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I'm not sure whether everything's clear from your description and from Geoff's Answer. If you haven't already worked it out, the essential problem that Feynman and Einstein are getting at is the rela …

To add to Robin Ekman's answer, the answer is "yes" as stated there, but you need to be very careful these days with the word "matter" - this is a word that is becoming outmoded in physics as we under …

We (physicists) believe the reason is this: known symmetries and conservation laws. For example, the mutual annihilation of a proton and positron would remove $2\,e$ charge units from the Universe. Th …

It's simply down to lack of observation: as the scientist community, we have never seen a reproducible perpetual motion machine, or a violation of the laws - particularly either conservation of energy …

You cannot walk at all if there is no horizontal component of the force of interaction between you and the ground; by this definition "no friction" is tantamount to "no ability to move horizontally". …

The astronaut can change his or her orientation in the same way that a cat does so whilst falling through the air. After the transformation, the astronaut is still and angular momentum is conserved. T …

Charge is a less awkward and probably more useful notion than mass. This is because the former is always conserved, and is linear additive in the sense that the charge of a composite system is exactly …

Given you've read only QED, this is a highly astute question. Conservation laws in the quantum world work a little differently from classical conservation grounded on Noether's theorem (there is a ki …

Whilst the motion you intend is not altogether clear, the motor will indeed move as you say. But the attached rigid bodies also move, such that the center of mass of the whole system is stationary (or …

There is no straightforward way to calculate the force of impact, because this latter is defined by the deceleration of the egg at the end of its journey as a function of time. In turn, this decelerat …

Their kinetic energy is in general different and is the same if and only if the masses are equal. If their oppositely directed velocities have magnitudes $v_1$ and $v_2$ respectively, then your first …

There are two and possibly three factors at work here: Factor 1: Nonscalar Inertia Matrix: Angular Momentum and Velocity have Generally Different Directions One which I don't think has been mentioned …

The statement is really about the transformation between inertial co-ordinates and co-ordinates fixed to the body. This is expressed by: $$D_t = d_t + \omega(t)\times\tag{1}$$ where $D_t$ is the "to …

The other answers are correct but it's worth stating, given your title's question, what the continuity equation is actually telling us. A continuity equation is the expression of balance between the …

Newton's third law is pretty near to the mark. All of the phenomena you cite stem from the principle of conservation of momentum in an isolated system, itself ultimately a result (through Noether's t …