Physics Questions - any thoughts?

[sdb09]

---

Members don't see this ad.

Hi everyone -

These were just some questions I was going over and was having difficulty with...I was wondering if anyone had any thoughts and could better explain them. Thanks sooo much!

1) Two objects of equal mass and center of gravity are placed at the top of an incline plane and given an initial velocity v. The .µs . and the .µk . of both objects are identical. One is a ball and one is a block. If both objects have the same contact surface area, which will hit the ground first?

a) The ball
b) The block
c) The same
d) Depends on what .µr. is

The correct answer is A and I didn't understand why because if they both have the same friction coefficients, mass, and height, why wouldn't they all be the same?



Also does anybody know what the .µr.. .referenced in answer choice d means? The explanation choice says to remember that .µs = µr > µk.. . but I didn't know what that meant??


2) Buoyant force is determined by all of the following factors except:
a) Gravitational constant
b) Size and shape of the object
c) Density of the fluid
d) Volume of the fluid

The correct answer is B - but why is D wrong? I thought that you need the volume of the submerged object, not of the fluid and couldn't you maybe determine that using the size and shape of the object?

3) The equations relating to circuits are the same equations in fluids and springs. If a spring is the same as a capacitor and a dashpot (the piston that slows a door) is the same as a resistor, which system stores more energy at equal tensions, two springs with constants 3 and 4 in series or in parallel? Both systems are equidistant and the length of stretch is the same.
a) Springs in series
b) Springs in parallel
c) Same
d) Depends on the temperature

This question has completely lost me. I selected answer choice B becuase I thought that equivalent capacitance would increase in parallel and then using U = 1/2CV^2 that would have higher energy, but it says the answer is C? Can anyone please explain this to me?

Thanks!

 

[mosquitoman]

for 1 - the ball has rolling momentum.. torque thats why

for 2. you are correct.. volume of fluid does not matter Fb = Vobj * dfluid * gravity

for 3. was there a diagram or something that is missing from the picture?

 

[spyderracing32]

For 2 it definitely seems like D should be the correct answer. Think if the mass of an aircraft carrier were put into the size/shape of a baseball. If the answer were B, this incredibly heavy baseball should float....

3 seems like the question contradicts itself. At equal tensions it seems like the one in series should have the greatest stored energy because for the tensions to be equal a spring with a lower spring constant will have to be stretched further and since the distance stretched is squared it would have a greater effect on the stored energy. However, at equal stretch lengths it seems like the springs in parallel should have the greater stored energy because they have a larger spring constant. Poorly written question?

 

[Kaustikos]

For 2 it definitely seems like D should be the correct answer. Think if the mass of an aircraft carrier were put into the size/shape of a baseball. If the answer were B, this incredibly heavy baseball should float....

3 seems like the question contradicts itself. At equal tensions it seems like the one in series should have the greatest stored energy because for the tensions to be equal a spring with a lower spring constant will have to be stretched further and since the distance stretched is squared it would have a greater effect on the stored energy. However, at equal stretch lengths it seems like the springs in parallel should have the greater stored energy because they have a larger spring constant. Poorly written question?

I think 2 maybe forgot to add the word "displaced" for D, which would make it correct. But as it stands, both choices could be argued based on the vagueness. I would be more inclined to believe the volume of the fluid, though.
For 3, I would tend to argue that it's parallel arrangement that would increase energy stored. Capacitance thrives on having as much area as possible to store the charge. Larger area = larger energy. Because of the analogy made here, the answer would have to be B