Third Law of Motion Class 9 Physics

Hey students, have thought ever that, why a cricket ball thrown on wall bounces back to you again? When you applied force, it strikes on wall, but does wall also applied the force on wall so it bounces towards you? It isn’t a strange fact that how wall will exert force on a ball? How a heavier satellite gets lifted up, though they have exerted thrust in downward direction? Answer lies in third law of motion. So let’s learn it in detail.

Newton’s third law states that ‘for every action force, there is equal, opposite and instantaneous reaction force.’

During the reaction, the direction of velocity of object is always gets reversed so as to reverse the direction of force too.

Very common but interesting examples of 3rd law can be explained below.

Eg: Coolie applied force of 5000 N to push a trolley containing luggage of mass 200 kg. Find the opposite force by trolley and resultant force on trolley. (Use, g= 10 m/s2)

Ans: F1=5000N, m= 200 kg, g= 10 m/s2

Here force exerted by trolley on coolie is equal to weight of trolley

W= mg= 200×10=2000 N

The direction of this force is opposite to force applied by coolie,

Hence the resultant force on trolley will be, F= F1 – W= 5000-2000 N.

Eg: If a boy of mass 50 kg skating on smooth surface and suddenly collides with rigid wall and exerts force of 50 N on wall, what will the force exerted by wall on boy? What will the acceleration of boy during the collision?

Ans: F = 50 N, m = 50 kg

According to Newton’s 3rd law of motion, force exerted by wall on boy will same as that of force exerted by boy on wall in opposite direction, hence F = 50 N

Now using Newton’s 2nd law of motion,

Acceleration of boy can be given as,

F = ma

A = F/m = 50/50 = 1 m/s2


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