34,534 reputation
347118
bio website inspirehep.net/…
location Duchy of Grand Fenwick
age 43
visits member for 3 years, 11 months
seen 5 hours ago

Experimental nuclear and particle physicist. These days I'm teaching, but I've spent a lot of time on nucleon structure in fixed target electron scattering and neutrino oscillations using reactors and beam sources.


9h
comment What causes the recessed blade of some folding knives to emerge in a kinetic opening?
In at least some dvices the blade starts at a position of stable equilibrium where the centrifugal pseudoforce alone won't cause it to move, but the Euler pseudoforce that develops as you begin the motion is enough to pull it past equilibrium.
10h
comment Two carts connected by spring on frictionless track
Now this is the kind of homework related questions that is good for Physics SE.
11h
comment Is the decay $B\rightarrow K^* \gamma$ decay allowed in the Standard Model?
The branching ratio quantifies it. Singly weak decays are much more common than doubly weak decays (by ratios of 10000:1 or more).
12h
comment Do we exist in multiple dimensions?
You just defined what you mean by "actually exist" by saying that it should actually exist. Define what consequences you think are required for an extra dimension to be "actual". I'm being persnickty about this because I sometimes get the idea that people asking this question don't have a clear idea what it means for a dimension to exist---especially in the context of string theory where the extras are currently presumed to be compact and small.
14h
comment Do we exist in multiple dimensions?
As usual, we'd need to know what you think "actually exist" means before we could answer this question even if we had string theory fully tested and the proper variant identified. Is it sufficient that the theory requires them?
21h
comment Is the decay $B\rightarrow K^* \gamma$ decay allowed in the Standard Model?
Any two-W process is going to be weak-surpressed relative the dominate singly weak decays. Looking is the copy of the particle physics booklet on my desk I see some other double weak decay channels listed with branching ration limits in the $10^{-4}$--$10^{-6}$ range. The listed limit for decay to $K^0 l^+ l^-$ is $3 \times 10^{-7}$.
1d
comment What is the mass of the unstable particle?
Conserve four momentum and rely on the relationship between mass, energy and momentum.
1d
comment Calculating the induced emf without knowing the change in magnetic flux
The problem expects you to engage on more than one level. You're doing fine employing Faraday's law, but you are also expected to deal with the problem in terms of energy.
1d
comment What is logical way to calculate percentage error?
The place where working scientists bother with fractional error is in comparing the size of uncertainties: "neglecting the foo asymmetry is about a 2% error, and we're looking at about 5% counting error anyway so I think it is OK."
1d
comment What is logical way to calculate percentage error?
Assuming that you are going to compute this thing at all, the sign carries useful information. But you certainly don't compute it in the case of an expected value of zero---that case always calls for careful treatment.
1d
comment What is the actual energy content of the Sun?
Most of the hydrogen is ionized, you're low by a factor of roughly two.
1d
comment How can randomness exist?
Right here "However, since it's caused by something," you have presumed your conclusion. The evidence on the ground can't absolutely rule this out, but it comes very close and a deterministic universe is out of favor right now.
1d
comment When a planet is heated through gravitational pull, where is the energy taken from?
Related: physics.stackexchange.com/questions/52132/…
2d
comment Underdetermined forces in a statics problem
@BenCrowell He's right for static friction with a non-deformation assumption (i.e. the sort we teach in PHYS 101). In that case it is always "just enough to prevent relative motion" up to the point where it fails and motion begins. The difficulty here is that examined at high resolution there can be deformation both normal to and across the surface so both the normal force on one side and the static friction on the other can be supporting the cylinder against motion along one direction (i.e. both can have non-zero values in a quiescent state), and ambiguity arises. Time to break out the FEA.
2d
comment How can we tell if a molecule is in thermodynamic equilibrium from scattering data?
I've a (roughly third-hand) story about people assuming equilibrium and later finding it wasn't so. Their system exhibited an anisotropic Doppler broading, which is one way to rule out equilibrium.
2d
comment Theory on what would happen if a proton touches anouther
This happens all the time. In every nucleus that is not hydrogen protons are "in contact".
2d
comment Electric Field due to volume charge
You integrate the expression for field (i.e. Coulomb's Law divided by the test charge) over the source volume, hopefully applying lots of mathematical trickery as you go. This problem is given to everyone at some point and working through it is good for you even if it is not much fun.
2d
comment Would using Cherenkov radiation for lighting be feasible?
@SteveJessop Those were the days when (in the words of a nuclear physics professor I had) "Men were men and radiation didn't hurt you." It was also the time when you could ship a 2 Curie AmBe source in a cardboard box by US post.
2d
comment What is the Weak force?
"they said that it's the interaction between Protons and Electrons(Electronegativity)" This is wrong. While there is a weak interaction between protons and neutrons it is swamped by the plain electromagnetic interaction and it has nothing to do with chemistry. "Electronegativity" is a chemistry idea and applies at much longer length scales and much lower energy scales than any manifestation of the weak force.
Oct
20
comment Underdetermined forces in a statics problem
@BenCrowell Dude, making me think when I'm trying to prepare lecture slides... but I think you're right.