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Only four weeks until exam day for AP Physics students. What should you do?
It's not all about working practice problems. You can't scheme to solve any problem if you don't know the fundamental facts and equations underlying that problem, any more than you can scheme to stop the Chiefs offense if your tackling skills are poor.
So spend the next month working on fundamentals - items you must recall quickly and accurately if you're to have any chance at dealing with a complex problem on the AP exam.
To help you out, I'm going to publish one five-minute fundamentals check each day until the exam. Take no more than five minutes! If you don't know how to do these, you ain't gonna figure them out by staring or thinking. They're straight-up recall, or sometimes simple questions about common situations.
And then a day or two later I'll publish my answers in the comments. Feel free to use these as quizzes in class, or for your own review! Don't look at my answers, though, until you've *written* - yes, I mean written down, not just thought about - your own answers.
These questions are perfect review for BOTH AP Physics 1 and AP Physics C - mechanics.
FUNDAMENTALS CHECK NUMBER 1:
Questions 1-6: In the diagram, a rope connects two object of different masses, where m1 > m2. The cart of mass m1 is on a frictionless surface. The mass of the pulley is negligible.
1. Is the tension in the rope greater, less than, or equal to m2g?
2. Is the acceleration of m1 greater than, less than, or equal to g?
3. Which object experiences a larger force of the rope on the object?
4. Which object has a larger magnitude of acceleration?
5. What is the direction of the acceleration of m2?
6. What is the direction of the velocity of m1?
7.
Write
two expressions for impulse.
8. What is determined by the area under a force vs. displacement graph?
9. When an object undergoes simple harmonic motion, what is the direction of the force that the object experiences?
10. On a position-time graph, how is the location of an object determined?
That's all - my answers will be posted in the comments on April 14 or 15.
Next level, once you're secure in those first 6 answers: now the pulley's mass isn't negligible. Do any of the answers to 1 through 6 change, and if so, how?
ReplyDeleteWonderful follow-up, *especially* for a physics C class!
DeleteHere's my attempt at your variant questions:
Delete1. Same answer: m2 still accelerates downwards, so the tension is less than m2g.
2. Same answer but more so: The system's acceleration from m2g is now decreased further by the rotational inertia of the pulley.
3. Unknown: this now depends on the coefficient of static friction between the string and the pulley. If it's large enough, the pulley is experiencing the greatest force.
4. Assuming this still means linear acceleration, the two masses both still experience the same acceleration.
5. Same answer, same reasoning.
6. Same answer, same reasoning.
You're right that #3 is the only one that changes, but the question (as I understood it) is specifically about tension. The tension in the hanging part of the string (attached to m2) must be greater than the tension in the string attached to m1. Otherwise, the pulley wouldn't accelerate clockwise.
DeleteJust a clarification to everyone that Will is discussing his additional, hypothetical question about whether the tension is the same or different in parts of the rope *WITH A MASSIVE PULLEY*. That's not what was asked in the original question! But it's an excellent physics C follow-up!
DeleteOkay, answer time! I'm giving justifications for some of these, but you didn't need to - just the answers are sufficient.
ReplyDelete1. less than. (If released from rest, block 2 speeds up downward, so acceleration is down, so forces are unbalanced down making the up force - tension - less than the downward weight.)
2. less than. (Block isn't in free fall - there's an up force as well as weight.)
3. neither, same rope force. (Jacobs law of tensions: one rope equals one tension. (The physics C corollary to Jacobs Law says "...unless interrupted by a massive object.) )
4. neither, same acceleration. (Acceleration is the change in speed every second. Since the objects are attached to one another, when one speeds up or slows down, the other one does too.)
5. down. (If released from rest, m2 will speed up downward; if given an initial upward speed, m2 will move up and slow down. In either case, acceleration is downward.)
6. unknown. (The unbalanced force on m1 is right, but that could EITHER mean speeding up and moving right OR slowing down and moving left.)
7. Impulse = change in momentum; Impulse = Ft.
8. work.
9. toward the equilibrium position.
10. from the vertical axis value.