However, it wasn't enough that people simply believe me on my birthday, so I was forced to break out the W/t of SCIENCE (If I can have a dogion joke I can have a power = energy/time joke). Unfortunately, this led to a point of contention involving the physics of gravity.
Some people are surprised to know that the Aristotelian view of gravity is not true (despite how cool Aristotle is). Heavier objects do not inherently accelerate faster than lighter objects. This belief persists because lighter objects tend to be more buoyant and have more air resistance, and hence drop more slowly on Earth.
Current scientific thinking links gravity with the curvature of spacetime. Unfortunately, quantum mechanics disagrees (but quantum mechanics is like that frizzy-haired uncle no one likes to talk to because he seems to only speak gibberish). Despite all that, most situations still work fine with Newton's "simple" Law of Gravity. Here we also see that laws are meant to be broken, even when they're scientific, since relativity and quantum mechanics have proven that Newton's law does not apply to all possible scenarios. Basically, given a vacuum (to negate air resistance and buoyancy) and objects of negligible mass (relative to a planet), objects will fall to the ground at the same rate (both would fall at approximately 9.8m/s2 on Earth).Astronauts make everything better, so fortunately the concept was illustrated by our good friend Commander David Scott during the Apollo 15 mission.
Transcript for people without video: "Well, in my left hand I have a feather. In my right hand, a hammer. I guess one of the reasons we got here today was because of a gentleman named Galileo a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought, 'Where would be a better place to confirm his findings than on the moon?' And so we thought we'd try it here for you. The feather happens to be appropriately a falcon feather...for our Falcon. And I'll drop the two of 'em here, and - hopefully - they'll hit the ground at the same time. [hammer and feather hit the ground simultaneously] How 'bout that? This proves that mister Galileo was correct in his findings."
Returning to our opening statements: people also tend to believe that horizontal motion negates gravity. If an object is shot horizontally and another object dropped simultaneously from the same height, both objects will hit the ground at the same time. Gravity's pull is uniform regardless of horizontal motion. This too, is illustrated by crazy science teachers around the world. Exhibit A:
(I apologize in advance for the lack of more astronauts)
...There is no Exhibit B.
Edit for 2013-2-06: There is an Exhibit B; I've since become aware that the Mythbusters have also performed this experiment using actual firearms with a result that is well within margin of error (basically the only improvements you could ask for is better timing on the drop/shot simultaneity and doing it in a vacuum).
Randomly teaching people about gravity since 2003; just another reason not to follow me when I walk home from school.
9 comments:
You could have gotten a less gay instructor vid, but I am going to stand by a bullet fired by gun will not hit at the same time as an equal dropped bullet.
So, would you say the bullet that hit your grandfather would have to have been fired at least slightly above perfectly horizontal? It seems like the affects of gravity would pull it down before it would run out if kinetic energy if it was fired flat, or slightly below horizontal.
There are three reasonable possibilities:
1. A bullet fired at an arc significant enough that it lost its kinetic energy. This is relatively unlikely, since there's not much reason for someone to fire a gun at such an arc in combat. It's also possible (depending on the angle) that the bullet may minimize air resistance and retain its velocity (this is why it's not wise to fire bullets straight up). A bullet fired from the horizontal takes about half a second to hit the ground; at that time air resistance has had a minimal impact on its velocity.
For those interested:
From an average person's height of 1.8m (5'9") an object takes 0.61s to hit the ground in a vacuum on Earth. On the moon, with 1.6g (or about one sixth of Earth's gravity), the same fall takes 1.5s.
2. A bullet that had passed through or bounced off of something. Very possible, especially if you're patrolling a jungle full of foliage. Piercing through things takes energy, slowing the bullet. Similarly, ricochets also slow a bullet substantially.
3. A misfired or insufficiently propelled bullet. The amount of propellant used in individual rounds varies, so the muzzle velocity of any individual bullet can vary substantially. In the event of a problem during firing (such as the breech being open, thus allowing the expanding propellant to escape without imparting much force to the bullet), a round may end up leaving a gun moving slow enough to cause the event described.
As for Jimmy...well, physics might just not be for you.
A ha, excellent. I like option 2, or at least that seems the most likely. I wasn't really aware of the third bit of info either, interesting.
Haha I'm telling you there is no way a fired bullet will hit the ground at the same time as a dropped bullet. That video you popped in there is showing something shot out with very little velocity and it didn't travel anywhere near as far a bullet would travel before it hit the ground. Alas there is no way for us to test this so we'll just go with me being right since you can't prove otherwise :-)
Don't worry Jimmy, plenty of high school students and middle school students also initially have problems circumventing their intuition with regard to the properties of kinematics. Perhaps this video will help you out. Or this one.
There're text sources too. One science teacher apparently had a student who had a thought process as obstinate (and wrong) as yours. This page has a diagram that might prove useful. You can look up plenty more sources if you want; you might want to look into projectile motion and gravity.
No matter how fast the bullet is moving parallel to the ground it will hit the ground at the same time as a bullet dropped in the same instant because gravity acts independently from the force propelling the bullet (in ideal conditions...but outside of ideal conditions we could always pretend some random updraft is keeping the dropped bullet in the air longer than the shot bullet or something equally crazy).
i am sure that if you shot the bullet FAR enough, the curvature of the earth would affect it and gravity itself would make it orbit a bit. it is true that gravity effects all things the same regardless of their lateral movement, but does that also apply here on our spherical planet???
http://instantmartians.blogspot.com/
In an ideal situation, modeling only gravity and a perpendicular force, gravity acts the same on both projectiles.
Other than forces noted in the post, there are other factors affecting projectiles fired on the Earth. The two more interesting for many people include:
1. The Coriolis Effect - the angular velocity of the surface of the Earth varies based on latitude (that is, different parts of the surface of the Earth are moving at different speeds (think about spinning a weight attached to a string, the weight at the end will be moving further than the middle of the string in the same amount of time). The effect is essentially negligible for anyone but snipers. Variation depends on latitude and the direction of the shot (this would also affect the dropped bullet, but it would essentially be immeasurable).
2. Curvature of the Earth - If a projectile is moving fast enough it would end up needing to fall further due to the shape of the Earth. I believe the basic measurement is just under one centimeter per kilometer (or about half an inch per mile). Again, for a bullet fired from the horizontal, this means a bullet would need to have a muzzle velocity over 2500m/s to even go one mile if fired horizontally (it has just over 0.6 seconds to do it). Keep in mind that very, very few weapon and ammunition combinations have a muzzle velocity even over 1000m/s. This is because muzzle velocity is not an indication of accuracy or power over distance; an M16A1 has a muzzle velocity of 990m/s (the highest muzzle velocity I could find on short notice - faster than the more common M16A2), a Barret M82A1 .50 caliber sniper rifle has a muzzle velocity of only 840m/s.
In case astronauts, teachers and professors aren't enough for you: Neil deGrasse Tyson also concurs:
"A bullet fired level from a gun will hit ground at same time as a bullet dropped from the same height. Do the Physics."
http://twitter.com/neiltyson/status/20894167897
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