exerts fuel with some force, the fuel in return produces an equal force on the rocket, which makes the rocket accelerate in the forward direction.
Example of Newtons Third Law

Newtons Laws of Motion Problems

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You can see the Newton's law problems below:

Solved Example

Question: (1) Calculate the force required to produce the acceleration of 5m/s in a body of mass 40kg.
Options :
  • 200N
  • 500N
  • 700N
  • 250N
  • 60N
F = m x a = 40 x 5

F = 200N


Correct answer is 200N
More topics in Newton's Laws of Motion
Inertia Newton's First Law of Motion
Newton's Second Law of Motion Newton's Third Law of Motion
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