# Kinematic #2

• Sep 7th 2009, 12:36 PM
jsu03
Kinematic #2
Hey, can someone help me with this problem. Thank you very much!

Question: Mitch is skydiving, he jumps out of the plane and reaches a terminal velocity of 20 m/s. When he is 1000 m above the ground, Skid, standing directly below Mitch fires a rocket at Mitch. The rocket has an initial velocity of 125 m/s.
a) How much time does Mitch have to figure out how to survive this predicament?
b) Assuming Mitch does not have his wits about him, at what location above the ground will the rocket intersect with him?

What I got so far:
a) x = 1000 - 20t + 1/2 (-9.8)t^2
x = 20 +/- squareroot 20^2 - 4(-4.9)(1000)/ 2(-4.9)
= 12.38 s , the correct answer is 10.9 but I got 12.38, so I have no idea what I did wrong.
b) x = 1000 - 20t + 1/2 (-9.8)t^2 when t = 10.9 = incorrect answer again.
b)
• Sep 7th 2009, 02:32 PM
skeeter
Quote:

Originally Posted by jsu03
Hey, can someone help me with this problem. Thank you very much!

Question: Mitch is skydiving, he jumps out of the plane and reaches a terminal velocity of 20 m/s. When he is 1000 m above the ground, Skid, standing directly below Mitch fires a rocket at Mitch. The rocket has an initial velocity of 125 m/s.
a) How much time does Mitch have to figure out how to survive this predicament?
b) Assuming Mitch does not have his wits about him, at what location above the ground will the rocket intersect with him?

assuming the rocket is a projectile the moment it leaves the ground ...

(distance skydiver falls) + (distance rocket goes up) = 1000 when they meet

\$\displaystyle 20t + (125t - 4.9t^2) = 1000\$

\$\displaystyle -4.9t^2 + 145t - 1000 = 0\$
• Sep 7th 2009, 05:20 PM
jsu03
part b
thanks a lot for part a, now for part b I got

v^2 = V not^2 + 2a delta y
delta y = v^2 - v not^2/2a
= 125^2/2(9.8) = 797 m , the correct answer is 782, Can u tell me what I did wrong?