What is meant by (a) free fall, and (b) acceleration due
to gravity
Answers
Answer:
A little lesson in general relativity (GR): Physical existence is in a medium called space-time, a 4-D continuum. A point-like object is a trajectory (Einstein called it a world-line) in S-T. Relative to an inertial rest frame, an object at rest in that frame is on a trajectory that runs parallel to the time, or ct, axis where units of time are multiplied by the speed of light to get a length to make it compatible with the three spatial dimensions. So you could say that time flows “at the speed of light”. Now if the object was at some speed relative to this inertial frame it would still be a straight line but with a slope compared to the one at rest, so for each successive point in time its space coordinates are different from the preceding coordinates, just as a car is at a different location at each instant. If the object is accelerating, this trajectory is no longer straight but becomes curved —- the slope is a measure of its speed, so increasing speed means increasing slope. Which brings up an important point: acceleration can be kinematic, like being in a car and punching the pedal from a stop, or it can be gravitational, in which case it’s in the state of free fall due to gravity. Absent any external forces, an object will always follow its local geodesic, which is the shortest line (but not the straightest) through 4-D S-T. Thus the bedrock rule of GR: Gravity curves space-time. As the late Prof. Wheeler said, “Matter tells space how to curve, space tells matter how to move”. It’s that simple. And the amount of curvature, for a massive object like the Earth, is very tiny: As a satellite orbits the earth in about 90 minutes, it’s traveled 25000 miles through 3-D space but just over ONE BILLION miles through 4-D S-T (90 minutes times the speed of light). So if you can imagine a helical line of one complete cycle with the helix 8000 miles in diameter but stretched out to a billion miles in length, you have some idea of how little Earth curves its local space (this is equivalent to taking a single coil, or turn, of a spring one inch in diameter and somehow stretching it until it’s two miles long). One other thing: I mentioned “Absent any external forces”. Sitting or standing on Earth’s surface, we are not following our local geodesic and are thus NOT in free fall, but we feel a force equivalent to being accelerated. That’s because we’re prevented from being in free fall due to 4000 miles of hard earth between us and the planet’s center. The point is, in the context of F = ma, gravity is NOT a force! It is a field which causes acceleration (free fall) but there are no forces on an object in free fall — the only forces are those interfering with a state of free fall. However, gravity does qualify as a force in the context of two other definitions, as the gradient of a potential and as the time derivative of momentum.
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Answer:
freefall is the condition in which the object is falling under the gravity without any external agent and
The acceleration which is gained by an object because of gravitational force is called its acceleration due to gravity
Explanation:
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