Why does gravity accelerate

This is often referred to as the acceleration due to gravity and is the value obtained if the air resistance force acting on the falling object is negligible. A Physics Narrative presents a storyline, showing a coherent path through a topic For 14 Resources. This activity provides a memorable experience relating to the gravitational pull of the Earth and establishes that objects Classroom Activity When was the last time you watched a young child playing that favourite game of throwing the toy out of the pram?

It was learned in the previous part of this lesson that a free-falling object is an object that is falling under the sole influence of gravity. A free-falling object has an acceleration of 9. This numerical value for the acceleration of a free-falling object is such an important value that it is given a special name. It is known as the acceleration of gravity - the acceleration for any object moving under the sole influence of gravity.

A matter of fact, this quantity known as the acceleration of gravity is such an important quantity that physicists have a special symbol to denote it - the symbol g.

Note that there is no mass in this equation - it doesn't matter what the mass of the object is, they all follow the same geodesic so long as it's not massless, in which case things are a little different. So, what good is this geodesic description of the force of gravity? Can't we just think of gravity as a force and be done with it? It turns out that there are two cases where this description of the effect of gravity gives vastly different results compared to the concept of gravity as a force.

The first is for objects moving very very fast, close to the speed of light. Newtonian gravity doesn't correctly account for the effect of the energy of the object in this case. A particularly important example is for exactly massless particles, such as photons light. One of the first experimental confirmations of general relativity was that light can be deflected by a mass, such as the sun. Another effect related to light is that as light travels up through the earth's gravitational field, it loses energy.

This was actually predicted before general relativity, by considering conservation of energy with a radioactive particle in the earth's gravitational field. However, although the effect was discovered, it had no description in terms of Newtonian gravity. The second case in which the effect of gravity vastly differs is in the realm of extremely strong gravitational fields, such as those around black holes. Here, the effect of gravity is so severe that not even light can escape from the gravitational pull of such an object.

Again, this effect was calculated in Newtonian gravity by thinking about the escape velocity of a body, and contemplating what happens when it gets larger than the speed of light. Surprisingly, the answer you arrive at is exactly the same as in general relativity.

However, as light is massless, you once again do not have a good description of this effect in terms of Newtonian gravity, which tells you that there has to be a more complete theory. So, to summarize, general relativity says that matter bends spacetime, and the effect of that bending of spacetime is to create a generalized kind of force that acts on objects. However, it isn't a force as such that acts on the object, but rather just the object following its geodesic path through spacetime.

Home Physics The Theory of Relativity. Advanced Einstein said there is no such thing as a gravitational force. Similar Questions that might Interest You If gravity is a "curvature of space" rather than a force, why do a ball and bullet follow different paths? Intermediate If light has no mass, then what draws it into a black hole? Intermediate How do gravitons escape black holes to tell the universe about their gravity?

Advanced If photons have zero mass, why do they feel the effects of gravity? Advanced Why are astronomers interested in gravitational waves? Random Question. How can two moons of Saturn share the same orbit? Most Recent. Is speed of light the same everywhere? The problem is this: if you have a central force, where a bound particle like for example the Earth is attracted to the Sun but moves around the Sun orbiting, or propagating at a finite speed, you will only get a purely elliptical orbit if that force's propagation speed is infinite.

If it's finite, then you don't just get a radial acceleration towards the other mass , but you also get a component that accelerates your particle tangentially. This would make orbits not only elliptical, but unstable. On the scale of a mere century, orbits would shift substantially. By , Laplace had used observations of the Moon to demonstrate that the speed of Newtonian gravity must be 7 million times greater than the speed of light. Modern constraints are now 20 billion times the speed of light, which is great for Newton.

But all of this placed a great burden on Einstein. One revolutionary aspect of relativistic motion, put forth by Einstein but previously built up by The faster you move relative to someone at rest, the greater your lengths appear to be contracted, while the more time appears to dilate for the outside world.

This picture, of relativistic mechanics, replaced the old Newtonian view of classical mechanics, but also carries tremendous implications for theories that aren't relativistically invariant, like Newtonian gravity. According to Einstein, there's a big problem, conceptually, with Newton's gravitational force law: the distance between any two objects is not an absolute quantity, but rather is dependent on the motion of the observer. If you're moving towards or away from any imaginary line you draw, distances in that direction will contract, depending on your relative velocities.

For the gravitational force to be a calculable quantity, all observers would have to derive consistent results, something that you cannot get by combining relativity with Newton's gravitational force law. Therefore, according to Einstein, you'd have to develop a theory that brought gravitation and relativistic motions together, and that meant developing General Relativity: a relativistic theory of motion that incorporated gravity into it.

Once completed, General Relativity told a dramatically different story. An animated look at how spacetime responds as a mass moves through it helps showcase exactly how, Note that spacetime can only be described if we include not only the position of the massive object, but where that mass is located throughout time.

Both instantaneous location and the past history of where that object was located determine the forces experienced by objects moving through the Universe. In order to get different observers to agree on how gravitation works, there can be no such thing as absolute space, absolute time, or a signal that propagates at infinite speed.

Instead, space and time must both be relative for different observers, and signals can only propagate at speeds that exactly equal the speed of light if the propagating particle is massless or at speeds that are below the speed of light if the particle has mass. In order for this to work out, though, there has to be an additional effect to cancel out the problem of a non-zero tangential acceleration, which is induced by a finite speed of gravity.

This phenomenon, known as gravitational aberration, is almost exactly cancelled by the fact that General Relativity also has velocity-dependent interactions. As the Earth moves through space, for example, it feels the force from the Sun change as it changes its position, the same way a boat traveling through the ocean will come down in a different position as it gets lifted up and lowered again by a passing wave.

Gravitational radiation gets emitted whenever a mass orbits another one, which means that over long Before the first black hole ever evaporates, the Earth will spiral into whatever's left of the Sun, assuming nothing else has ejected it previously. Earth is attracted to where the Sun was approximately 8 minutes ago, not to where it is today.