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Re: newbie here,, need help with parametrics

  • To: mathgroup at smc.vnet.net
  • Subject: [mg96503] Re: newbie here,, need help with parametrics
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sun, 15 Feb 2009 03:19:34 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <gn5u3e$gko$1@smc.vnet.net>

In article <gn5u3e$gko$1 at smc.vnet.net>, "R.J." <nemocamaro at epix.net> 
wrote:

> I am using Mathematica and i'd rather be using derive,,,,, but anyways,,  
> heres the problem i have to do, a baseball is thrown from the stands 32 feet 
> from the ground at an angle of 30 degrees above the horizontal, initial 
> velocity is 32 feet per second.
> 
> what i want to do is find how to plot this in mathematica,, the graph i keep 
> getting says it hits the ground after .8 seconds,   and i know thats not 
> right,,, here is the formula i am using
> 
> r = (x + v Cos a) t) i + (y + (v Sin a) t - 1/2 gt^2) j
> 
> where x is the initial x position and y is the initial height, v is the 
> initial velocity a is the angle (30*) t is time and g is the gravitational 
> constant..
> 
> i'm stuck,,, any help?

Note that you should have posted the actual code you used, so it would 
have been possible to give you precise recommendations on what you did. 

Anyway, assuming I have correctly interpreted your requirements and 
notation (as well as correctly filled in the missing information), the 
following expressions produce two plots that suggest that the ball hits 
the ground for 4 < t < 5 seconds. Now, why not using Solve[] to get the 
precise value?

x[t_] = (x + v*Cos [a])* t;
y[t_] = (y + (v*Sin[a])*t - 1/2 *g*t^2);
ParametricPlot[{x[t], y[t]} /. {x -> 0, y -> 30, v -> 32, a -> Pi/6, 
   g -> 9.81}, {t, 0, 4}]
ParametricPlot[{x[t], y[t]} /. {x -> 0, y -> 30, v -> 32, a -> Pi/6, 
   g -> 9.81}, {t, 0, 5}]

Regards,
--Jean-Marc


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