Quite simple.


Use the same equation and substitute t = 1 to find the distance the second object is above the ground first after 1 second.

y = 17(1) - 5(1²)
y = 17 - 5
y = 12


So the second object is 12m above ground.


But for the first object, it is already 2.5 seconds after it has been projected upwards.

(1.5 seconds + 1 second = 2.5 seconds)


So substitute t = 2.5


y = 17(2.5) - 5(2.5²)
y = 42.5 - 31.25
y = 11.25

So the first object is 11.25m above ground.



Distance between them = 12m - 11.25m = 0.75m

Edited