Free Fall Acceleration: Better Term?

[JohnDubYa]

We have already discussed the definition of g to death, but I have another question regarding the use of "free fall." This is also a term that is misleading, since a body doesn't have to be falling to be truly in free-fall.

Can anyone come up with a better term?

 

[jamie]

please explain what could be in freefall without falling

 

[JohnDubYa]

An object rising.

 

[ArmoSkater87]

? then it won't be falling...

 

[ArmoSkater87]

and if something is rising, then ur going against gravity, which is something completely different

 

[DeShiznit44]

When it is rising the object, let's say a rock, is still considered to be in the "free fall" scenario. This is because even though it is has been given enough force to counteract its weight, gravity is still doing work on the rock. So you could say it is falling in the negative direction. This is what JohnDubYa is trying to say i think. The term free fall is not a very good term because it gives us the impression that the object must be falling.

 

[chroot]

To be in "free fall" means to have no forces other than gravity acting upon you. In the parlance of general relativity, it means you're following a geodesic (a straightest-possible line) through curved spacetime.

Perhaps you should just call it "geodesic motion."

- Warren

 

[ArmoSkater87]

hum...good point, it accelerates upwards but negatively, -9.81m/s^2.

 

[JohnDubYa]

RE: "then it won't be falling..."

Precisely the problem.

 

[chroot]

Consider also just calling it "inertial motion."

- Warren

 

[JohnDubYa]

I would think inertial motion would apply where NO forces act on the object.

 

[chroot]

No, an inertial frame is one in which Newton's laws hold. In the parlance of general relativity, gravity is not a force. The only situations that forces are involved are those situations in which a body is not allowed to follow its natural trajectory. The chair you're sitting on is preventing you from following the trajectory you'd otherwise follow, onto the ground. When you're freely falling, you don't feel your own weight, which means no forces are acting upon you.

Einstein's principle of equivalence states quite simply that the physics in an inertial frame is indistinguishable from that in a freely falling frame -- so calling free fall "inertial motion" is entirely valid.

- Warren

 

[robphy]

In the context of Newtonian mechanics, a "free-falling projectile" is falling with respect to an object starting at the launch point and moving with constant velocity equal to the projectile's launch velocity. Pictorially, draw the projectile's parabolic trajectory and the tangent line to that parabola at the launch point.

 

[JohnDubYa]

I am not saying that "free fall acceleration" is inaccurate, but simply misleading.

 

[robphy]

I guess the problem is that "falling" suggests decreasing.

Thinking about it more, it seems to me that what is falling (decreasing) is the y-velocity. As the y-velocity decreases, one can picture the velocity vector turning downwards.

In terms of the trajectory itself, it seems that one may need to capture the notion of "concave down".

 

[JohnDubYa]

Well, that assumes the object isn't thrown perfectly vertical. :)

 

[ArmoSkater87]

how about ...gravity acceleration :D

 

[jamie]

The definition of freefall is "no other forces are acting on it apart from gravity".
so therefore it could not be rising, going sideways or anything else but falling

 

[LURCH]

Sure it could; A sattelite going to a higher orbit burns its engine for a relatively short period of time, then the engine is shut off. But the sattelite is still gaining altitude (relative to the Earth's surface). It is, quite litterally, "falling up". Once the transfer orbit burn is finished, no force (other than the pseudoforce of gravity) is acting on the sattelite, but it continues to climb.

 

[chroot]

Or, even simpler, consider the first half of a baseball's trajectory, just after you've thrown it. It's going away from the earth, but is acted upon by no other forces besides gravity. It is therefore in free-fall.

- Warren

 

[jamie]

There is still a force acting against gravity so therefore it is not in freefall

 

[Doc Al]

There is still a force acting against gravity so therefore it is not in freefall

And what force would that be (ignoring air resistance)?

 

[salamander]

In a books on s. relativity by John A. Wheeler (Spacetime Physics, Freeman) the frame in which no gravitational acceleration is experienced is referred to as the "free float frame". I think the frame is officially called inertial or Lorentz frame, but free float is pretty good.

Why not call free fall "free float" in the general case?

//Cheers

 

[jamie]

could it be momentum?

 

[Doc Al]

could it be momentum?

Could what be momentum? Momentum is not a force.

 

[jamie]

momentum is defined as the mass x velocity
p=mv
so therefore a force applied to it would give it momentum
and you are quite right momentum is not a force

 

[chroot]

You still haven't answered the question -- what forces, besides gravity, act on a baseball that has just been tossed upwards? (Neglecting wind resistance.)

- Warren

 

[jamie]

We have kind of lost track
firstly the force that has been exerted to get it going up in the first place and while it is going up it is not in freefall

 

[chroot]

The force that made it go up in the first place stops acting on it as soon as it leaves your hand, and is not relevant.

By the definition of free-fall, the ball is in free-fall from the time it leaves your hand to the time it strikes the ground.

- Warren

 

[jamie]

all free-falling objects (on Earth) accelerate downwards at a rate of approximately 10 m/s/s (to be exact, 9.8 m/s/s)
A free-falling object is an object which is falling under the sole influence of gravity.
so back to the question how can it be in freefall if it is acting against gravity, ie going up