# why do things accelerate when they fall and not stay the same speed?

Hi

I'd like to check i'm understanding what I'm reading. Hopefully someone on this forum will confirm or disconfirm the following:

I have just read that objects accelerate when they fall. No explanation was given as to why they accelerate and not stay the same speed. However the book has previously mentioned Newton's first law so I am thinking the reason is as follows:

According to 1st law, moving objects travel at constant speed if the net forces are 0, and accelerate/decelerate if forces are not equal.

When things fall they have a downward force which is their weight due to gravity(?) and an upward force (friction from air resistance?). If they are accelerating then these forces aren't equal so the weight due to gravity exceeds air resistance?

I presume then that terminal velocity is when air resistance matches downward force and you don't travel any faster.

Please let that be right :) (at a simple level)
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Billing EngineerCommented:
> If they are accelerating then these forces aren't equal so the weight due to gravity exceeds air resistance?

EXACTLY.
air resistance is very low, as force, compared to the "gravity"
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Commented:
If you throw a large object horizontally out of an airplane, its initial vertical velocity is 0. There is a downward gravitational force on it. So it starts accelerating down. The initial air resistance force is very little. I believe that the air resistance force is proportional to the speed. However the truth is more likely here: http://en.wikipedia.org/wiki/Drag_%28physics%29

But, in any case, as the speed increases, so does the air resistance force. Eventually, there is a terminal speed where the upward air resistance force is equal to the gravitational force resulting in a net 0 force on the object.

Because the object has inertia and is already moving downwards, it continues to do so at a constant speed until it hits the ground. After that impact, the results are determined by a number of different factors, strength of materials in the object, the compositional makeup on the ground at the point of impact.
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Author Commented:
I don't know what inertia is :) Haven't got to that yet. I'm sure i'll be asking questions about that in a day or so.
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Author Commented:
many thanks
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Commented:
>> air resistance is very low, as force, compared to the "gravity"
It appears that our answers are opposite since I was saying that the air resistance force becomes the same magnitude as the object's speed increases.
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Commented:
Wonderful article on "Drag" in Wikipedia.

Terminal velocities for balloons, feathers, ping pong balls, ...  are on the order of 1 m/s.
You could probably race a balloon down an open circular stairwell.

Drag is related to the size/shape of a projectile.  Inertia is related to the mass.

Bullets are made of lead (uranium is even better) to mitigate effects of air resistance.
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