For those who didn't read the introductory entry, head over to http://www.teamliquid.net/blogs/viewblog.php?topic_id=79275

After a hiatus, the physics blog is back. Last time there was some disagreement and interesting discussion about if the author of the question I borrowed was in fact wrong. I'm honestly still not 100% sure, but I'll try to pick a question that is less likely to create disagreement that won't be resolved.

Last week the question of the week was:

Question 3
Momentum and Energy

An open railroad car is rolling without friction in a vertically-falling downpour and an appreciable amount of rain falls into the car and accumulates there. Consider the effect of accumulating rain on the speed, momentum, and kinetic energy of the car.

1) The speed of the car will
(a) increase
(b) decrease
(c) not change

2) The momentum of the car will
(a) increase
(b) decrease
(c) not change

3) And the kinetic energy of the car will
(a) increase
(b) decrease
(c) not change

The rain has now stopped. A drain plug is opened in the bottom of the rolling car allowing the accumulated water to run out. Consider the effects of the draining water on the speed, momentum, and kinetic energy of the rolling car.

4) The speed of the car will
(a) increase
(b) decrease
(c) remain the same

5) The momentum of the car will
(a) increase
(b) decrease
(c) remain the same

6) And the kinetic energy will
(a) increase
(b) decrease
(c) remain the same

Go ahead and look back to your post in the last thread here. Rethink your reasoning, then proceed to the answers. You need to make the assumption that he's referring to the speed/momentum/energy of the car including its contents, but that makes sense as the default assumption anyway.

The answers are:
+ Show Spoiler +

b
c
b
c
b
b

I'd assign this a 2/5 difficulty rating because it's basic applications of simple physics principles, but it's still quite easy to slip up. betaben was close, but fight_or_flight won.

We'll settle any disagreements with a bo9 on lost temple. But seriously, if the answers don't make sense, please ask!

Question 4
Rotation

Please remember to post answers in SPOILERS, and preferably with an explanation.

VIB

Brazil3567 Posts

October 10 2008 08:21 GMT

Wait, is the wheel spinning by itself due to the movement of the porter is there some kind of engine spinning it in the direction of the drawing?

edit: If it's due to the movement of the porter then none of the above would happen so I suppose it's spinning of an engine, if so then I think it's:
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B) Opposing to the force done by the porter. I'm sorry I don't know how it's called in english :S but one thingy will oppose the other thingy to keep the sum zero ^^

fight_or_flight

United States3988 Posts

October 10 2008 08:28 GMT

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c).....after staring at this for 5 minutes

skyglow1

New Zealand3962 Posts

October 10 2008 08:54 GMT

On October 10 2008 17:21 VIB wrote:
Wait, is the wheel spinning by itself due to the movement of the porter is there some kind of engine spinning it in the direction of the drawing?

edit: If it's due to the movement of the porter then none of the above would happen so I suppose it's spinning of an engine, if so then I think it's:
+ Show Spoiler +

B) Opposing to the force done by the porter. I'm sorry I don't know how it's called in english :S but one thingy will oppose the other thingy to keep the sum zero ^^

It's driven by a motor. I've played with one of these before and they're really fun, but it was a long time ago so I can't remember how it moved when you went around in a circle.

micronesia

United States24513 Posts

October 10 2008 17:55 GMT

C'mon TL I want an explanation!

MasterOfChaos

Germany2896 Posts

October 10 2008 18:03 GMT

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D
The angular momentum points to the left. (easy to see from r cross p)
when the suitcase is turned the angular momentum points partially to the front of the suitcase.
Thus the suitcase tries to rotate around this axis which means that the top tries to turn to the right which is depicted in D.

LaLuSh

Sweden2358 Posts

October 10 2008 18:30 GMT

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b) The spinning weel will want to retain its movement. Sorry don't know all the physics terms in English. But you might imagine it as a person sitting in a car making a sharp turn to the left. The bodies in the vehicle will continue their motion forward but will be pressed towards the front --> right side of the car.

LiAlH4

New Zealand111 Posts

October 11 2008 01:13 GMT

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c, i think... it's been a few years since I did angular momentum problems in physics.
When he turns the corner, about a vertical axis (z), the total angular momentum of the system must be conserved!
i.e. Ltotal = Lspin + Lorbit
(Lspin is the angular moment of the wheel
Lorbit is the angular momentum generated by turning the corner.)
He will be turning the corner in an anti-clockwise-direction about the z-axis. Therefore, to keep the total angular momentum about the z-axis constant (0), the wheel must adjust to have some finite opposite angular momentum about the z-axis (i.e. spinning in a clockwise direction about z). This happens in case c, where the wheel's spinning distorts to spin clockwise around z

evanthebouncy!

United States12796 Posts

October 11 2008 09:57 GMT

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I hope...
D)
ANgular velocity to the left initially
Turn to left, means angular acceleration up
So angular velocity changes from left to up, so top suitcase tilt down(right), bottom suitcase tilt up(left).

VIB

Brazil3567 Posts

October 12 2008 06:05 GMT

#10

On October 11 2008 02:55 micronesia wrote:
C'mon TL I want an explanation!

lol I'm sorry I never did physics nor any math whatsoever in english I'm completely lost in how to explain it ^^ But after some wikipedia I think I can explain it better in english terms:
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B) It's Newton's third law ("every action has a reaction"), the movement described in alternative B) is due to the reaction caused by the force applied by the porter on the briefcase. In english, this particular case of Newton's third law is also referred to as "Precession"

Good enough now?

I've learned lots of new english words in this thread lol ^^