First slide

Third law

Question

Conservation of momentum in a collision between particles can be understood from

Moderate

A

conservation of energy
B

Newton's first law only
C

Newton's second law only
D

Both Newton's second and third law

Solution

We know that for a system, $F_{ext} = \frac{d p}{d t}$
(from Newton's second law)
If F_ext = 0, dp = 0 $\Rightarrow$ p = constant
Hence, momentum of a system will remain conserved, if external force on the system is zero.
In case of collision between particles equal and opposite forces will act on individual particles by Newton's third law.
Hence, total force on the system will be zero.
Note We should not confuse with system and individual
particles. As total force on the system of both particles is zero, but
force acts on individual particles.

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