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Wednesday, October 8, 2008
The following is a conversation I had during physics tutorial.
The question being discussed is as follows:
If the plane is frictionless and the plank isn't attached to the plane on an axle, what is the final velocity of the ball, Vf, after the collision.Me: Hmmm... so the board falls and translates forward at the same time.
Alwin: Oh, you know what I did for this question?
Me: No. What did you do?
Alwin: Basically the board slips right?
Me: Yeah....
Alwin: So I assumed that the board started spinning around in a circle about the center of mass.
Me: (gives him the "are-you-shitting-me" look)
Me: Dude, how can the board start spinning around in a circle?
Alwin: Because the plane is frictionless.... So the board slips what.....
(Hugh and I proceed to demonstrate how the board falls flat and thus CANNOT start spinning in a circle)
Me: Yeah, how the hell did you come up with that man?
Alwin: Tsk.... Well, the plane is frictionless.... the board slips.... but it's just that I forgot about gravity....
WTS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Don't worry Alwin, I forgive you. I've had such moments myself.
The following took place during lecture today, which happened to be on special relativity:
(Professor Chew, in an attempt to explain time dilation, draws the picture below. The man, Mr O' is sitting in the middle of a bus/boxcar that's moving at a constant speed v.)
Me: OMG! O' IS PEEING OUT OF THE BUS!!!!
(Professor Chew then proceeds to draw the second diagram, which shows Mr O' again, this time with observer, Mr O who is not moving relative to earth. Suddenly, two balls from either side of the bus fly towards O', both with speed u, but travelling in opposite directions.)
Me: OMG!!! O' IS PEEING ON O!!!!
(After that he moves on to explain that if you replace the two ball shooters on the side of the bus with lasers, O' will observe that both lasers reach him at the same time whilst O will see that one laser reaches O' first.)
(After that he moves on to explain that if you replace the two ball shooters on the side of the bus with lasers, O' will observe that both lasers reach him at the same time whilst O will see that one laser reaches O' first.)
(He thus moves on to another example. A girl is in a train travelling at constant velocity and she fires a laser vertically upwards towards a mirror. She observes that the laser beam rises to the top, hits the mirror, and returns back to the sensor on the ground. The observer who is not moving relative to the earth on the other hand, sees that the laser beam is travelling in the manner as shown in the picture below)
Me: Hugh, help me out here. If light can't go faster than c, then for the girl in the train, the instant the light leaves the laser, the beam of light it doesn't take on the initial speed of the train right? So doesn't that mean that while the train is translating forward, the light beam gets "left behind"?
Hugh: No.... The thing is that the girl isn't supposed to know that she is in a moving reference frame what.
Me: No, but whether she knows or not, she's still moving what. The light leaves the laser vertically upward at a speed c, and thus ends up behind her upon being reflected by the mirror.
Hugh: No... The girl isn't supposed to know that she is in a moving reference frame.
Me: But it doesn't matter what.... the light goes up, the girl moves forward.
Hugh: Then the light doesn't know that it's in a moving reference frame also lor....
Me: Then why doesn't the light know?
Hugh: Err.... Liddis liddis liddis then liddat liddat liddat, then.... Liddat lor.
Me: ........
Hugh was right apparently, because I went to ask Prof Chew after lecture who then made me realise that I had a Michelson-Morley luminiferous aether mindset [ for those who don't know what that means, you can wiki if you're interested]. But, apparently, Hugh was right in his explanation of why light doesn't know it's in a moving reference frame too..... Because just liddat lor...... God, I love physics....
Well.... after all was said and done, we decided to take a photo with Professor Chew
And that's the Prof, Alwin, Hugh and Andrew. PAP students reading this, please join the Prof Chew Lock Yue fan club if you have a facebook account.____________________________________________________________________________________
