I have heard the expression before and as a physics educator I want to make sure I am using the right language.

By describing momentum this way it seems to be modelling momentum as a "thing" that can be transferred or flow around a system, like "energy" being transferred. And that fact that it is conserved during collisions makes this language even more tempting to me.

However I have also heard that is not right to think of it this way at all, it is far better to just restrict its description to being a mathematical quantity (mv). ]]>

I have a question, and I hope it's in the right place. If not, hopefully the mods won't mind too much moving it....

I have a basic understanding of the concept of a space elevator, where a line is tethered to the Earth at one end and "floating" in space at the other (presumably at a terminus type spacecraft or counterweight. This would allow payloads to climb to or descend from Space.

So to my question. Why don't we re-enter space craft vertically from a geostationary orbit? Am I correct in thinking that the amount of ful that would be required to brake the craft so that it descended slowly enough to not overheat would be too impractical to carry? Or am I missing something more fundamental?

Thanks for reading, and apologies if this is a stupid question.

Best

DeltaIV ]]>

An old commercial jet is fitted out with long 'gun'/launcher the length of the craft... about 70 meters .

The plane flies to it's maximum altitude , about 13Km , where atmospheric pressure is 10% that at sea level.

The launcher uses compressed air for initial stage , then coil gun or rail gun principles to get a rocket up to perhaps mach 5 or higher ...

The rocket and launcher need be no more than about 10 cm diameter , once clear of the aircraft the rocket ignites , taking the 'soda can' sized satellite into orbit ... there is a big market for putting small satellites into earth orbit ... 15,000 dollars per Kg ....

Perhaps the mass of each rocket is about 50kg (500 dollars to make) ...payload perhaps 2 kg ... plane could launch 200 of these on one flight.

Is it feasible??? ]]>

So here it is:

BULLET movement is defined by his spike!

Dont consider BULLET's mass, gravitational pull, or any kind of loss of BULLET's speed from other sorts of forces, except acceleration, that is always decressing.If BULLET stopped moving becose he lost his speed, he can be fired again at the same place where he stopped, but must be fired in direction that is facing.Remember: Only invisble force can change BULLET's angel after its fired in time (T).

So for any angel ANGEL, rotation of a BULLET to angel ANGEL is also happening in time (T) whether clockwise or anit-clockwise.

If BULLET's rotation is in motion, and he got rotated for -PI/6, the last rotation of a BULLET stops(last rotation in this case is Pi/2) and new on to -PI/6 strats to increse/decrese.

I'm having trouble with these two questions, can anyone help.

Thank

Pbot = ptop + density*g*height

pbot = 101300 + (1030)(9.81)(4)

F/A = 141717.2

F = 141717.2 * pi * .6^2

F = 160197.1N

Is this force the correct force which I am trying to find? ]]>

A large sealed container full of air falls into the ocean. It has internal dimensions of 6.06 2.59 2.43 m, with walls made of steel (8050 kg m3 ) which is 2.00 cm thick. Does it float?

2. Relevant equations

FBuoyancy = density*volume*g

weight = mg

m = density*volume

density of salt water = 1030 kg/m^-3

3. The attempt at a solution

Volume = (6.06)(2.59)(2.43) = 38.139

FBuoyancy = (1030)(38.139)(9.81)

= 385,367.9 N

= 385,000 N

Weight:

Wtot = Wair + Wsteel

Wair = m*g = density*volume*g = (1.29)(38.139)(9.81) = 482.65 N

Wsteel = (8050)(38.139)(.02meters)(9.81) = 60,237.1 N <- I don't understand what to do about the thickness.

Weight = 60,719. 8N which is less than FBuoyancy which means it floats.

Is this right? ]]>

We are university students working on a pet project, we are trying to pick up an electrical current passing through a conductor via a capacitive sensor (i.e. attached to a charge amplifier with filters) that is not in direct contact with the circuit.

The signal we detect seems to be larger when the conductor is in contact with one electrode only.

As soon as the circuit is completed (we connect the conductor at both ends) the signal we detect on our sensor is either attenuated or disappears altogether…

The signal source is a battery powered generator with no direct connection to the sensing circuit. We are simply trying to detect an electric field.

The signal is about 2 volts over 0.45ms

Does anyone know why this is or what is happening?

Any help really appreciated... ]]>

I know there is a nice experiment where you project a 'bullet' horizontally and drop one at the same time, the two land at the same time. This demonstrates the independence of horizontal and vertical motion and allows us to separate the motion when working on projectile problems.

However, i do not understand WHY this works and WHY i am allowed to do this?

Is it because they are right angles to each other so simply cannot affect each other? ]]>