Free Fall Acceleration: Better Term?

[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!

:)

 

[Rampantbaboon]

Armo is a failure. It is just constant gravitaional acceleration. no matter what direction its moving...that the only clarification that's necessary. It might be difficult to grasp the concept of accelerating in the opposite direction of movement. but then there is a general comprehension problem as its a basic mechanics concept, similar to braking in a car.

 

[JohnDubYa]

Armo is a failure.

You're not exactly subtle, are you?

It is just constant gravitaional acceleration. no matter what direction its moving...that the only clarification that's necessary. It might be difficult to grasp the concept of accelerating in the opposite direction of movement. but then there is a general comprehension problem as its a basic mechanics concept, similar to braking in a car.

That's all fine, but irrelevant to the topic.

 

[Rampantbaboon]

Armo is a friend of mine...its all in jest. Anyway, its true but not irrelevant. the confusing terminoligy comes about when when people don't get that the gravitational acceleration can still be in the other direction even when an object has 0 velocity. my initial cofusion way back when i still had a problem grasping basic mechanics was that the acceleration due to gravity is the same in any case. and the term falling is a reference to acceleration not velocity.

 

[Phymath]

I think the orginal post, is referring to say an astronaut, that is in a free fall in space, but not actually falling, he/she is just falling at the same rate the Earth is spinning (orbit)...idk just to help you all out, the other word for free fall...under the influnce of a gravtational force

 

[JohnDubYa]

Rampant, the problem is that students have associated falling with downward motion all their lives. To assign it to cases where an object is rising is counter-intuitive to THEM.

Take a bunch of schoolchildren outside and throw a rock in the air. On the way up, ask them if the rock is falling. What do you think they will say?

And the idea of falling is ingrained in their heads for 20 years, and now you are going to say that the rock is actually falling, when they "know" that such isn't the case.

One of the complaints that students have about physics is that we use their common-day lingo in odd ways. To them, it is like showing a picture of a rhino and telling them it is an elephant.

 

[jamie]

This discussion regarding free fall has taught me to be sure of the facts before argueing. I apologise i got it wrong.
something can be in freefall if it is going up but it has to be traveling a an angle of 0 degrees. if it is not then it is undergoing projectile motion,which means that the only force acting on it is gravity which is the same as freefall but i think it is important that we do distinguish between the two.
My respect for the forum has increased endlessly and i will be contributing as soon as my new credit card comes through.
regards all and keep on talking. it feeds our brains and gets us thinking
jamie

 

[JohnDubYa]

something can be in freefall if it is going up but it has to be traveling a an angle of 0 degrees. if it is not then it is undergoing projectile motion,which means that the only force acting on it is gravity which is the same as freefall but i think it is important that we do distinguish between the two.

You need to work on it a little more. Simple projectile motion IS an example of a body in free fall. An artillery shell shot out of a cannon is in free-fall motion (ignoring air friction).

Free-fall acceleration is defined as motion where the only force acting on the object is gravity. A projectile satisfies this criterion.

I think everyone can see why the term free-fall acceleration leads to errors. :)

 

[BobG]

You could just say the object's experiencing gravitational acceleration.

Oh, wait. How can it be accelerating if it's slowing down?

Terminology's a problem, no matter how you slice it. Common language evolves over time, sometimes for good, sometimes for bad (what could be wrong about being a little more specific about exactly what type of acceleration you're experiencing - positive or negative?). Science and math tend to hold their terms constant and wind up using some rather archaic definitions.

I get the same problem explaining orbital element sets. Argument of perigee, true or mean anomaly, flight path angle. If you're creative, you can use three different words for angle in one sentence (argument, anomaly, angle). And just try explaining how the First Point of Aries can be found in the Pisces constellation - you're sure to wind up arguing about when will it be the dawning of the age of aquarius.