Circular motion kid-on-a-swing problem

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Homework Statement

http://img98.imageshack.us/img98/4571/kidswing.jpg [Broken]

Mass (m) of the kid sitting on the swing is 35 kg, and its center of gravity is at point C (the meeting place of the swing's arm EC with the arch p-r, created by the movement of the center of gravity of the kid). If the kid swings to a max height (when angle Psi is 55 degrees), calculate:

A) The speed of the swing as it passes point D (Psi = 0 degrees)

(There are other clauses but I really wanna get A first)

The Attempt at a Solution

I used the same formula that I used in my last non-uniform circular motion. From some reason this time it didn't work!

http://img684.imageshack.us/img684/205/567wg.jpg [Broken]

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ehild
Homework Helper
What is h?
And I was in the belief that g is about 9.8 m/s2.

ehild

Gold Member
And I was in the belief that g is about 9.8
Isn't this exactly what I wrote? 9.81?

h is delta y...oh wait, I need delta y not delta x. One moment.

Okay, my delta y (or "h") should've been

2cos(55) = 1.147
h (or delta y) = 2-1.147 = 0.8528

ehild
Homework Helper
I see "g=2cos(35) " written on the sheet of paper just below the triangle

That h should be delta y, you are right. And the angle is 55° instead of 35 which is the mass of the child. Getting the correct result?

ehild

the kid swings to max height and makes angle of 55
so now find the height of this point from the lowest point in its motion (where you need to find velocity) (use radius, if i see correctly its given 2m?)
use energy conservation ... mgh = 0.5mv2

Gold Member
use energy conservation
What is the difference between the formula I used and "energy conservation"? I was under the impression it's one and the same.

That h should be delta y, you are right. And the angle is 55° instead of 35 which is the mass of the child. Getting the correct result?
Yes, thank you, I also solved the other clauses asking me to calculate tension and reaction forces :) !! Mechanics is fun!

What is the difference between the formula I used and "energy conservation"? I was under the impression it's one and the same.
sorry i didn't saw your answer :p
its same :)

anyway have you solved it now?

Gold Member
the kid swings to max height and makes angle of 55
so now find the height of this point from the lowest point in its motion (where you need to find velocity) (use radius, if i see correctly its given 2m?)
use energy conservation ... mgh = 0.5mv2

Btw why did your equation for "energy conservation" has mass in it and mine doesn't?

anyway have you solved it now?
Yes :) Thank you. I even solved the following clauses asking for the tension and the reaction forces at each of the 4 support beams.

http://img831.imageshack.us/img831/2180/physfor.jpg [Broken]

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Btw why did your equation for "energy conservation" has mass in it and mine doesn't?
they are same
mine has mass on both sides ... so they just gets cancelled.

Gold Member
I see :) thanks.