A good one for all you physicists

xsnv

xsnv

xsnv

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Damn..got answered. Got another one:

Got another one for u then:

A factory worker has 50 chocolate making machines. Each makes chocolate bars that weight 50g. however he knows one machine makes 55g bars but he doesn't know which. He has only got one weighing machine that can only make one measurement (after which it explodes!). How does he know which one is dodgy ;)
 
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From what I remember from my physics yes the fly does reach 0 velocity. Makes sense if you break it down...

Say the fly was moving at -10mph [minus because it is opposite direction to train], for it to end up going the same speed as the train it needs to go from -10 to to +100. You can't do that without passing zero.
 
Jokester said:
Why would the trains velocity be 0, it keeps going in the same direction?

Jokester
Click to expand...

my thoughts too, the trains does not stop hence no zero velocity.
 
xsnv said:
Assumptions: both the train and fly are solid and inelastic objects, the train weights 1,000,000 times as much as the fly,no air resistance etc.
Click to expand...
If solid means "perfectly solid", then according to classical physics, the fly/train can change velocity instantaneously (formally, have a jump discontinuity). And in fact, the standard (i.e. classical) calculation everybody does for A-level mechanics assumes an instantaneous jump in velocity that doesn't have 0 as an intermediate value. In other words your claim:

according to classical physics, for the fly to change direction from left to right there is a time where it is actually stationary ie it's velocity is 0.
Click to expand...
is false.

N.B. The speed of sound in a "perfectly solid" object is infinite, which is a bit of a blow to those who believe in relativity. Since we're talking classical physics, however...
 
ohh...forgot one tit bit. I'l edit the first post to reflect this. When the fly hits the train they shoud be moving in the same direction together. If they are together and the fly's velocity drops to 0 then surely so must the trains...
 
xsnv said:
My question is this: according to classical physics, for the fly to change direction from left to right there is a time where it is actually stationary ie it's velocity is 0.
Click to expand...

But is that true? Not too sure on that one, but let's assume that it is. In which case we can use the same style of arguement. i.e. In order to go from 10mph in one direction to 100mph in the other, then the fly must slow from 10mph down to 5mph, then must at some time slow from 5 down to 2.5 etc. If the rate of deceleration is the same (and then the rate of acceleration in the opposite direction) then the time interval between the two speeds gets shorter and shorter (halving each time on deceleration and doubling each time on acceleration). As the time interval is halving each time, it approaches a length of 0 (seconds / hours, whatever) and it is during this shortest possible time interval that the fly changes direction - at which point both the train and fly are stationary.

Look at this another way (same type of arguement): if the train is doing 100mph, then it is also doing 1.66' m/minute, then it is also doing 0.027' m/second etc to the point that as the time interval approaches 0 (the time during which the fly changes direction), so does the distance travelled.

Phew!

(I could be wrong though).
 
DaveF said:
If solid means "perfectly solid", then according to classical physics, the fly/train can change velocity instantaneously (formally, have a jump discontinuity). And in fact, the standard (i.e. classical) calculation everybody does for A-level mechanics assumes an instantaneous jump in velocity that doesn't have 0 as an intermediate value. In other words your claim:

is false.

N.B. The speed of sound in a "perfectly solid" object is infinite, which is a bit of a blow to those who believe in relativity. Since we're talking classical physics, however...
Click to expand...
Show me a perfectly solid object and I'll let you ride in my time machine. ;)
 
xsnv said:
ohh...forgot one tit bit. I'l edit the first post to reflect this. When the fly hits the train they shoud be moving in the same direction together. If they are together and the fly's velocity drops to 0 then surely so must the trains...
Click to expand...

But in order to acheive that they can't be perfectly solid, hence the fly squishes (and it velocity goes -ve > 0 > +ve) and the train just keeps going in the same direction but ever so slightly slower.

Jokester
 
Ignore my previous post.

(using 100000kg as weight of train)
Due to the fact that:
Mass*velocity of object1 + mass*velocity of object2 = final mass of the 2 combined*velocity of the 2 combined
If fly's velocity = 0
100000*100 + 10*0 = velocity*mass
10000000 = velocity*100010
10000000/100010 = 99.990000999900009999000099990001 (trains velocity when fly's velocity = 0)




In OPs example:
100000*100 + 10*-10 = 100010*velocity
velocity= 9999900/100010 = 99.989mph (speed of train and fly combined after collision)
 
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touch said:
Mass*accel of object1 + mass*velocity of object2 = final mass of the 2 combined*velocity of the 2 combined
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That bit doesn't make sense. The units on either side don't match up with acceleration there. I'm assuming you meant velocity.
 
Psyk said:
That bit doesn't make sense. The units on either side don't match up with acceleration there. I'm assuming you meant velocity.
Click to expand...

Idd, i used accel instead of velocity, then realised my mistake and i thot i'd changed them all. Missed that 1.
(edited it now :p)
 
Jokester said:
But in order to acheive that they can't be perfectly solid, hence the fly squishes (and it velocity goes -ve > 0 > +ve) and the train just keeps going in the same direction but ever so slightly slower.

Jokester
Click to expand...

Damn...that was quick. You're completely right. A few smart people on here I see. Didn't think anyone would spot the inconsistent assumptions.

Got another one for u then:

A factory worker has 50 chocolate making machines. Each makes chocolate bars that weight 50g. however he knows one machine makes 55g bars but he doesn't know which. He has only got one weighing machine that can only make one measurement (after which it explodes!). How does he know which one is dodgy ;)
 
Who cares, just turn on your wipers. :p

xsnv said:
A factory worker has 50 chocolate making machines. Each makes chocolate bars that weight 50g. however he knows one machine makes 55g bars but he doesn't know which. He has only got one weighing machine that can only make one measurement (after which it explodes!). How does he know which one is dodgy ;)
Click to expand...

Someone asked that yesterday but with a bag of coins. Forgot the answer already :o
 
xsnv said:
Damn...that was quick. You're completely right. A few smart people on here I see. Didn't think anyone would spot the inconsistent assumptions.

Got another one for u then:

A factory worker has 50 chocolate making machines. Each makes chocolate bars that weight 50g. however he knows one machine makes 55g bars but he doesn't know which. He has only got one weighing machine that can only make one measurement (after which it explodes!). How does he know which one is dodgy ;)
Click to expand...
Take 1 bar from the 1st machine, 2 bars from the 2nd machine, .... 50 bars from the 50th machine and weigh them together. Take the answer, subtract 1275 and divide by 5g. The answer will be the number of the machine.
 
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