# Free-fall acceleration

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
Staff Emeritus
Gold Member
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

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
Staff Emeritus
Gold Member
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

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
Staff Emeritus
Gold Member
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
Mentor
jamie said:
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.

what would cause it to move sideways

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"?

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.

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LURCH
JohnDubYa said:
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.

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.

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
Homework Helper
Gold Member
JohnDubYa said:
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 [Broken]
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.

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free fall

even below straight line doesn't do it for something travelling 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
Homework Helper
Gold Member
rayjohn01 said:
even below straight line doesn't do it for something travelling 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.

What has your comment got to do with mine??

robphy
Homework Helper
Gold Member
Maybe I should have clarified that
"falling" [below straight line motion]
means "falling" [below the trajectory of inertial motion].

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.

Consider the situation where the space ship 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
Mentor
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, lets 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.

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.