This is probably child's play to you guys but I'm fucking struggling with this shit.

A girl weighing 450 N sits on one end of a see-saw that is 3 meters long, and is pivoted 1.3m away from the child. If the see-saw is just balanced when a boy sits at the opposite end, what is his weight? Neglect the weight of the see-saw.

How the fuck do I figure this shit out.

Well I think the first step is to figure out what N means.

For the see-saw to balance, the pivot has to be at the center of mass. I'm hoping you know how to do center of mass in 1 dimension?

Use moments, if you don't know what they are I don't know why you were given this problem... but anyway

For there to be equilibrium, the sum of the moments around a point have to equal zero (i.e. get cancelled, so...

(450 x 1.3) + (boy x 1.7) = 0

thefore, boy = 344.18N

I hope you understood what I did, if not I suggest you research moments.

Oh and to the second poster, N = Newtons

yep moments is the way to go. basically its torque.

a 'moment' is the turning force on an axis. in this case, its the pivot on the see saw.

moment = force * distance from axis. thats why its easier to turn a spanner if you hold on the end rather than close to the head.

also, know that anticlockwise moment is positive.

so if the seesaw is equal, the moment made by the girl = moment from the boy

moment made by girl = 450 * 1.3
moment made by boy = x*1.7 (since the see saw is 3, distance between the pivot and the boy is 3-1.3)

450*1.3 + -(x*1.7) = 0 (its 0 because there is no moment when both people are on it. that is, its stable)
450*1.3 = x*1.7
x = 450*1.3/1.7
=344N

Ah, understood- thanks for the help guys. My professor is totally worthless.