The way a javelin falls

[Humberto]

Everything else being the same, heavier objects fall at the same speed as lighter objects.

Why is it, then, that the heavier end of a javelin will usually lead the lighter end when falling to the ground?

I guess it has to do with the center of gravity of the javelin.

 

[Sinbad]

Everything else being the same, heavier objects fall at the same speed as lighter objects.

Why is it, then, that the heavier end of a javelin will usually lead the lighter end when falling to the ground?

I guess it has to do with the center of gravity of the javelin.

Not everything else is the same. Heavier objects only fall at the same speed in a vacuum. There is air to contend with, as well as the aerodynamics of the javelin which tend to keep the nose pointed in the direction of travel, which is pretty much downward at the end of the flight.

 

[zippy]

I'm guessing its gravity and momentum. The heavier end isn't falling faster, it's just turning towards the center of the earth before the lighter end. Yes center of gravity and the fact the heavy end was in front when thrown

 

[zippy]

Actually when I read my first reply it doesn't make sense. I think Sinbad is spot on. It's momentum

 

[froot]

Actually when I read my first reply it doesn't make sense. I think Sinbad is spot on. It's momentum

This.
The momentum is pulling the javelin along until such a time that the momentum is not enough to contend with the center of gravity, at which time the javelin will no long do the spear-motion.

 

[Keeper]

Heavier objects only fall at the same speed in a vacuum.

That's not correct.
Aerodynamics asides two balls made of different materials with different weights will fall at the same speed, even if one is heavier than the other

It's only in a vacuum where aerodynamics don't apply, where you can drop two completely different shaped items, that will fall at the same speed.
(eg: a feather will fall at the same speed as a bowling ball, because you've taken aerodynamics out of the equation as well)

 

[Sinbad]

That's not correct.
Aerodynamics asides two balls made of different materials with different weights will fall at the same speed, even if one is heavier than the other

It's only in a vacuum where aerodynamics don't apply, where you can drop two completely different shaped items, that will fall at the same speed.
(eg: a feather will fall at the same speed as a bowling ball, because you've taken aerodynamics out of the equation as well)

You can't say aerodynamics aside when not in a vacuum. End of story. There will be a slight difference if drag is the same and weight is current.

 

[Keeper]

You can't say aerodynamics aside when not in a vacuum. End of story.

Negative. Two spheres/balls will have the same aerodynamic properties.
(that's Why I used it as an example, to put aerodynamics aside)

So, a Ping-Pong ball and a bowling ball will fall at the same speed, even if not in a vacuum.

Therefore saying the following is incorrect:

Heavier objects only fall at the same speed in a vacuum.

What I suppose you were thinking about is a feather and a bowling ball falling at the same speed in a vacuum... but that has nothing to do with weight, that's because
aerodynamics are taken out of the equation.

A vacuum equalizes the aerodynamics between two different objects - Nothing to do with the weight.

 

[Shake&Bake]

there

http://engineeringsport.co.uk/2012/09/21/the-story-of-the-javelin-bringing-it-back-down-to-earth/

 

[Fulcrum29]

Back to school: http://www.physicsclassroom.com/class/newtlaws/

 

[Sinbad]

Negative. Two spheres/balls will have the same aerodynamic properties.
(that's Why I used it as an example, to put aerodynamics aside)

So, a Ping-Pong ball and a bowling ball will fall at the same speed, even if not in a vacuum.

Therefore saying the following is incorrect:

So they have the same drag. They do NOT have the same gravitational force acting on them as they are NOT the same weight. They will NOT accelerate at the same speed. They will also have different terminal velocities.

In the absence of ANY drag, F=ma, or a = f/m. Since gravitational force is directly proportional to mass, the acceleration is constant no matter the mass.

If there is drag, then the F (force) changes, and the f/m ratio does therefore NOT stay the same for all values of m.

 

[Sinbad]

By saying that you are essentially saying:



If I drop a bowling ball and a tennis ball, they will fall at the same speed and land at the same time.
Is that not correct?

Surely the bowling ball will not fall faster than the tennis ball :erm:

It is not correct. They will fall at different speeds in air. They will fall at the same speed in a vacuum.

 

[Keeper]

Heavier objects only fall at the same speed in a vacuum.

By saying that you are essentially saying:

Heavier objects fall at different speeds when not in a vacuum.

If I drop a bowling ball and a tennis ball, they will fall at the same speed and land at the same time, is it not?

Surely the bowling ball will not fall faster than the tennis ball :erm:

 

[Keeper]

It is not correct. They will fall at different speeds in air.

Bull dust, I don't believe this. you telling me a bowling ball will fall faster than a tennis ball?

C'mon, this can't be right.... :wtf:

 

[Keeper]

They will NOT accelerate at the same speed.

I'm also not sure about that quote either.
afaik acceleration under gravity is a constant (9.8 m/s/s), even if the objects weight different amounts. Is it not?


Here is a video:
[video=youtube;miZfGjIgtg0]http://www.youtube.com/watch?v=miZfGjIgtg0[/video]

 

[Sinbad]

Bull dust, I don't believe this. you telling me a bowling ball will fall faster than a tennis ball?

C'mon, this can't be right.... :wtf:

Ok, you know what terminal velocity is, right? The maximum speed an object can reach when falling through air (ie, where gravitational force equals drag)

Terminal velocity is calculated as: Sqrt ((2*m*g) / (rho * A * C))
m = mass
g = gravitational acceleration
rho = density of the fluid/air
A = projected (frontal) area
C = coefficient of drag of the object

So it's obvious from this that increasing the mass, keeping others constant, will increase terminal velocity.
If you double the mass AND double the projected area, keeping all else constant, the terminal velocity of the two objects will be the same.

Radius of tennis ball = 3.85cm, so area = 46.5 cm^2 (pi * r^2)
Radius of bowling ball = 10.8cm, so area = 366cm^2, or 7.8 times the projected area of the tennis ball.

Assume the coefficient of drag is the same (It's not, the tennis ball is fluffy, so will fall slower)

Weight of tennis ball 56g
Weight of bowling ball 7kg or 125x the weight of the tennis ball.

It's obvious from the formula that when you multiply the weight by 125, you need to multiply the area by 125 as well (all else being equal) to get the same terminal velocity. Tennis ball vs bowling ball you don't, so the bowling ball's terminal velocity will be much higher than that of a tennis ball. Add the increased C of the tennis ball into the equation and it's an even bigger difference!

 

[Sinbad]

I'm also not sure about that quote either.
afaik acceleration under gravity is a constant (9.8 m/s/s), even if the objects weight different amounts. Is it not?


Here is a video:
[video=youtube;miZfGjIgtg0]http://www.youtube.com/watch?v=miZfGjIgtg0[/video]

9.8m/s/s without taking drag into account.
On such a short drop, the difference is too small for the naked eye to make out. The objects need to reach a significant fraction of terminal velocity for the difference to be made out easily.

 

[Keeper]

But are you taking inertia into account?
Doesn't innertia cancel out the gravitational force/etc of the bowling ball and you end up with the same speed and same acceleration, and same time the hit the ground?

 

[DJ...]

No...

 

[Sinbad]

But are you taking inertia into account?
Doesn't innertia cancel out the gravitational force/etc of the bowling ball and you end up with the same speed and same acceleration, and same time the hit the ground?

Where did I mention inertia at all? Inertia is what keeps the gravitational acceleration constant a constant. If inertia was zero the acceleration would be infinite. It's not relevant in these calculations.
Inertia can be thought of as a function of the mass of an object, and thus is cancelled out by the fact that gravitational force is also a function of the mass. Hard to explain here, just read the formula and do your own substitutions.