# Projectile Motion Question

• Feb 10th 2011, 07:11 AM
ksinger
Projectile Motion Question
I'm Stuck with how to approach this problem, can anyone help!

A ball is thrown at from a point on the ground 10m from a vertical wall and just clears this wall and another parallel wall. The distance between the walls is 20m and the height of each wall is 2m. Find the angle of projection of the ball and the greatest height above the tops of the walls attained.
• Feb 10th 2011, 07:47 AM
TheEmptySet
Quote:

Originally Posted by ksinger
I'm Stuck with how to approach this problem, can anyone help!

A ball is thrown at from a point on the ground 10m from a vertical wall and just clears this wall and another parallel wall. The distance between the walls is 20m and the height of each wall is 2m. Find the angle of projection of the ball and the greatest height above the tops of the walls attained.

This should get you started.

First I am assuming that the only force acting on the ball is gravity. If that is the cases then the ball flies in the form of a parabola.

So a parabola can be written in the form $\displaystyle y=ax^2+bx+c$

Now from the problem we know that it must pass through these three points (if we put the origin at the launch point) $\displaystyle (0,0), \quad (10,2), \quad (30,2)$

Plugging these ordered pairs into the equation gives a system of 3 equations in three unknows.
$\displaystyle \begin{array}{r r r r} 0= & a(0)^2&+b(0)& +c \\ 2= & a(10)^2&+b(10)& +c \\ 2= & a(30)^2&+b(30)& +c \end{array}$

If you solve this you will have the equation of the parabola.
• Feb 13th 2011, 01:07 PM
ksinger
Thanks for your help - i managed to get it!
I solved the equation of the parabola then calculated the height over the walls at x=20, giving us the greatest height.
We know the range = 40 so its a case of dividing the Greatest Height formula by the Range formula the expanding to calculate the angle.
I'm just about to post another question - any help would be much appreciated although i've just noticed that i posted this thread in the pre-univertsiy math help forum by mistake!