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  • 152 votes cast
Jan
18
comment Expansion of the Universe
What do you mean by "bound by gravity"? Do you mean that, for example, a comet on a parabolic trajectory around our Sun will be affected by space expanding (since it is not bound to a closed orbit), but a comet on an elliptical trajectory will not? Or do you mean that if the gravitational force is of sufficient strength then you won't notice the effects due to space expanding?
Jan
13
comment Violation of the Newton's first law of motion in the movement of spacecrafts in the vacuum of space
You went wrong with: "The molecules of the gas are moving in any direction in the vacuum of space; they won't move in the space the same way that a metal ball will move." I'm not sure why you assume that the gas from a rocket sprays out in all directions. Maybe you are picturing the turbulence exhibited by rocket exhaust when the rocket is first leaving the launch pad?
Dec
7
comment How can angular velocity be constant even when there is a torque by friction?
"How can friction vanish?" - Friction disappears when there is no relative motion between the cylinder and the surface. So there is no torque being applied by friction. If the cylinder were to suddenly hit an ice patch, it would continue to roll exactly the same as it was before.
Nov
18
comment Explaining conservation of angular momentum with a disconnect
Nevermind - I was confused.
Nov
18
comment Explaining conservation of angular momentum with a disconnect
I think that the only revolving @Dylanthepiguy is asking about is around the probe. I don't think he's talking about the probe/instrument system revolving around a planet or anything like that.
Nov
18
comment Explaining conservation of angular momentum with a disconnect
I don't think this is correct. If I spin a baseball on a string, then let go of the string, the baseball does not continue to revolve around me - it travels in a straight line, tangent to the original orbit.
Nov
13
awarded  Citizen Patrol
Nov
5
comment Why is walking up stairs harder than walking normally?
To walk, you just have to lift one foot barely above the ground, tilt forward, and land on the foot. To climb a stair, you have to lift a foot to the next step, and then that leg has to lift your entire body up so that your other foot can be lifted.
Oct
26
suggested rejected edit on Why don't surfboards fly out the back of a utility truck when driving on the highway?
Sep
9
comment Source of energy for magnetic work?
"Well, I didn't do any work to separate the magnets." - Yes you did, when "I set two magnets on my desk such that...". Assuming the magnets were initially stuck together, you did work to pull them apart and place them on the desk with some separation between them.
Sep
9
comment Source of energy for magnetic work?
You have used up the magnetic potential energy due to the initial separation of the magnets.
Aug
20
comment Why is an Alcubierre drive shaped like it is
What is the difference between "contract space in front of you" vs "pancake space in front of you"?
Jul
31
comment Intro Mechanics: Finding ball speeds after collision
In real collisions, how does nature choose v1? If the collision is completely elastic, then both momentum and kinetic energy are conserved. Those two equations give a unique solution for the final v1 and v2. For a completely inelastic collision, momentum is conserved, but energy is not. However, we have one more piece of info - the masses are stuck together after the collision, so their final velocities are the same. This also gives us a unique solution. So, nature does not have any wiggle room to arbitrarily distribute momentum to one mass or the other - there is only one solution.
Jul
22
comment Is there a general rule for determining the direction of tension force?
@ja72 - Yes, that would be true for a rigid rod. But for homework-type problems, I think you can rule out compression for strings and ropes.
Jul
22
comment Is there a general rule for determining the direction of tension force?
No, the string never pushes the mass.
Jul
22
comment Is there a general rule for determining the direction of tension force?
Try it yourself - tie a string to a block, and grab the other end of the string with your hand. Now, try to use the string to move the block away from your hand. You can't, because the string goes limp. Now try to use the string to move the block towards your hand.
Jul
22
comment Is there a general rule for determining the direction of tension force?
Because the string "pulls" upward - if the force were pointing down, the string would be "pushing" the block.
Jul
22
comment Is there a general rule for determining the direction of tension force?
Here's an easy way to get it right, without worrying about "why": a string can only pull. If you try to push with a string, it will just fold up.
Jul
7
comment Radiation from home heaters
Are you asking whether the radiation from home heating units causes radioactive contamination to build up in your house?
Jul
1
comment Reduction of Earth's rotation due to increase in humidity
But the atmosphere is not a rigid body. If the atmosphere rotates more slowly than the earth, then the earth's rotation might have to actually increase to conserve angular momentum.