M2 projectile Q help

A vertical cliff is 73.5 m high. Two stones A and B are projected simultaneously. Stone A is projected horizontally from the top of the cliff with speed 28 m s–1. Stone B is projected from the bottom of the cliff with speed 35 m s–1 at an angle  above the horizontal. The stones move freely under gravity in the same vertical plane and collide in mid-air.

(b) Find the time which elapses between the instant when the stones are projected and the instant when they collide.

cos=4/5

so how would you do this? i can't put the image of the stones colliding with one motion, like horizontal or vertical.

Reply 1

TenOfThem

Find an expression in terms of t for the horizontal displacement of each and equate

Is that box an unknown angle

If so do the same for vertical displacement

2 equations in 2 unknowns, solve simultaneously

Reply 2

cooldudeman

Original post by TenOfThem

box isnt unknown, cos=4/5
and i tried that, if we use the s=ut+0.5at^2 equation with horizontal motion, both equations give s=28t.
and with vert, i get s=0.5gt^2 and s= 21 - 0.5gt^2 which doesnt give the correct ans when equating.

(edited 12 years ago)

Reply 3

TenOfThem

Original post by cooldudeman

box isnt unknown, cos=4/5
and i tried that, if we use the s=ut+0.5at^2 equation with horizontal motion, both equations give s=28t.
and with vert, i get s=0.5gt and s= 21 - 0.5gt^2 which doesnt give the correct ans when equating.