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

 

[chroot]

A ball going upwards is still accelerating downward at 9.81 m/s^2. That's why it slows down, stops, and begins falling.

By your own definitions, a ball that is going up is in free fall. It's always accelerating downward, and, once the ball has left your hand, gravity is its sole influence.

- Warren

 

[russ_watters]

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

"An object in motion will remain in motion unless acted upon by an outside force." So warren's baseball can translate (move sideways) once it has left his hand and still be considered to be in freefall. No other force is required. A baseball can also be rising even while it has no other forces acting on it apart from gravity. If it couldn't, it would drop out of your hand and never go up.

 

[jamie]

what would cause it to move sideways

 

[JohnDubYa]

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

When a shell is launched, what forces other than gravity act on it? (Ignoring air friction, of course.)

The term "free fall" is already used to describe simple projectile motion, no matter if the ball is rising or falling. That is not in dispute. The question is, "Is there a better term available?"

Another example are satellites in orbit. They are said to be in free-fall because only gravity acts on them. But isn't that misleading to many students? After all, the distance between the astronauts and Earth is not changing, so are they realling "falling"?

 

[JohnDubYa]

BTW, 9.8 is not an exact value for g. I use this inaccuracy to let g = 10 to simplify computations. Students like not having to pull out their calculators all the time.

 

[jamie]

I agree the term free fall is misleading, The definition is given on the following website
http://www.wordiq.com/definition/Free-fall.
This will probably explain the term better than i can
regards jamie

 

[LURCH]

When a shell is launched, what forces other than gravity act on it? (Ignoring air friction, of course.)

The term "free fall" is already used to describe simple projectile motion, no matter if the ball is rising or falling. That is not in dispute. The question is, "Is there a better term available?"

Another example are satellites in orbit. They are said to be in free-fall because only gravity acts on them. But isn't that misleading to many students? After all, the distance between the astronauts and Earth is not changing, so are they realling "falling"?

On the contrary, I think the term is very enlightening. Astronauts in orbit truly are falling, not only in the technical sense but also in terms of the "gut feeling" we usually associated with the idea of falling. Ask any astronaut who's been in orbit and he'll tell you; he was falling. Watching the event on our television sets, or looking at a chart or diagram of it, we tend to miss this simple truth. Using the term "free fall" to describe it helps us to remind ourselves of what is actually happening. IMHO, anyway.

 

[JohnDubYa]

So if they are falling, when do they hit the ground? That is the fundamental problem with the term "falling."

No one here is saying the term is incorrect -- only misleading to those struggling to learn physics.

And having an object rising upwards and, at the same time, be in free fall is truly confusing. Sure, WE know what is taking place.

 

[JohnDubYa]

what would cause it to move sideways

Newton's first law: Just because an object is moving, don't think that something necessarily had to cause it to move. After all, such motion is just as natural for an object as sitting still.

But to be more precise, the initial cause of the motion is irrelevant since this force no longer acts on the object during the time under consideration. So an artillery shell* fired at an angle is in free fall once it leaves the barrel of the gun.

* The artillery shells in my physics examples are not duds, and they maim and kill when they hit the ground. Stupid PC textbooks! :)

 

[robphy]

So if they are falling, when do they hit the ground? That is the fundamental problem with the term "falling."

No one here is saying the term is incorrect -- only misleading to those struggling to learn physics.

And having an object rising upwards and, at the same time, be in free fall is truly confusing. Sure, WE know what is taking place.

http://galileoandeinstein.physics.virginia.edu/lectures/Newton.html
http://www.shef.ac.uk/physics/people/vdhillon/teaching/phy105/phy105_gravitation.html
have an answer to the question you ask... the Earth's surface falls away sufficiently fast for their motion.

On these webpages, there are references to Newton's determination that the moon "falls" [below straight line motion] 1/20th of an inch (1.37mm) each second. (This is consistent with my description above of falling with respect to the tangent to the trajectory at the launch position.)

It may just be that "falling" [below straight line motion], however imperfect, is the best term.

 

[rayjohn01]

free fall

even below straight line doesn't do it for something traveling straight up and down. The problem with a 'simple' term is that it has to describe several rather different motions including circular or eliptical -- I'm not sure there is a 'simple term' to do that. However the student will study all these motions and they obey one set of rules , I do not think they are confusing, Ray.

 

[robphy]

even below straight line doesn't do it for something traveling straight up and down.

Really?

Traveling vertically with constant velocity V₀:
y_A=y_0+v_0t

Launched vertically with initial velocity V₀:
y_B=y_0+v_0t + \frac{1}{2}\left(-g \right)t^2

The magnitude of the term \frac{1}{2}\left(-g \right)t^2 is how much the projectile has fallen, relative to the constant velocity particle.

 

[rayjohn01]

What has your comment got to do with mine??

 

[robphy]

Maybe I should have clarified that

"falling" [below straight line motion]

means "falling" [below the trajectory of inertial motion].

 

[JohnDubYa]

have an answer to the question you ask... the Earth's surface falls away sufficiently fast for their motion.

Yes, I know that. I'm not the one struggling with the concepts. We are talking about clarifying the language for students.

 

[JohnDubYa]

Consider the situation where the spaceship has more energy than necessary to maintain circular motion, so that it spirals away from the Earth. Doesn't everyone here find calling its motion "free fall" counterintuitive? It sure doesn't seem to me that the term "falling" is a very apt description of its motion.

 

[russ_watters]

I'm generally not in favor of simplifying language to make things easier for students to understand. What that ends up meaning is they never learn the correct terminology. The term "free-fall" works just fine if it is explained.

I've heard people have difficulty with the concept of Relativity because the term "relative" makes them think the theory says the speed of light is relative. So to avoid confusion, let's just call it 'Einstein's theory of Bob.' Problem solved - until the future physics majors in the class go to college and wonder what the heck this theory of "Relativity" is.

Another thing that irritates me: oversimplification. The classic example is "the speed of light in a vacuum..." We see someone misinterpret this almost once a week. There are probably a good dozen threads here on it. Students learn in high school that the speed of light is C in a vacuum, but it varies in different media. Imagine their surprise (and confusion) when they first read about Relativity and the constancy of the speed of light. Avoiding that requires a tiny, simple clarification when teaching optics in high scool physics.

 

[JohnDubYa]

I'm generally not in favor of simplifying language to make things easier for students to understand. What that ends up meaning is they never learn the correct terminology.

Which is why I am not in favor of changing the term "work." But "free-fall" is misleading, not just hard to understand.

I think we should adapt the names of things until we get it right. Some concepts are so deeply ingrained that we best leave them alone, such as "work" and "force." But I think replacing the term "free-fall" with something better (if it exists) can only have positive effects on physics teaching. At least, IMHO.

 

[ArmoSkater87]

how about, Force-Free gravity acceleration...LOL

 

[JohnDubYa]

I Don't Think That's Funny At All!

:)