Physics at 1.45AM
I was watching TV at odd hour asusual. And stumbled upon this interesting bit of program
This is what they showed first. An ice skater (a kid) started spinning by applying force and after spinning with a reasonable speed for few seconds by keeping her one leg on the ground and other perpendicular to it (can you imagine it?) she started shifting her position to standing straight and started spinning really fast without applying any extra force. (I am sure the visualization would have made this whole thing more interesting). I was surprised to see her spin so fast and thought this is some kind of trick she is doing. But then a person asked her how she is doing that and asked her to stop since she is violating the newton’s 2nd law of motion. (A body at motion will be going at same speed unless an external force acted upon it)
This kid said she learnt to do it and not sure about any physics behind it. Then the narrator (I assume she is some kind of techie) starts explaining step by step.
I am trying my best to reproduce it before it evaporates from my brain.
Momentum works only for body in straight line. For bodies rotating it is called angular momentum. Where momentum depends on Mass, angular momentum depends on I (angular inertia). Angular inertia depends on Mass and Shape of the body. In case of disc (CD), the mass is equally distributed and distance from the center to end of the object is R (radius). So the totally angular momentum is 1/2mR2 (read as half m r square).
The inertia depends on axis of rotation too. For example, a cylinder spinning with a axis going through its center (center of circle) will have less inertia than the cylinder spinning in its length axis. vs --- . For the first case the I = ½ m r^2 and I = 1/12 m l ^2
I = ½ (40kg) (.15m)^2 = 0.45 kgm2
I = 1/12 (40kg) (1.2m)^2 = 4.8 kg m2
That explains why her inertia is more when she was spinning with her legs in horizontal position and why it is less when she is spinning in upright position. That in turn explains why she could spin faster when she is in upright position
But still that doesn’t explain how she could increase her speed when she shifted from horizontal to vertical without applying any extra pressure.
She continued. According to law of conservation of momentum, momentum should be maintained somehow. And her is the formula for it
L (angular momentum) = I (angular interia) * w (angular velocity)
Since she shifted from horizontal to vertical her inertia changed from 4.8 to .45 and only way to maintain/conserve the angular momentum is by increasing the angular velocity so her velocity went up.
WOW! Sometimes it useful to stay late (1.45AM) to learn physics, other times you may find interesting “Magic bullet” advertisements
Have a good new year!
This is what they showed first. An ice skater (a kid) started spinning by applying force and after spinning with a reasonable speed for few seconds by keeping her one leg on the ground and other perpendicular to it (can you imagine it?) she started shifting her position to standing straight and started spinning really fast without applying any extra force. (I am sure the visualization would have made this whole thing more interesting). I was surprised to see her spin so fast and thought this is some kind of trick she is doing. But then a person asked her how she is doing that and asked her to stop since she is violating the newton’s 2nd law of motion. (A body at motion will be going at same speed unless an external force acted upon it)
This kid said she learnt to do it and not sure about any physics behind it. Then the narrator (I assume she is some kind of techie) starts explaining step by step.
I am trying my best to reproduce it before it evaporates from my brain.
Momentum works only for body in straight line. For bodies rotating it is called angular momentum. Where momentum depends on Mass, angular momentum depends on I (angular inertia). Angular inertia depends on Mass and Shape of the body. In case of disc (CD), the mass is equally distributed and distance from the center to end of the object is R (radius). So the totally angular momentum is 1/2mR2 (read as half m r square).
The inertia depends on axis of rotation too. For example, a cylinder spinning with a axis going through its center (center of circle) will have less inertia than the cylinder spinning in its length axis. vs --- . For the first case the I = ½ m r^2 and I = 1/12 m l ^2
I = ½ (40kg) (.15m)^2 = 0.45 kgm2
I = 1/12 (40kg) (1.2m)^2 = 4.8 kg m2
That explains why her inertia is more when she was spinning with her legs in horizontal position and why it is less when she is spinning in upright position. That in turn explains why she could spin faster when she is in upright position
But still that doesn’t explain how she could increase her speed when she shifted from horizontal to vertical without applying any extra pressure.
She continued. According to law of conservation of momentum, momentum should be maintained somehow. And her is the formula for it
L (angular momentum) = I (angular interia) * w (angular velocity)
Since she shifted from horizontal to vertical her inertia changed from 4.8 to .45 and only way to maintain/conserve the angular momentum is by increasing the angular velocity so her velocity went up.
WOW! Sometimes it useful to stay late (1.45AM) to learn physics, other times you may find interesting “Magic bullet” advertisements
Have a good new year!
posted by BrainWaves at
2:33 AM
1 Comments:
very very interesting indeed...hehe never thought about the physics of skating..
By
Mad Max, At
11:07 AM
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