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22047
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location United States
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visits member for 4 years, 8 months
seen 14 mins ago

Started programming on a ZX spectrum in the 80's and have moved through Assembly, Turbo Pascal, C++, C#, Fortran. My main area of focus is engineering and scientific computing like numerical methods and 3D graphics.


8h
comment Why aren't all points on a rolling ball moving?
Any body moving in a plane will have one point with no instantaneous velocity. This point might be on the body, or away from the body. At that instant all distances from the points on the body to this point remain constant. The body spins about this point.
16h
comment What happens when two photons collide with one another, head on, dead center?
Where is the center? Heisenbeg would prevent you from specifying dead center aiming.
16h
comment Creating a structural dynamic model using only modal test data
I think I did this in college with an impact hammer connected to a computer which reconstructed the modal shapes. See youtube.com/watch?v=tBRjPN8m6zE
18h
comment Creating a structural dynamic model using only modal test data
Isn't $\phi^\intercal K \phi$ a matrix with the natural frequencies in the diagonals. So you can construct this experimentally by finding said natural frequencies.
Jul
30
comment Why spinning a pen makes it easier to remove it from the stand?
Is there suction created when not spun?
Jul
30
comment What happens to the kinetic energy of a dropped ball when it comes to rest on the ground?
Did you hear that BAM? Is the sand hot afterwards? Did you have to wear goggles to prevent sand grains getting into your eyes?
Jul
30
comment Ball flying towards me or me flying towards ball
Maybe an equation that shows that force/impulse is a result of "relative momentum" (however you define that) would improve the answer.
Jul
30
comment How can we account for a friction force changing sign in a conservation of energy equation?
If there is friction, how is there conservation of energy?
Jul
30
comment Rotation matrix of Euler's equations of rotation relative to inertial reference frame
I thought you don't integrate $\dot{R}$ directly, but integrate the euler angles (and their derivatives).
Jul
30
comment Rotation matrix of Euler's equations of rotation relative to inertial reference frame
It is all explained here: cs.cmu.edu/~baraff/sigcourse/notesd1.pdf
Jul
29
comment Is there a situation where $\left|d\vec{v} / dt\right|$ is non-zero while $d|\vec{v}| / dt$ is zero?
Point taken. I will correct.
Jul
29
comment Is there a situation where $\left|d\vec{v} / dt\right|$ is non-zero while $d|\vec{v}| / dt$ is zero?
${\rm d} | \vec{v} | $ is confusing notation. Did you mean ${\rm d} \sqrt{ \vec{v}\cdot \vec{v}}$? Because I took it as $\sqrt{ {\rm d}\vec{v} \cdot {\rm d}\vec{v} }$.
Jul
29
comment Is there a situation where $\left|d\vec{v} / dt\right|$ is non-zero while $d|\vec{v}| / dt$ is zero?
Since $t$ is a scalar, $$\| \frac{{\rm d}\vec{v}}{{\rm d}t} \| = \frac{\| {\rm d}\vec{v}\|}{{\rm d}t} = \frac{{\rm d}\| \vec{v}\|}{{\rm d}t} $$
Jul
29
comment Virtual work (generalized forces) for rotation with Euler angles
Equation 7.2.31 in ocw.mit.edu/courses/mechanical-engineering/…
Jul
29
comment Virtual work (generalized forces) for rotation with Euler angles
Actually it comes from the power calculation. It is the unique value that maintains the powerflow through a joint. I also found it in slide 21 of diku.dk/OLD/undervisning/2005f/101/lecture11.pdf and slide 32 of homes.cs.washington.edu/~todorov/courses/amath533/…
Jul
29
comment Virtual work (generalized forces) for rotation with Euler angles
Did you mean $\dot{\omega}=$ or ${\omega}=$ ? Angular velocity is $\omega=f(\dot{\phi},\ldots)$ and angular acceleration is $\dot{\omega}=f(\dot{\phi},\ddot{\phi},\ldots)$
Jul
28
comment Can forces in N be thought of as a length?
Not like lengths, but like vectors.
Jul
28
comment system to reposition a car
You want to use RCS? See the equations here, en.wikipedia.org/wiki/Spacecraft_propulsion.
Jul
27
comment Inelastic collision and conservation of linear and angular momentum
I agree, there is a momentum exchange between the two colliding bodies so the total momentum is maintained.
Jul
22
comment Trajectory of rolling billiard ball after collision with another billiard ball with same mass
\begin{pmatrix} x & y & z \end{pmatrix} => $\begin{pmatrix} x & y & z \end{pmatrix}$ See physics.stackexchange.com/help/notation