Question

One day I decide I want to see how high I can throw a rock. I throw a rock straight up in the air and it has a speed of 30 m/s when it leaves my hand. I then realize this was foolish, as I need to get out of the way before the rock comes back down and hits me on the head. How many seconds do I have to move out of the way after I throw the rock?

Details and assumptions
The acceleration of gravity is −9.8 m/s^2.
Treat me as a point mass.
Upward motion:
h=v₀t-gt²/2
v= v₀-gt
v=0 => t=v₀/g
h= v²/2g =30²/2•9.8=45.9 m
t=sqrt{2h/g} =sqrt{2•45.9/9.8} = 3.06 s.
t(total)=2•t = 2•3.06 =6.12 s.

Related Questions

Bob, who has a mass of 60 , can throw a 800 rock with a speed of 26 . The distance through which his... A boy wishes to throw a rock into the river below the cliff where he is standing. The cliff is 50 M... To estimate how high the cliff is you throw a rock off the edge and 7.8 s later you hear the rock hi... Bob, who has a mass of 75 kg, can throw a 500 g rock with a speed of 30 m/s. The distance through...