al phy mc 問題

1. The answer is option A
When M=m, since the collision is elastic, sphere A will be at rest after collision, then sphere B will acquire all the momentum and kinetic energy from A.

2. Spring constant k = mg/e
when the spring is cut into two half, spring constant is doubled, i.e. equal to 2k.

The two new springs are in parallel, hence equivalent spring constant = 2 x 2k = 4k

Extension when a mass of 2m is suspended = 2mg/4k = mg/2k = mg/2(mg/e)
= e/2


2010-01-25 16:48:37 補充：
Sorry...I made a mistake to the first question. The answer should be C, as the change of momentum is largest (=2mu) if sphere A rebound with the same speed.
For Q2, spring constant is inversely proportional to the length of spring. A long spring has small spring constant and vice versa.