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## Homework Statement

A spring system is set up as follows: a platform with a weight of 10 N is on top of two springs, each with spring constant 75 N/m. On top of the platform is a third spring with spring constant 75 N/m. If a ball with a weight of 5.0 N is then fastened to the top of the third spring and then slowly lowered, by how much does the height of the spring system change?

http://img856.imageshack.us/img856/4352/screenshot20130127at101.png [Broken]

## Homework Equations

K

_{total parallel}= K

_{1}+ K

_{2}+ ... K

_{n}

F = -kx

## The Attempt at a Solution

Initially, I tried adding up all the springs in the system for an equivalent spring of 210 N/m, and then solving. That didn't work and the answer was incorrect. I feel like the 5 N ball will make the first spring push back some, and the 2nd and 3rd springs push back with a different amount of force. The trouble is, I'm lacking the conceptual understanding of how to go about calculating those values.

## Homework Statement

Consider the two orbits around the sun shown below. Orbit P is circular with radius R, orbit Q is elliptical such that the farthest point b is between 2R and 3R, and the nearest point a is between R/3 and R/2. Consider the magnitudes of the velocity of the circular orbit vc, the velocity of the comet in the elliptical orbit at the farthest point vb, and the velocity of the comet in the elliptical orbit at the nearest point va. Which of the following rankings

is correct?

(A) vb > vc > 2va

(B) 2vc > vb > va

(C) 10vb > va > vc

(D) vc > va > 4vb

(E) 2va >vb√2 > vc

http://img835.imageshack.us/img835/4352/screenshot20130127at101.png [Broken]

## Homework Equations

v

_{ellipse}= √GM ((2/R)-(1/a))

## The Attempt at a Solution

I honestly have no idea how to solve this one. With this equation, I can calculate the velocity of an elliptical orbit, but the way they worded the problem, so that it's

*between*two points—well, that honestly has me stumped. I thought about taking the minimum and maximum values and finding a range, but the thing is, I don't know for certain what the semi major axis is.

All help on either of these is very much appreciated!

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