Newton’s law of motion Class 9
Newton’s law of motion:
Sir Isaac Newton studied motion, extended the idea of Galileo and gave three laws of motion known as Newton’s laws of motion.
- Newton’s 1st law of motion
- Newton’s 2nd law of motion
- Newton’s 3rd law of motion
Newton’s first law of motion or Law of inertia:
Newton’s first law of motion or Law of inertia:
A body maintains its state until an external force is applied on it.
Force and laws of motion 🖱️👉 NCERT in-text questions solution
Newton’s second law of motion:
The rate of change of momentum of a body is directly proportional to the amount of force applied to the body, and it takes place in the direction of the applied force.
F ∝ Change in momentum / time taken
F = (final momentum – initial momentum) / time taken (t)
F = (mv-mu) / t
F = m (v-u) / t From equation of motion 1st ⇒ a = (v-u) / t
F = m (v-u) / t = ma
F = ma
Newton’s second law of motion is the real law of motion, and the other two laws can be derived from it.
Unit of force:
SI Unit of force is newton(N).
F = ma
1N = kg m/s2
Definition of 1N:
Force acting on a body is said to be 1 N if it can produce an acceleration of 1 m/s2 in a body of mass 1 kg.
Newton’s third law of motion:
To every action, there is always an equal and opposite reaction.
A single force can never exist. The forces always exist in pairs; one is called an “action force” and the other is a “reaction force.”
Action force = – Reaction force
Here, negative sign indicate that both forces are in opposite direction.
QUIZ-1 Force and Laws of Motion 🖱️👉 START
Newton’s laws: Important Questions
Question 1:
Why is Newton’s first law of motion also called law of Inertia ?
Ans: According to Newton’s first law of motion, a body cannot change its state of rest or state of motion in the absence of external force, and this inability is also called inertia. That’s why Newton’s first law of motion is called the law of Inertia.
Question 2:
Why Newton’s second law is called real law of motion?
Ans: Newton’s second law is called real law of motion because Newton’s 1st law and 3rd law are derived from it.