Question

A ball is thrown up with a velocity of 15 ms^-1 from the top of a building of height 20 m from the ground level. The time taken by the ball to reach the ground is (take g = 10 ms^-2):
1. 3.5 s
2. 6.0 s
3. 4.5 s
4. 4.0 s

Answer 1

Correct Answer - Option 4 : 4.0 s

CONCEPT:

• When a body moves in a straight line under constant acceleration, it follows three equations of motion.
• All unknowns are found by these equations.

1. v = u + at;
2. s = ut + (1/2) at2 
3. v2 = u2 + 2as

Where u is initial velocity, v is final velocity, a is constant acceleration, t is time and s is displacement.

CALCULATION:

Taking upward direction as +ve and downward -ve.

Given that u = 15 ms-1;  h = -20 m; a = -g = -10 ms-2

s = ut + (1/2) at2 ​

-20 = 15 × t + (1/2) (-10) t2 ​

Solving equation t = -1 (not possible) and 4

So t = 4 sec

Hence the correct answer is option 4.