assn12query

012. `query 12

Physics I

07-20-2008

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21:01:48

Query set 3 #'s 13-14 If an object of mass m1 rests on a frictionless tabletop and a mass m2 hangs over a good pulley by a string attached to the first object, then what forces act on the two-mass system and what is the net force on the system? What would be the acceleration of the system? How much would gravitational PE change if the hanging mass descended a distance `dy?

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RESPONSE -->

fnet = 9.8m/s^2 * m2

a = fnet/ total mass

PE = fnet * 'dy

confidence assessment: 3

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21:02:50

07-20-2008 21:02:50

** The net force on the system is the force of gravity on the suspended weight: Fnet = m2*9.8m/s/s

Gravity also acts on m1 which is balanced by force of table on m1, so this force makes no contribution to Fnet.

Acceleration=net force/total mass = 9.8 m/s^2 * m2 / (m1+m2).

If the mass m2 descends distance `dy then gravitational PE decreases by - m2 g * `dy.

COMMON MISCONCEPTIONS AND INSTRUCTOR COMMENTS:

The forces acting on the system are the forces which keep the mass on the table, the tension in the string joining the two masses, and the weight of the suspended mass. The net force should be the suspended mass * accel due to gravity + Tension.

INSTRUCTOR COMMENT:

String tension shouldn't be counted among the forces contributing to the net force on the system.

The string tension is internal to the two-mass system. It doesn't act on the system but within the system.

Net force is therefore suspended mass * accel due to gravity only

'The forces which keep the mass on the table' is too vague and probably not appropriate in any case. Gravity pulls down, slightly bending the table, which response with an elastic force that exactly balances the gravitational force. **

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21:04:00

How would friction change your answers to the preceding question?

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RESPONSE -->

fnet would be reduced in the amount of the force of friction

confidence assessment: 3

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21:04:20

07-20-2008 21:04:20

**Friction would act to oppose the motion of the mass m1 as it slides across the table, so the net force would be m2 * g - frictional resistance. **

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21:12:27

Explain how you use a graph of force vs. stretch for a rubber band to determine the elastic potential energy stored at a given stretch.

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RESPONSE -->

PE is the area under the graph because it is the energy required to stretch the rubberband

confidence assessment: 2

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21:13:02

07-20-2008 21:13:02

** If we ignore thermal effects, which you should note are in fact significant with rubber bands and cannot in practice be ignored if we want very accurate results, PE is the work required to stretch the rubber band. This work is the sum of all F * `ds contributions from small increments `ds from the initial to the final position. These contributions are represented by the areas of narrow trapezoids on a graph of F vs. stretch. As the trapezoids get thinner and thinner, the total area of these trapezoids approaches, the area under the curve between the two stretches.

So the PE stored is the area under the graph of force vs. stretch. **

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21:14:03

07-20-2008 21:14:03

STUDENT QUESTIONS: Does the slope of the F vs stretch graph represent something? Does the area under the curve represent the work done? If so, is it work done BY or work done ON the rbber bands?

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21:14:24

07-20-2008 21:14:24

** Slope isn't directly related to any physical quantity. The area is indeed with work done (work is integral of force with respect to displacement).

If the rubber band pulls against an object as is returns to equilibrium then the force it exerts is in the direction of motion and it therefore does positive work on the object as the object does negative work on it.

If an object stretches the rubber band then it exerts a force on the rubber band in the direction of the rubber band's displacement, and the object does positive work on the rubber band, while the rubber band does negative work on it. **

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21:15:28

Query Add comments on any surprises or insights you experienced as a result of this assignment.

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RESPONSE -->

My misunderstandings are results of the fact that the queries and q_a seed questions do not seem to correlate with the lab work and the reading assignments completely. I have had to research several chapters ahead to gain some understanding to most of my current assignments.

self critique assessment: 2

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By the time you read a topic in the text you will typically have experienced it a number of times in introductory problem sets, qa's, experiments, etc.. One reason for this strategy is that most students taking this course cannot effectively read a college-level textbook and need a lot of experience with a topic before they can begin to understand the text. Another is that physics consists of difficult ideas and concepts that take time to 'sink in'. If topics are 'covered' sequentially students quickly reach a saturation point; the net result is that the topics being built upon have not been mastered at the necessary level, and student learn less and less as the course progresses. So each idea is spread out over several assignments, culminating in the text assignments and problems. This requires that each assignment addresses a number of different, but usually related, topics at various levels of understanding.

For example, the topic of energy conservation is challenging to almost all students, so it is spread out of approximately ten different assignments. This topic is closely related to the topic of forces, which are introduced just a little earlier then developed largely in tandem with energy conservation. The idea of impulse-momentum arises logically at the same point as the concept of energy, just after Newton's Second Law is introduced. This idea is introduced a little later than that of energy but while both force and energy concepts are still being developed, so that it can be contrasted with the concept of energy (the two are often confused).

In your case reading level does not appear to be an issue, and if all eligible students were capable of working at your level the course would be designed differently. However it would still be desirable to spread out the learning of various topics, though not to the extent it is done in this course, and multiple threads would still be a likely strategy.

If you do the assignments in the recommended order it shouldn't be necessary for you to rely on sources outside the course, though if this turns out to be the most efficient approach for you it's fine to do so. For example you don't need to master the conservation of energy to do Assignment 12, though if you haven't mastered it by Assignment 22 you should be concerned.

One thing you should consider doing is responding in detail to solutions you aren't sure you completely understand, telling me very specifically what you do and do not understand about each part of the given solution. This isn't required, but it's an effective learning strategy for most students.