course Phy 201
The statements you sent me regarding steps to go through in determining sources and magnitude of forces and changes in the different categories of energy helped alot.Thanks.
|uj?????????h???assignment #012012. `query 12
Good. I'm finding that it's also helping my in-class students.
Physics I
10-10-2007
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14:34:55
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 -->
I first define the ""system"" as being the two masses, the pulley, and the cable.
I think the only forces acting ON the system are gravity pulling m2 down and the natural force of the table keeping the m1 mass in vertical equilibrium.
Since the table top is non-friction and the pulley is high quality (zero friction), is the Fnet the acceleration of gravity acting on m2?
Is the acceleration of the system the gravitational constant of 9.8 m/s^2?
I think the PE would change if m2 descended by dy would e the amount of work represented by the force of m2 moving the distance dy or m2*9.8m/s^2*dy.
confidence assessment: 2
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14:44:11
** 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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RESPONSE -->
I understand all this and got ir right except for the accel of the system. Where I'm confused by your answer which says it should equal the Fnet/total mass of the system which I would take to mean Fnet/(m1+m2)
But your answer shows Fnet*m2/(m1+m2)
Which is correct? Please explain.
My solution isn't showing above. However it's easy to summarize:
Fnet = m2 g, the force exerted on the suspended mass by gravity so
Fnet / (m1 + m2) = m2 g / (m1 + m2), which could also be written as m2 / (m1 + m2) * g.
I've also inserted the given solution to this problem below:
The net force on the system is the force of gravity on the suspended weight: Fnet = m2*9.8m/s/s" + vbCrLf + vbCrLf + "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. **
How would friction change your answers to the preceding question?
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.
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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14:44:41
&#Good work. Let me know if you have questions. &#