Physics Questions!!!

<div class='quotetop'>QUOTE(Jonathan Rohr @ Apr 29 2007, 09:50 PM) [snapback]432390[/snapback]</div>

Without knowing which came first, the bird or the egg, I'm afraid I can't help you! <_<

Anything with math or numbers was by far my worst subject in school. Good luck with the test tommorrow, er... today.


Wildkow

 

<div class='quotetop'>QUOTE(Jonathan Rohr @ Apr 29 2007, 08:50 PM) [snapback]432390[/snapback]</div>

B)Mechanical Energy is conserved

E) 1g upward

On the third one, I don't understand what Bill's configuration looks like, so can't answer without further clarification.

 

<div class='quotetop'>QUOTE(Jonathan Rohr @ Apr 30 2007, 04:50 AM) [snapback]432390[/snapback]</div>

This Doesn't sound right. If indeed Mechanical energy is defined as the potential plus the kinetic then Kinitial + Pinitial = Kfinal + Pfinal.

As an object falls kinetic increases but potential decreases.

E) 1g upward

I can't remember exactly what tension is but suspect the more ropes between the pulley and bucket the less tension in the rope. It doesen't matter if it is one rope traversing twice or two identical ropes.

 

<div class='quotetop'>QUOTE(Hobbs @ Apr 30 2007, 08:29 AM) [snapback]432466[/snapback]</div>

That is correct. No energy is being added or removed, just converted from potential to kinetic.

The problems always come down to what is the system. In this case I assume the OP is talking about just the egg, so all of the above answers are correct. If the system is the egg and the earth, then energy and momentum are conserved. As the egg falls toward the earth, the earth falls up toward the egg. The downward momentum of the egg is canceled by the upward momentum of the earth.

When analyzing classical physics problems, always draw a line around your system, then remember these three things:

1) Energy is never created or destroyed, just converted from one form to another, or given from one object to another.

2) In elastic collisions, kinetic energy is conserved. Elastic collisions are ones like colliding billiard balls.

3) In plastic collisions, kinetic energy is not conserved, only momentum is conserved. The lost energy is converted to heat. Examples of plastic collisions are car wrecks and bean bags; ones that go splat and smush, instead of whack and crack.

Good luck on the test,
Tom

 

1. Mechanical

2. 1g at the top of the loop. Imagine swinging a yo-yo in a vertical circle (same principle as the car). If you swing it very fast, it stays in a circle and you can feel the g's at the bottom and the top. If you swing it slowly, it doesn't make it all the way up and instead falls back down.

Now, if you swing it just barely fast enough, it'll travel in a circle. Slow it down any, and it won't fall straight down to the ground, instead it'll do a little "skip" at the top of the arc where it doesn't complete the circle. This *should* show you that the car can be traveling so as to be "weightless" while still maintaining forward velocity enough to regain weight the very next second as it starts down the other side of the track.

3. Free body diagrams are great... but require drawing. Take the first example where the guy is holding the rope. You have a force g acting downwards at the bucket, and at the guy, and a force of 2g acting upwards at the pulley. Each section of the rope thus has a tension of 1g on it (if the tension was greater then it would overcome the force pulling it down, moving the object).

In the second situation, you have a force g pulling down at the bucket, and another force g pulling up at the pulley. Since there are two lengths of rope between the two, you can infer that each length has a tension of 0.5g. Any greater, and the over all tension pulling upwards at the bucket would overcome the mass and move the bucket.

 

I agree with your answers on the first two questions. I think they are clear and obvious. In the second question, the weightless roller coaster maintains contact with the track, but just barely.

In the third question, I analyse as follows:

In the first situation, when one end of the rope attaches to the bucket, and the other end is held in someone's hand, the bucket end of the rope supports the bucket's entire weight. The hand-held end of the rope has to balance that weight, so it also supports a weight equal to the bucket. A comparable situation would be if EACH end had a bucket of equal mass tied to it.

In the second situation, the weight of the bucket is supported by both ends of the rope, which therefore share the load, and each end supports only half the total weight.

Therefore the tension on the rope is greater in the first situation, where the rope ends are separate, one with a bucket, and one held by a hand.

I like these kinds of problems. I am sure you could come up with ones that would baffle me, but these three seem very straightforward, though in the second one I had to rely on your definition of mechanical energy. And I think you described them extremely well. I had no problem visualizing what you described.

And good luck on your exam.

 

<div class='quotetop'>QUOTE(Jonathan Rohr @ Apr 29 2007, 09:50 PM) [snapback]432390[/snapback]</div>

If this is a college exam and not high-school, I hope it's "Physics for non-science majors".

 

If I did not know an answer I wrote down "green". I figured someday it had to be right. B)

 

<div class='quotetop'>QUOTE(Jonathan Rohr @ Apr 30 2007, 01:23 PM) [snapback]432748[/snapback]</div>

Sweet!