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  • Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground.

Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of the initial and final momentums of the ball are the same. Yet, it is not an example of conservation of momentum. Explain why?

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Sol. When we throw a ball upwards with some speed, it’s its speed decreases because the acceleration of gravity acts downward. It goes up to a height till its velocity becomes zero. Force of gravity still acts in the downward direction, which increases the speed in the downward direction. When the ball reaches the same level, it acquires the same speed but in opposite direction. We know that momentum is a vector quantity, which has the same direction as of velocity.

So in the above example, the momentum has the same magnitude but its direction is totally reversed in the journey.

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Gurleen Kaur

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