# Homework Help: Projectile motion graphs

1. Sep 30, 2005

### 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

2. Sep 30, 2005

### 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

3. Sep 30, 2005

### 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

Last edited: Sep 30, 2005
4. Sep 30, 2005

### 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
$$\rho = v(x)^2/a_n$$
$$\rho = v(x)^2/(gv_x(x)/v(x)) = v(x)^3/(gv_x(x))$$

5. Sep 30, 2005

### ehild

You can be sure in your derivation.

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

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

ehild