Newton's Laws of Motion

The laws of motion help us understand the behaviour of objects when they are moving, standing still and their behaviour when forces act upon them. There are three laws of motion. Here in this article, we have provided a brief description of each of the laws clearly.

Newton’s Law of Motion

Sir Isaac Newton was an English mathematician, astronomer and physicist who gave three laws which proved to be fundamental laws for describing the motion of a body. These are generally known as Newton’s laws of motion. We will discuss each law of motion one by one in detail.

Newton’s First Law of Motion:

Newton's First Law

Newton’s first law of motion states that “A body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it”. Suppose a block is kept on the floor, it will remain at rest until we apply some external force to it. Also, we know that it takes us more effort or force to move a heavy mass. This is directly related to a property known as Inertia. This law is also known as the law of inertia.

Newton’s Second Law of Motion:

The first law has already given us a qualitative definition of force. Now we are interested in finding out its magnitude. According to Newton’s second law of motion, the net force experienced by a body is directly proportional to the rate of change of momentum of the body. It can be written as:

Newton's Second Law

\(f\propto \frac{dP}{dt}\\ \\ \Rightarrow f\propto \frac{mv-mu}{t}\\ \\ \Rightarrow f\propto\frac{m(v-u)}{t}\\ \\ \Rightarrow f\: \propto \: ma\\ \\ \Rightarrow f=k\, ma\)
Where k is the constant of proportionality and it comes out to be 1 when the values are taken in SI unit. Hence the final expression will be,

F = ma

Problems On Newton’s Second Law Of Motion

Suppose a body of mass 10kg is having an acceleration of 5m/s2. What is the net force experienced by it?

Solution:

From the second law of motion, we know,

F = ma

Putting the values we get,

F = (10 x 5) N

F = 50N

So the body is experiencing a net force of 50N.

Newton’s Third Law of Motion:

Newton's Third Law

According to Newton’s third law of motion, for every action, there is an equal and opposite reaction. Forces are always found in pairs. For instance, when you sit on a chair, your body exerts a force downward and that chair needs to exert an equal force upward or else the chair will collapse.


Important questions on Newton’s law of motion

Q1: State Newton’s first law of motion.

Ans: Newton’s first law of motion states that A body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it.

Q2: What is the SI unit of Force

Ans: SI unit of Force is newton

Q3: State Newton’s second law of motion.

Ans: Newton’s second law of motion states that the net force experienced by a body is directly proportional to the rate of change of momentum of the body.

Q4: How do force, mass, and acceleration are related?

Ans: The force, mass, and acceleration are related as F=ma

Q5: If the mass of the body increases keeping the acceleration constant. What happens to the net force?

Ans: If the mass of the body increases keeping the acceleration constant. The net force increases.

Q6: For a constant mass. If acceleration decreases, what happens to net force?

Ans: For a constant mass. If acceleration decreases, the net force also decreases.

Q7: State Newton’s third law of motion.

Ans: Newton’s third law of motion states that for every action, there is an equal and opposite reaction.

Q8: If the rate of change of momentum is small. What will be the net resultant force?

Ans: If the rate of change of momentum is small. The net resultant force will also be small.

Q9: If Mr.Shankha wants to move a 10 kg and 20 kg solid block from the rest. Which block needs more force?

Ans: If Mr.Shankha wants to move a 10 kg and 20 kg solid block from the rest block with 20kg needs higher force.

Q10: Fill in the blank. Newton’s first law of motion is alternatively called as _________.

Ans: Newton’s first law of motion is alternatively called a law of inertia.

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Practise This Question

When a football player kicks a ball, he experiences a force on his leg which is equal to the force he exerted on the ball.