# 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.
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
Original post by 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

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 11 years ago)
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.

That first s should be negative
Original post by TenOfThem
That first s should be negative

so -s=0.5gt^2 and s= 21 - 0.5gt^2? but that would give no solution.
Sorry

Misread

Had not realised one was from base of cliff

Perhaps you ad not either
Original post by cooldudeman
so -s=0.5gt^2 and s= 21 - 0.5gt^2? but that would give no solution.

21t

I get t=3.74
Original post by TenOfThem
21t

I get t=3.74

say the answer is t=3.5
Original post by cooldudeman
say the answer is t=3.5

Ok I will find some paper
Original post by cooldudeman
say the answer is t=3.5

Yep 3.5

With a diagram it is clear

Downward displacement of A plus upward displacement of B = height of cliff
(edited 11 years ago)
Original post by TenOfThem
Yep 3.5

With a diagram it is clear

Downward displacement of A plus upward displacement of B = height of cliff

oook..........
Crazy I'm using an 11 year old post to do my hwk!!
Original post by LoneWonder
Crazy I'm using an 11 year old post to do my hwk!!

fr
Original post by LoneWonder
Crazy I'm using an 11 year old post to do my hwk!!
hello is there any chance you can help me with this question part b
Original post by teewizzle07
hello is there any chance you can help me with this question part b
you need to find when the two distances add up to 73.5 m