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March 29, 2015

March 29, 2015

Posted by **Roberto** on Saturday, February 12, 2011 at 3:22pm.

(1st C. BCE). You are responsible for a catapult, a device with a long spoonlike arm used

to lob stones or pumpkins over the walls of Gallic oppida. These fortresses are protected by

high walls rising from a deep, wide moat; you must fire your missiles from the far side of the

moat. You are expected to fire forward.

a) Caesar has asked you for an algebraic formula for the maximum height of wall you

can clear from across a moat of width x, if the initial speed of your projectile is v0,

the magnitude of the acceleration of gravity is g, and you can launch at any angle you

choose. What formula do you give him?* Assume aerodynamic forces are negligible

and disregard the height of the catapult itself.

b) For a certain value of x your formula gives zero height. Explain why-to what does

this correspond?

c)When you shoot to clear a wall of maximum height per the formula of part a, is your

missile ascending, descending, or at the peak of its trajectory when it clears the wall?

- Physics -
**bobpursley**, Saturday, February 12, 2011 at 3:42pmIn this case, Caesar doesn't know the purpose of the Catapult. It is supposed to knock down the walls, not go over them.

hf=hi+vo*sintheta*t-1/2 g t^2

but horizontal distance is "x", or

x=vo*cosTheta*t

or t=x/(vo*cosTheta)

then

h= vo*sinTheta*x/(vocostheta)-g/2 *(x/vocostheta)^2

h= x*tanTheta-g/2 * (x/vo)^2 sec^2 theta

now when it h zero?

=x(tantheta -g/2 *x/vo^2*sec^2Theta)

so h is zero when x is zero, and when

x=2vo^2*sintheta*costheta /g

but is not that the max distance?

check my work.

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