Question

An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. the initial and the final points of the path of the object lie on the same horizontal line. what is the work done by the force of gravity on the object ?

Answer 1

(i) When an object is thrown at a certain angle, it rises to a certain height and then comes back while moving in a parabolic path. Let h be the maximum height to which it rises.
As it rises, its kinetic energy decreases but its potential energy increases.
Potential energy gained by it in rising to the highest point = work done against gravity. 
= - mg x h = -mgh
As the body falls down, it gains kinetic energy = work done by the force due to gravity.
mg x h = mgh
Net work done by the force due to gravity
= - mgh + mgh = 0

Answer 2

┏─━─━─━─━∞◆∞━─━─━─━─┓ ✭✮ӇЄƦЄ ƖƧ ƳƠƲƦ ƛƝƧƜЄƦ✮✭ ┗─━─━─━─━∞◆∞━─━─━─━─┛

Since, the initial and final point of the path of the object lie on the same horizontal line, perpendicular to the direction of the force of gravity.

θ = 90º, W = F s cos 90º = 0.

Thus, no work is done.

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