Where Will It Sag To?

A mass (of negligible size) is attached to two initially un-stretched springs. The other end of each spring is anchored at a fix point, as shown in the picture. Spring parameters are shown in the picture. The mass falls from its initial position under the influence of gravity and of the springs.

Assuming that air resistance eventually brings the mass to rest, what is the sum of the absolute values of the equilibrium x x and y y coordinates?

All quantities are in SI units.

Feel free to solve using any method of your choosing.


The answer is 16.198.

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1 solution

Steven Chase
Jul 12, 2016

As far as I can tell, this one is actually rather tricky, because the equations depend non-linearly on the variables. I'm curious to see what people propose as solutions. I used a hill-climbing algorithm to solve it (or more properly, valley-descent). Essentially, randomly perturb the x and y coordinates of the mass slightly and see if the net force on the mass goes down relative to its previous position. Only accept alterations which lessen the net force. After a great many small perturbations, the program settles on a set of (x,y) coordinates that yields a net force of approximately zero.

@Matt Owings Would you mind telling me how you solved this one?

Steven Chase - 4 years, 5 months ago

Spring 1 has a neutral length of 1 meter, K=2 N/M, and let x1 be its elongation and a1 be the angle it forms with the horizontal. Similarly, spring 2 has neutral length 2 with K=6 N/M, and let x2 be its elongation and a2 be the angle it forms with the horizontal. Then there are 3 conditions that must be met:

2 x1 cos(a1) = 6 x2 cos(a2) 2 x1 sin(a1) + 6 x2 sin(a2) = 98 (1 + x1) sin(a1) = (2 + x2) sin(a2)

By preliminary examination it is obvious that the elongation of the springs will be large in relation to the neutral lengths. The sines of a1 and a2 will be close to 1. So I picked a starting estimate for mass position below the x axis by assuming it was hanging form a spring of neutral length 2 and K=8. Then I gradually reduced this value until the 3 equilibrium conditions were met.

Matt Owings - 4 years, 5 months ago

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Thanks. So it sounds like you employed a search process as well.

Steven Chase - 4 years, 5 months ago

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