# Homework Help: Bouncing Half Pendulum Math (Arccos problems)

1. Mar 25, 2013

### brandonhil7

So what I've got is a ball on a string hanging against a wall. When lifted and dropped, it forms a sort of half pendulum motion, but it looses energy very quickly as compared to a full pendulum. I need to calculate energy loss on each bounce, given only the time between each bounce. The solution to me seems to be relating height to half the time between each bounce, and then using height to calculate potential energy at the maximum point in each swing. Its the math that is giving me problems.

v = $\frac{s}{t}$
s = rθ
∴ v = $\frac{rθ}{t}$

E = 0.5mv2 = mgh
0.5v2 = gh
v2 = 19.62h
∴ $\frac{r^{2}θ^{2}}{t^{2}}$ = 19.62h

to relate theta to height, a horizontal line can be imagined from the wall to the ball creating a right angled triangle with the hypotenuse being r, the adjacent wall being equal to r-h.

∴ cos(θ)r = r - h
θ = arccos($\frac{r-h}{r}$)

given this:
$\frac{r^{2}(arccos(\frac{r-h}{r}))^{2}}{t^{2}}$ = 19.62h

and if r is then assumed to be 1 m:
$\frac{(arccos(1-h))^{2}}{t^{2}}$ = 19.62h

And this is where I'm stuck. I cant figure out how to get rid of the arccos, and i cant find any identities to help.
$(arccos(1-h))^{2}= 19.62ht^{2}$
what i need to do is solve for H
Please note that i do indeed understand what arccos is, it is the squared that id like to get rid of.

Any help would be appreciated.