A stone is projected vertically upward with a velocity of 20ms'1. Two seconds later a second stone is similarly projected with the same velocity. When the two stones meet, the second one is rising at a velocity of 10m/S². Neglecting air resistance

Physics Jamb, Waec, Neco Past Questions & Answers on Speed, Velocity and Acceleration

QUESTIONS 31. A stone is projected vertically upward with a velocity of 20ms'¹. Two seconds later a second stone is similarly projected with the same velocity. When the two stones meet, the second one is rising at a velocity of 10m/S². Neglecting air resistance, calculate the 

(i)length of time the second stone is in motion before they meet, 

(ii) Velocity of the first stone when they meet (g= 10m/s²)

WAEC 1998  ANS: I. 1s; ii) 10m/s²

SOLUTIONS 

The second stone moves upwards and its acceleration g = 10m/s2. 

Final velocity v= 10m/s

Initial velocity u= 20m/s

Time t= ?

From equation v = u - gt

10 = 20 - 10t 

10t = 20 - 10

10t = 10

10t/10 = 10/10

t = 1

The second stone has been in air for 1s before they met.

(ii) The first stone has been moving for (1 + 2)s = 3s

From equation v = u + gt

V = 2 - (10 x 3) 

V = 20 - 30 

V = -10m/s

The minus sign means the first stone is moving down and its velocity is 10m/s.