Rewatching the mythbusters where they test helium in footballs. They determine the heavier air in a ball makes it go further through the air than a lighter helium ball when both have the same energy applied at the throw. The quote was “the greater the mass, the more inertia, and this inertia causes the ball to travel through the air further, counter acting drag.”
I’m a pretty big idiot, but its momentum in this case, not inertia. Correct?
If I understand correctly, a car sitting still has more inertia than a basketball, which is why it is so hard to push and get rolling. Once you do get it moving, it has greater momentum, so it is harder to stop.
This applies to a helium ball vs air ball, but once the ball is in flight, the physics pertain to momentum? The inertia pertains to it before velocity is acted on it. Yeah? Or do I not get it?
Both apply to an object in motion; only inertia applies to an object at rest.
Inertia is the tendency of matter to stay in its existing kinematic state (i.e. objects at rest want to stay at rest; objects in motion want to stay in motion). Momentum is specifically the product of an object’s mass and velocity. Inertia is a general tendency of matter while momentum is a specific aspect of kinematics. Both are a function of mass.
A helium-filled football will have a lower mass, so it will have lesser momentum when traveling at the same velocity as an air-filled football. Its lower mass will also allow it to be accelerated to a greater velocity than an air-filled football (assuming the same throwing force).
When the ball is at rest, only inertia applies. When the ball is in motion, both inertia and momentum apply.
At least that’s how I understand it. Perhaps someone with a more physics-based background can provide a more thorough explanation.
Inertia is the resistance to change in state of motion; e.g. momentum. The explanation is poorly worded, but essentially correct.
Stranger
The ball’s inertia is fixed, whereas its momentum isn’t. To say “The greater the mass, the more momentum” would be misleading at best, as the momentum depends on the ball’s velocity too.
i didn’t want to reply until i could post the verbatim quote. here is the segment in question:
Jamie: “Physics determines that if we launch the balls at the same speed and one has more mass, that one is going to go further.”
Narrator: "It’s all to do with inertia. Which, according to Newton’s First Law of Motion, states that the heavier an object, the more inertia it has–and the more resistance to drag.
in other words, a helium filled ball shouldn't travel as far as an air filled one, thrown at the same **speed**."
bolding mine to emphasize my confusion. he is talking about both the mass and velocity, which by definition (i think) refers to momentum. granted, the narrator’s first sentence alone isn’t incorrect, alone. but it’s bookended by Jamie talking about the speed it is thrown and the narrator explaining that speed was a necessary constant to test the myth.
i understand inertia is the resistance to change in motion or stasis, but since speed is the key factor in determining the difference, shouldn’t he have said momentum?
He intended that to only apply to militias.
And this is where I have trouble with the experiment.
Assuming the same kinetic energy from the foot:
0.5 m1 x v1^2 = 0.5 m2 x v2^2 => v1/v2 = sqrt (m2/m1)
or as a comparison kicking a football with half the mass will cause it to travel about 1.4 faster off the foot. It seems to me that there is a pro to using helium (less mass = higher speed off the kick) and a con (less mass = less inertia) and then you take away the pro and say helium is not as good.
Am I missing something?
Only that an American football is primarily launched by the hand. 
But, yeah, this is what bothers me, too. Why would they assume the same launch speed for projectiles of different mass when the launcher is presumably capable of (and exerting) the same maximum force?
they first tested w a professional kicker and saw no noticeable difference. (helium balls seemed to fall a bit short and had slightly lesser hangtime).
next they excluded the human element and used a football launching machine in a giant NASA hangar (exact same force enacted on each ball, no external weather factors).
the difference between the two was, as decided by a professional statistician, “not statistically significant.” that is to say air balls went ever so slightly further but not by any major degree. it was statistically within the margin of error based on the sample size, but observably the air balls went just a smidge further.
so that is why i am confused. they never once say “momentum,” ever–even tho they make sure to point out velocity as a constant on both variable balls.
The ball-throwing machine doesn’t really impart the same energy to the different mass footballs, so they were essentially correct on them being of different inertia at the same speed. If I recall, the machine used a heavy flywheel rotating at a certain speed to eject the balls. Considering the flywheel was much more massive than either of the footballs, and given the footballs were not apparently slipping in the chute, they were coming out at pretty much the tangential velocity of the flywheel. The momentum carried off by the footballs would be a small bit of the total of the flywheel, its speed would be reduced only a little by either football, so the only speed difference in their departure would be the small difference between the two, multiplied by the equally small slowdown of the flywheel; so, not much difference.
This manner of propelling isn’t like getting kicked, it’s going to eject anything that is small compared to the flywheel at pretty much the same speed.