'Impossible' rocket drive works and could get to Moon in four hours

[TinselKoala]

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It is all dependent upon that one singular moment in time when a mass reaches the peak of it's upward motion then stops and becomes weightless. For a brief moment in time it acts like it's in outer space with little or no gravity rather than here on Earth. Think about that... no matter it's weight for a brief moment in time it is weightless.

For an instant, the velocity is zero.  The acceleration is in fact at a maximum at that same instant.

No, that is not right either, neither one of you. Neglecting air friction, in a gravitational field like that of the Earth, if a mass is launched upward from the surface by, say, a spring, or out of a gun muzzle, then the mass is in _free fall_ and only under the acceleration of gravity, from the instant it departs the spring or gun muzzle until the instant it hits the ground again... where it is _still_ under the constant acceleration of gravity even though its velocity is once again zero.  The gravitational acceleration is always constant and always downward. During the upward _coasting_ travel of the object, the downward gravitational acceleration acts to reduce the velocity of the object until it comes to an "instantaneous" stop at the top of its arc. The gravitational acceleration is still there, is still constant, and now acts to accelerate the object downward.

[MarkE]

No, that is not right either, neither one of you. Neglecting air friction, in a gravitational field like that of the Earth, if a mass is launched upward from the surface by, say, a spring, or out of a gun muzzle, then the mass is in _free fall_ and only under the acceleration of gravity, from the instant it departs the spring or gun muzzle until the instant it hits the ground again... where it is _still_ under the constant acceleration of gravity even though its velocity is once again zero.  The gravitational acceleration is always constant and always downward. During the upward _coasting_ travel of the object, the downward gravitational acceleration acts to reduce the velocity of the object until it comes to an "instantaneous" stop at the top of its arc. The gravitational acceleration is still there, is still constant, and now acts to accelerate the object downward.

I agree that is absolutely true for all projectiles.  My understanding was that AC was talking about building a harmonic oscillator.

[gravityblock]

Both TK and MarkE have conveniently left out the acceleration equation that is inside F=mA.  It's a common practice for MarkE and his minions to conveniently leave things out.  If the initial and final speed are both the same, then there is no acceleration, regardless of the value for the time interval. The acceleration equation and the result of that equation speaks for itself, as we see below.

Gravock

[gravityblock]

If a force is being applied to an object (such as a gravitational force), and the object is resisting this force with the same intensity (such as being on the ground), then there is no net force and there is no acceleration, thus F=mA is false and Net F=mA is true.

Gravock

[MarkE]

Both TK and MarkE have conveniently left out the acceleration equation that is inside F=mA.  It's a common practice for MarkE and his minions to conveniently leave things out.  If the initial and final speed are both the same, then there is no acceleration, regardless of the value for the time interval. The acceleration equation and the result of that equation speaks for itself, as we see below.


Gravock

Once again you are worse than wrong.  Things can and do accelerate in one direction and later accelerate in the opposite direction with the effect that the starting and ending velocities are zero.  Yet, the acceleration and velocity values during the movement are both far from zero.  The simplest motion profile that a typical digital servo can execute is a triangular velocity ramp and double half parabolic relative position move:  A_MAX from X_START to ( X_START + X_END )/2 and -A_MAX from ( X_START + X_END )/2 to X_END.