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Golf ball question, kinematics

  • Thread starter devanlevin
  • Start date
  • #1
devanlevin
a golf ball passes on its course exactly over the top of a tree standing 20m high and 40m away from the tee, the ball eventually lands 100m past the tree. what is the original velocity, speed and direction, of the stroke?

this is a question in kinematics and the only force to take into account is gravity,


using
V(t)=Vo+at
X(t)=Xo+Vot+½at²
V²-Vo²=2aΔx

Vx=const=cosθ*Vo
Vy(t)=sinθ*Vo+at

knowing that the ball passed through (40,20) i use the equation for X,
X(t)=40m=cosθ*Vo*t
t=40/cosθ*Vo

Y(t)=20m=sinθ*Vo*(40/cosθ*Vo)-4.9(40/cosθ*Vo)²
=40*tanθ-4.9(40/cosθ*Vo)²

now from here i dont know what to do, one equation with both values i need to find, θ and Vo, have i overlooked something, done something wrong or used the wrong equations for the case?

hope my english and the terms i used are correct, it is a question i have had to translate, thanks
 

Answers and Replies

  • #2
tiny-tim
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i dont know what to do, one equation with both values i need to find…
Hi devanlevin! :smile:

(btw, the English was fine, except just say "a golf ball passes exactly over the top of a tree …" :smile: )

Yes, you're right … you need one more equation …

you haven't used:
the ball eventually lands 100m past the tree
 
  • #3
devanlevin
how can i work this in to get an answer??
 
  • #4
tiny-tim
Science Advisor
Homework Helper
25,832
249
how can i work this in to get an answer??
Just do it!

You'll have two more equations, and only one more unknown (t2). :smile:

(it may … or may not … help to note that the maximum height is after 50 m horizontally)
 

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