# basic physics - terminal velocity again

Hi

Quesiton about terminal velocity

When a sky diver jumps out of a plane he accelerates due to gravity until he reaches terminal velocity when force of air resistance = weight of man due to gravity

When you do free body diagrams to show the forces as the sky diver falls the downward force of the man is always mass * acceleration due to gravity.

So lets say the man is 70kg and his weight is ~ 700N, his force is always 700N. So at terminal velocity when he travels at a constant speed his force = mass * accleration due to gravity.

From a teaching perspective (and from my own too a bit) i think its confusing to explain something is travelling at a constant velocity yet accelerating due to gravity. Any thoughts on this welcomed. In reality i need to ask other teachers about this but I don't have that opportunity yet so i value your expert opinion until then

Thanks
Asked:
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Commented:
>> travelling at a constant velocity yet accelerating due to gravity

Constant velocity means balanced forces, and no acceleration.
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Commented:
He only travels at constant veloctity once the man reaches his terminal velocity where there is no acceleration as glitch says...
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Commented:
"From a teaching perspective (and from my own too a bit) i think its confusing to explain something is travelling at a constant velocity yet accelerating due to gravity"
--
Be precise in the use of your words.

"he accelerates due to gravity"
He does NOT experience the acceleration due to gravity.
He experiences the FORCE due to gravity.
That force will provide him with an acceleration (due to gravity) IF there is no other force operating.
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Author Commented:
Hi again

This is similar to my other question and I've caused the same issue here as in that question by not being precise, sorry.

Wil try again:

When a sky diver jumps out of a plane he experience force due to gravity until he reaches terminal velocity and travels at constant speed when force of air resistance = weight of man due to gravity

At terminal velocity (and at any point during the fall) the force of the man is his mass * acceleration due to gravity.

So at terminal velocity the man is travelling at a constant speed but the equation to calculate his downward force, f = ma, includes a term for acceleration. It's not the man that's accelerating but i think its confusing because you are calculating the force of the man using the mass of the man so you automatically think its the acceleration of the man. (i think kids will anyhow)

I don't think i would know how to explain that this acceleration term. I don't know if it would be wrong to avoid the word acceleration at all when calculating the force due to gravity.

i.e.  the downward force of gravity depends on your weight. The force of gravity = mass times a constant. That constant is the acceleration of all objects if they fall under the sole influence of gravity.
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Commented:
You have hit upon the wonderful mystery of gravity.

F = ma should more reasonably be taught as a = F/m in the general sense.  If a force F acts on a body of mass m, then that body will experience an acceleration a.  This makes clear sense.

But gravity is special.  Gravity will magically figure out how to apply just exactly the force F necssary to always accelerate any body of any mass m with an acceleration of a.  The day you figure out how it does that you will be world famous and very rich.  You just have to teach that this is one of the great remaining mysteries of the universe.

So, our skydiver is being pulled to earth by a force F which gravity, because it always makes bodies fall at an acceleration a, has made equal to ma.  It is always pulling him to earth with that force.  But at some point the resistive force of the air equals the force of gravity, so the net unbalanced force is zero so he stops accelerating.

He is not constantly accelerating under gravity - he is having a force F=ma applied to him, and gravity, by some unkown magic, has adjusted that force to be ma.
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Commented:
"i.e.  the downward force of gravity depends on your weight. The force of gravity = mass times a constant. That constant is the acceleration of all objects if they fall under the sole influence of gravity."
the downward force of gravity IS your weight.
Even the above statement is not accepted by everyone. Some books differ.

"gravity, by some unkown magic, has adjusted that force to be ma."
That statement is truer than you might imagine.
Ask me about it again in 6 weeks and I might have a 1000 word paper on the subject.
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Author Commented:
aburr - can i see the paper? I'd love to show it to the person who is due to teach me physics
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Author Commented:
xfvgdrthbdtyvhgscv:, that's a very interesting user name

Gravity will magically figure out how to apply just exactly the force F necssary to always accelerate any body of any mass m with an acceleration of a.

That had never occured to me, because i am new to physics. That;s really quite fascinating.

So i wouldn't be 'wrong' at a basic level of physics to avoid using force = ma when it comes to gravity and treat gravity as a special condition whereby force = mg?
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Commented:
"gravity, by some unkown magic, has adjusted that force to be ma"
Put another way
The mystery is, "why is inertial mass the same as gravitational mass?"
The General Theory of Relativity has done much to clear this up but I think some difficulties still remain.
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Author Commented:
thanks
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