# Elementary physics of falling bodies.

1. Apr 22, 2007

### Randomer Guy

It is possible to calculate the velocity of falling bodies (ignoring air resistance of course) using a gravitational acceleration constant.

If there is some other resistance, other than air, is it reasonable to simply reduce the acceleration by some amount to account for that resistance, if one is trying to get at a reasonable range for kinetic energy?

Assume:

You don't know what the resistance is

You DO know there is downwards motion

I am trying to get at an ending velocity for about 3-10 meters of accleration if that helps. Using that ending velocity, I can then easily calculate kinetic energy.

2. Apr 22, 2007

If you have an acceleration, what is its velocity after that amount of distance?

3. Apr 22, 2007

### Randomer Guy

Ah, there is the burn.

I am trying to get the acceleration or a close approximation of it.

The people I am talking to say that it is not reasonable to simply reduce or "fudge" the gravitational acceleration to account for some unknown quantity of resistance.

The falling object is meeting some resistance that is not due to air.

4. Apr 22, 2007

### Randomer Guy

Going back to the OP, the only known is that the object is moving.

I think the resistance is rather small compared to the energy of motion, so the ending velocity is probably pretty close to what one would expect from a free fall. (say about 75% of g, acting over a known distance)

5. Apr 22, 2007

### Randomer Guy

I do know that the maximum acceleration is that of gravity though.

6. Apr 22, 2007

how big is the body?? I say, if the problem doesn't say it, they don't want you to have that much fun.

7. Apr 22, 2007

### Randomer Guy

The question ultimately hinges on how much force, relative to the mass, is required to stop the fall.

The thing doing the catching is designed to hold the falling mass stationary plus some.

I am trying to demonstrate that the kinetic energy/force imparted of a falling body is MUCH higher than that of the body held stationary.

8. Apr 22, 2007

### Randomer Guy

X is falling from 11 meters.

Y is an object designed to support X+(unknown variable) against gravity when it is stationary.

How much force/resistance must Y apply to X to stop its downwards motion?

Assume X+(unknown variable)= X*2

What is not known in this case is the ending velocity. If two times the mass of the falling object X is all that Y can stop/catch, the ending velocity would have to be VERY low, around 5% (less actually) of what one would expect if the object had been accelerated for 11 meters at 9.8 m/s/s

I have figured all of this out, but simply want to know if there is anything I am missing.

Again, I have been told my calculations (by my dubious pupils) are meaningless. I think they don't believe that moving bodies really impart as much force as they do.

9. Apr 22, 2007

Well, force does equal mass times acceleration. so bigger the mass, and g will always be the same for the most part, the bigger the force. Its really not that complex really. In terms of kinetic energy, I think the velocity would have a bigger impact at a certain speeds because of the v squared. But it takes alot of time to reach a powerful energetic speed when the acceleration is g. but that object has a alot of mass so, its from all that mass.

Last edited: Apr 23, 2007
10. Apr 22, 2007

I think you should research momentum, collisions etc... You derive the kinetic equations from momentum relationships. Newton's equation is actually rate of change in momentum

11. Apr 23, 2007

### Randomer Guy

But Y's ability to catch that mass is a function of the mass in the first place.

So the mass is less important than the amount of energy relative to the mass.