Projectile Motion Graphs: Radius of Curvature as a Function of X and Time

[Warr]

A projectile is fired at an angle 30 degrees to the horizontal at a muzzle velocity of 450 m/s. Graph the radius of curvature as a function of x, and as a function of time.

I'm pretty clueless here:

Here's what I think:

\vec a = (v^2/\rho)\hat {e_n} + \dot v\hat {e_t}

where v is the velocity in the direction of motion for any given point in space along the parabola, \rho is the radius of curvature

so then from this

a_n = v^2/\rho

where a_n is the acceleration at any point in space along the parabola in the direction normal to the direction of motion

so therefore to find a plot of \rho in terms of x, I must find

\rho = v(x)^2/a_n(x)^2

so what I need to know is how to find the v(x) and a_n(x)

or am I going abouit this all wrong..thanks

 

[Warr]

ok I think I may have gotten v(x)^2

by setting
v_x(x) = v_ocos(30) (1)
and
v_y(x) = v_osin(30)-gt (2)

then substituing

t = x/{v_ocos(30)} into (2)

then I used

v(x)^2 = v_x(x)^2 + v_y(x)^2

after simplifying, got

v(x)^2 = v_o^2 - 2gxtan(30) - (gx/{v_ocos(30)})^2

so this is in parabolic form..which would make sense for the magnitude of v with respect to x..

but if this is right..I'm still confused about how to get a_n

:S

 

[Warr]

I actually just found a mistake on my v(x)..the last term should have a plus in front of it

still having trouble with a_n though

 

[Warr]

ok, I think I solved it, but I can't be completely sure

assuming my v(x) is right (with the exception that the sign before the 3rd term should be +ve and not -ve),

would it make sense that a_n (acceleration normal to the motion) would be given by
a_n = gcos(\theta), where \theta is the angle of the motion of the projectile relative to the horizontal at any x

such that I could calculated cos(\theta)

by trigonometry

cos(\theta) = v_x(x)/v(x)

so subbing back into the a_n equation

a_n=gv_x(x)/v(x)

and therefore I would be graphing the function
\rho = v(x)^2/a_n
\rho = v(x)^2/(gv_x(x)/v(x)) = v(x)^3/(gv_x(x))

 

[ehild]

ok, I think I solved it, but I can't be completely sure
assuming my v(x) is right (with the exception that the sign before the 3rd term should be +ve and not -ve),

You can be sure in your derivation.

v(x)^2 = v_o^2 - 2gxtan(30) + (gx/{v_ocos(30)})^2

would it make sense that a_n (acceleration normal to the motion) would be given by
a_n = gcos(\theta), where \theta is the angle of the motion of the projectile relative to the horizontal at any x

such that I could calculated cos(\theta)

by trigonometry

cos(\theta) = v_x(x)/v(x)

so subbing back into the a_n equation

a_n=gv_x(x)/v(x)

and therefore I would be graphing the function
\rho = v(x)^2/a_n
\rho = v(x)^2/(gv_x(x)/v(x)) = v(x)^3/(gv_x(x))

It is correct, you solved your problem by yourself, congratulation!

ehild