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#1 Posted : Friday, May 21, 2021 5:18:46 PM
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I had some physics questions:

1) w = delta KE + delta PE, both of these together or separately. How then does w = delta KE? Also, can we conclude that w = delta PE for forces involving a potential energy?

2) How do we know the MA of a pulley? By counting the number of circles? Also, if you increase pulley diameter does that increase MA?

3) Can you please explain the answer choice for Q32 in the ICE #2? What is the differentiating factor between conservative and non-conservative forces? I'm not sure how to determine if it is path dependant or not because the distance it travels seems significant.

4) Is the energy stored in muscles always chemical potential? Does this get converted to KE?

5) For Q 41 in the ICE#2 how can we not determine work? I thought if force increases (as seen in the graph) work would also increase?

6) Can you please explain how to do #46 for the ICE #2? When given an angle for a pulley should we ever consider that? How is the lever (In) twice the distance than the lever out?

7) Review q 1 for the first physics lecture: If it is being crushed how do you know that it is pulling in/pushing out?

8) Any good way to know the differences between conservative/nonconservative force?
#2 Posted : Sunday, May 23, 2021 7:37:06 PM
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Hi Moiz,

1) I found this document for you, which goes into really great detail about work, potential energy, kinetic energy, conservative forces, non-conservative forces, and some example calculations. I hope this helps answer your questions.


2) For a pulley system, the mechanical advantage is easy to find! Just remember this:

MA = (tensions pulling UP)/(tensions pulling DOWN)

Here is an example:


The rope on the left and in the middle are pulling UP and the rope on the right is pulling DOWN.

Therefore, MA = 2 / 1 = 2.

3) I think the key with this question is that the passage mentions the idea of FATIGUE, implying that performing this action (knee extensions) results in a REDUCTION in force generation -- i.e., it's a non-conservative force! So, the mechanical energy isn't conserved.

4) Muscles used stored CHEMICAL energy (like you said) to both kinetic energy AND heat.

5) Although it is true that work increases as a function of distance, the graph is plotting FORCE vs. POWER.

To get from force to work, we need distance! Neither the passage nor the graph gives us distance.

To get from power to work, we need time! Although the passage mentions time, this value of time is NOT connected to the graph. The graph doesn't give us time, either.

So, we cannot actually determine which curve has the "highest work". There isn't enough info. :(

6) First, the mass is 10 kg, so it WEIGHS (force) 100 N.
Next, we gain a MA of 2/1 using our pulley system, so the force "feels" like 100 N / 2 = 50 N by using the pulley system, but this would be if we had the rope labelled F parallel to the other ropes in the pulley system.
Because the rope labelled F is at a wonky angle, it actually makes the force that we need to apply EVEN LESS! Since sin(30°) = 0.5 (provided), then the applied force is 0.5(50 N) = 25 N.

7) We are looking at this question from the perspective of the cell.

If the cell is being crushed by something else, then the cell wants to NOT be crushed (because... who wants to be crushed?). So, the cell will resist this force, and PUSH OUT against the crushing force.

8) Non-conservative forces dissipate energy into other forms (usually heat). Conservative forces DO NOT do this.

Or... you can memorize a list! :D

Conservative forces:
- gravitational force
- spring force
- electrostatic force
- buoyancy force

Non-Conservative forces:
- friction
- air resistance
- drag
- material stress
- viscosity
- resistance to electric current
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