Posted by Anon on Wednesday, September 18, 2013 at 1:17am.
Mary wants to throw a can straight up into the air and then hit it with a second can. She wants the collision to occur at height h=5.0 m above the throw point. In addition, she knows that she needs t1=4.0 s between successive throws. Assuming that she throws both cans with the same speed. Take g to be 9.81 m/s2.
(a) How long it takes (in s) after the first can has been thrown into the air for the two cans to collide?
(b) Find the initial speed of the cans (in m/s).

Classical Mechanics  Graham, Wednesday, September 18, 2013 at 7:00am
Let the height of the first can be x, that of the second can be y; and both cans be thrown at speed v.
x(t) = v t  g t^2 /2
y(t) = v (tt1)  g (tt1)^2 /2
Find t when x(t) = y(t) = h
From the first height equation:
v = h/t + gt/2
Substituted into the second.
h = (h/t + gt/2)(tt1)  g(tt1)^2/2
Simplify into a quadratic and solve for t.
...
Once you have t then use (from above):
v = h/t + gt/2 
Classical Mechanics  Anon, Wednesday, September 18, 2013 at 2:37pm
thank you it help me!