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.

(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.

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

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.

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.

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

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

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!!

Original post by teewizzle07

hello is there any chance you can help me with this question part b

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