Question

a stone is thrown vertically upward with a speed of 20m/s the time (in seconds) taken by it to come back to its point of projection is(take g=10)

Answer 1

Let us take the point of projection as the origin of coordinate system. Let the up direction be taken as positive.

The initial velocity of the body = + 20 m/ß

Acceleration due to gravity a= - 10 m/s²

Let the time taken to return to the ground be t second

Since the objects return to the ground, the displacement s= 0 m

Using the relation; s = u × t + ½ a t².

Substituting various values

0 = 20 t - ½ × 10× t²; ==> 20 t - 5 t² = 0;

5 t² - 20 t = 0; ==> 5 t (t - 4) = 0; ==> t= 0 sec. or t = 4 sec.

The body would be at the ground at the beginning ie at t =0 and again at t = 4 s

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