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visits member for 2 years, 9 months
seen 12 hours ago

12h
comment How does escape velocity relate to energy and speed?
@Jim: For bodies that are not rotationally symmetrical the expression remains complicated even above the surface. Even the definition of "centre" is not obvious there.
13h
comment How does escape velocity relate to energy and speed?
@Jim: Well, under the surface one would have to substitute proper formula for gravity inside a body. Trying to talk about what happens without that is somewhat misleading, yes.
14h
comment How does escape velocity relate to energy and speed?
@Jim: If you are below the surface, the material above you cancels some of the gravity and the forces perfectly balance in the centre, so effectively $M$ goes to $0$.
2d
comment Why is it easier to walk diagonally upstairs
Whether power is lower depends on speed. It needs to be explained why the speed will not increase on the shallower slope to make up for the reduced force.
Aug
27
accepted Conservation of energy in a different frame of reference
Aug
27
comment Conservation of energy in a different frame of reference
@dmckee: I understand why work. I would welcome some hint at how to qualify it and verification it comes out $-mv_0\Delta v$.
Aug
26
asked Conservation of energy in a different frame of reference
Aug
20
comment Are voltages discrete when we zoom in enough?
Since $e$ is a charge and $k$ is dimensionless (natural number), $k\cdot e$ is still a charge, not an electric potential (voltage).
Aug
5
comment What does centre of lift depend on?
@MikeDunlavey: Make that an answer please; the bit about the curvature of the flow is what I was missing.
Aug
5
revised What does centre of lift depend on?
added 120 characters in body
Aug
5
comment What does centre of lift depend on?
@MikeDunlavey: I think the lower pressure above explains the upwash adequately. The part I don't understand is why the pressure reduction is bigger in the forward part rather than spread out over the surface. Hm, I guess I'll rewrite the question.
Aug
5
comment What does centre of lift depend on?
@MikeDunlavey: I've read that source some time already. It shows the pressure field and that, but then it just glosses over that it depends on the angle of attack and shape and does not really explain it much.
Aug
5
comment What does centre of lift depend on?
@Floris: The image shows that the pressure field is indeed such that the centre of lift is in forward quarter of length. It does not explain why it is the case.
Aug
5
comment Integrating pressure over a surface
@Floris: It would be nice if you converted the last comment to an edit to the answer.
Aug
5
asked What does centre of lift depend on?
Jul
30
comment Why aren't units with powers, like cm³, surrounded by parentheses?
@PhilFrost: I've never seen anyone write that and would not consider it either correct or readable. To cancel d in dm I would do a substitution of $\mathrm{dm}$ to $0.1\mathrm{m}$, which does not involve a stand-alone $\mathrm{d}$.
Jul
29
comment Why aren't units with powers, like cm³, surrounded by parentheses?
@Nit: Or maybe better because the prefix does not have a meaning of it's own, so it's precedence can't be mistaken.
Jul
25
comment Which ball touches the ground first?
It would be nice to add buoyancy too. In air it will be significantly smaller than drag (friction), but it would make the answer more general.
Jun
6
awarded  Critic
Jun
6
comment Why do heavier objects fall faster in air?
@ja72: But the results are answering the question what is the speed after time $t$.