A car (mass mA) is driving at velocity V, when it smashed into an unmoving car (mass mB), looking bumpers. Both cars move together at the same velocity. The common velocity will be by A) mAV/mB B) mBV/mA C) mAV/(mA+mB) D) (mA+mB)V/mA

It is said that we can't study quantum gravity because gravity is a weak force. But gravity and acceleration are the same. Why can't we study quantum gravity in a strongly accelerated frame of reference?

What reference frame is used to measure the velocity, $v=\frac{distance}{time}$?
$\mathbf{p}=\frac{mv}{\sqrt[2]{1-\frac{v^2}{c^2}}}$
Suppose the momentum of an electron is being observed in a lab. It could be observed in a lab that as the speed of electron increases, it becomes harder to accelerate it. To measure the velocity of electron, we need to measure distance or displacement and time.

Why is it hopeless to view differential geometry as the limit of a discrete geometry?
Classical mechanics can be understood as the limit of relativistic mechanics $R{M}_c$ for $c\to \mathrm{\infty }$.
Classical mechanics can be understood as the limit of quantum mechanics $Q{M}_h$ for $h\to 0$.
As a limit of which discrete geometry ${\mathrm{\Gamma }}_{\lambda }$ can classical mechanics be understood for $\lambda \to 0$?

The reason the galaxy does not spin around me (relative to me) at a trillion times the speed of light when I do a pirouette is the inertial frame of reference to which I am confined along with the rest of humanity.
Where does this frame of reference end?
To the best of my limited understanding, gravity doesn't just stop at some point. Anything that has mass has gravity; gravitational waves (or whatever) extend from the mass spherically. They weaken as they get further away from the source, they get pushed around by other, more powerful, gravity waves, they become progressively weaker as the distance increases, but do they ever vanish completely, i.e. get reduced to absolute zero? Or do they become weaker and weaker forever, without zeroing out? And if so, where does a frame of reference end?

How do you find radioactive decay half life?

An original sample of the radioisotope fluorine-21 had a mass of 80.0 milligrams. Only 20.0 milligrams of this original sample remain unchanged after 8.32 seconds. What is the half life of fluorine-21?