# Homework Help: Projectile motion on a hemisphere

1. Mar 11, 2012

1. The problem statement, all variables and given/known data
A person standing on the top of a hemispherical rock of radius R kicks a ball (initially at rest on the top of the rock) to give it horizontal velocity $v_i$
What must be it's minimum initial speed if the ball is never to hit the rock after it is kicked?

2. Relevant equations
$v=v_i+at,\\ v^2=v_i ^2 + 2ar, \\ r=v_i t +\frac{1}{2}at^2$

3. The attempt at a solution

I'm not sure how to do this one as a parabola and a semicircle are 2 different shaped curves, a little nudge in the right direction would be helpful, thanks guys.

2. Mar 11, 2012

### tiny-tim

Write out the two equations, and see where they intersect …

what do you get?

3. Mar 11, 2012

Hmm ok...
Well the eqn of the semicircle is $y=\sqrt{R^2 - x^2}$

The eqn of motion is $y = \frac{1}{2}at^2 \\ x = v_it \\ y = \frac{1}{2}a(\frac{x}{v_i})^2 \\ 2yv_i^2 = a x^2?$

4. Mar 11, 2012

### tiny-tim

= a(R2 - y2) ?

5. Mar 11, 2012

$ay^2 + 2yv_i^2 - aR^2 = 0 \\ 4v_i^4 +4a^2R^2 < 0 \\ v_i^4 < -a^2R^2$

and that's not right...

Last edited: Mar 11, 2012
6. Mar 11, 2012

### tiny-tim

after that, you've lost me

7. Mar 11, 2012

Solve the quadratic, they don't touch so the discriminate needs to be < 0 (typo in the other post, I'll fix it)

Last edited: Mar 11, 2012
8. Mar 11, 2012

### tiny-tim

ah, sorry, i'd forgotten what the question asked for!
yes, that's fine

9. Mar 11, 2012

Yes but it's a^2, so it cancels out the negative, leaving me with -g^2R^2?

10. Mar 11, 2012

### tiny-tim

hmm … you're right!

ok, let's go back and check your original equations …
ah! should be y = R + 1/2 at2 !

(no wonder it couldn't avoid hitting the circle!! )

11. Mar 11, 2012

Haha well spotted

So, now with that fix, I get $2v_i^2(y-R)=ax^2 \\ 2v_i^2(y-R)=a(R^2 - y^2) \\ 2v_i^2 = -a(R+y) \\ v_i^2 = \frac{g}{2}(R+y)$

12. Mar 11, 2012

### tiny-tim

looks good!

(but on my recent performance, i could be missing something! )

13. Mar 11, 2012

Haha yeah, we've all been missing simple things..
I don't see how this ties in with it not hitting the semi circle though.

14. Mar 11, 2012

### tiny-tim

because …
gives you the y-coordinate where it hits the circle

for example, if v = 0, the ball drops straight down, and hits the circle at y = -R !

ok, now what happens to y as you increase v ?

(btw, the other solution, y = R for any value of v was eliminated from the equation when you divided it by (y - R) )

15. Mar 11, 2012

So I want y = 0?

16. Mar 11, 2012

### tiny-tim

in a moment, i'm going out for an hour or so

draw yourself some diagrams of a parabola touching and cutting a circle, and see what happens as the parabola changes shape

(also, think how many times can a parabola touch and cut a circle?)

17. Mar 11, 2012