A boy throws a stone with a horizontal velocity of 10 m s-1 from the top of a building. The height of the building is 8.0 m.

 Question 38

A boy throws a stone with a horizontal velocity of 10 m s^-1 from the top of a building. The height of the building is 8.0 m. The stone travels along a curved path until it hits the ground, as shown in the diagram.

How long does it take the stone to reach the ground? (Air resistance can be neglected.)

A 0.61 s                       B 0.80 s                       C 1.3 s                         D 1.6 s

 

 

 

Reference: Past Exam Paper – November 2015 Paper 13 Q9

 

 

 

Solution:

Answer: C.

Since the stone is thrown horizontally, its vertical component of initial velocity is zero.

Consider the vertical motion.

Initial velocity, u = 0

Distance, s = 8.0 m

Acceleration, a = 9.81 m s^-2

Time, t = ???

Consider the equation of uniformly accelerated motion.

s = ut + ½ at²

8 = 0 + ½ × 9.81 × t²

Time t = √(2×8 / 9.81) = 1.27 s