A small metal ball thrown vertically upwards from the top of a tower with an initial velocity of 20ms-1. If the ball took a total of 6s to reach ground level, determine the height of the tower [g = 10ms-2]
-
A.
60m -
B.
80m -
C.
100m -
D.
120m
Correct Answer: Option A
Explanation
Time from tower to top, t = (frac{u – v}{g})
= (frac{20 – 0}{10})
= 2s
distance from tower to top
x = ut – (frac{1}{2})gt2
= (20 x 2) – ((frac{1}{2}) x 10 x 2 x 2)
= 20m
Time from the top to the ground, t = 6 – 2
= 4s
distance from top to ground,
x = ut – (frac{1}{2})gt2
= (0 x 4) + ((frac{1}{2}) x 10 x 4 x 4)
= 80cm
therefore, height of tower = 80 – 20
= 60m