View Full Version : Balance the seesaw
aca123
Oct 7, 2009, 11:07 AM
Abe and Cindy are sitting on a seesaw. Abe weighs 680 newtons and sits 1.20 meters from the pivot point on the left. Cindy weighs 450 newtons and sits 1.50 meters from the pivot point on the right. The seesaw tips counter clockwise. How far back on the seesaw would Cindy need to be in order to balance the seesaw.
Unknown008
Oct 7, 2009, 11:49 AM
Use Moment = Force\, \times\, Perpendicular\, Distance
Counter Clockwise moment = Fd = 680 * 1.2 = 816 Nm
Clockwise moment = Fd = 450 * 1.5 = 675 Nm
The resultant is 141Nm counter clockwise (816-675). Now, that is not required. But if they have to be balanced, both moments have to be equal.
Clockwise Fd = Counterclockwise Fd
680 * 1.2 = 450 * d
Where d is the distance of Cindy from the pivot point.
Solve for d, and you're done. :)
sukarsimba
Oct 11, 2009, 02:40 PM
hi jerry.
I know I must sound thick to you, but given that:
680 * 1.2 = 450 * d
I still don't get how you find solve for d... lol
for a thickhead like me that's another equation to learn... lol
wish I could get my head around all this physics stuff
sukarsimba
Oct 11, 2009, 02:59 PM
so...
450 * "something" = 816..
that's got to be a large decimal hasn't it??
or am I missing something
sukarsimba
Oct 11, 2009, 03:12 PM
I get 680 x 1.2 = 450 * 1.8133333333
surly that can't be right can it?
god I am thick... sorry for bugging you...
its just that this is bugging me...
Unknown008
Oct 14, 2009, 08:42 AM
Sorry for late reply. I had some trouble with my internet connection...
Well, yes, that's the answer if the values you gave were the correct ones, with no typo :)
Do not be too surprised by figures with many decimals. In practice, we round up to some figures. Here for example, we will consider 1.81 m only, for I don't think one would bother to measure up to several nanometres for a see saw, lol! Once they got the balanced point, they can play with it with no trouble.