course phy201 ЋæŠIö€„qª»x—Õzéäö~ÊÄassignment #014
......!!!!!!!!...................................
18:49:07 class notes: rubber band and rail How does the work done to stretch the rubber band compare to the work done by the rubber band on the rail, and how does the latter compare to the work done by the rail against friction from release of the rubber band to the rail coming to rest?
......!!!!!!!!...................................
RESPONSE --> confidence assessment:
.................................................
......!!!!!!!!...................................
18:52:06 ** The work done to stretch the rubber band would in an ideal situation be available when the rubber band is released. Assuming that the only forces acting on the rail are friction and the force exerted by the rubber band, the work done by the rail against friction, up through the instant the rail stops, will equal the work done by the rubber band on the rail. Note that in reality there is some heating and cooling of the rubber band, so some of the energy gets lost and the rubber band ends up doing less work on the rail than the work required to stretch it. **
......!!!!!!!!...................................
RESPONSE --> the work by the rubber band being streched would be equal to the work done by the rubber band when the rubber band would be released from its stretched position self critique assessment: 1
.................................................
......!!!!!!!!...................................
18:52:50 Why should the distance traveled by the rail be proportional to the F * `ds total for the rubber band?
......!!!!!!!!...................................
RESPONSE --> the distance should be proportional due to friction confidence assessment: 1
.................................................
......!!!!!!!!...................................
18:53:27 ** The F `ds total of the rail when it is accelerated by the rubber band is equal Fave `ds, which is equal to to m * aAve * `ds. Here aAve is the average acceleration of the rail by the rubber band. 2 aAve `ds = vf^2 - v0^2 by the fourth equation of motion. So the F `ds total is proportional to the change in v^2. The rail is then stopped by the frictional force f; since f `ds is equal to m * a * `ds, where a is the acceleration of the sliding rail, it follows that f `ds is also proportional to the change in v^2. Change in v^2 under the influence of the rubber band (rest to max vel) is equal and opposite to the change in v^2 while sliding against friction (max vel back to rest), so work f `ds done by friction must be equal and opposite to F `ds. This ignores the small work done by friction while the rubber band is accelerating the rail. **
......!!!!!!!!...................................
RESPONSE --> self critique assessment: 1
.................................................