Misconceptions About Falling Objects
We know from physics that two objects dropped from the same height should hit the ground at exactly the same time, assuming they don’t get held back by air resistance. But do you remember exactly why that is?
Derek Muller from Veritasium helps explain how two differently-weighted balls can have completely different gravitational forces acting upon them, but still hit the ground at the same time. Now you will never forget it again, right?
Buzzing the Moon
NASA’s twin GRAIL spacecraft, Ebb and Flow, crashed into the Moon recently. Their fuel was exhausted, their mission to map lunar gravity complete. Fare thee well, fine ships. The video above is a view of their final days, skimming a mere 6 miles above the gorgeous lunar surface. I’m jealous.
“You are go for fly-by, GRAIL. The pattern is not full.”
The two spacecraft orbited our rocky satellite, one lagging behind the other, sensing slight fluctuations in each other’s orbits caused by slight differences in the Moon’s gravity. For instance one passed over a spot with slightly stronger pull, it would dip ever so slightly. Communicating via microwaves, the other spacecraft would sense that dip. And so they flew, bobbing and weaving and mapping.
Technically, the Earth and the Moon aren’t perfect spheres. However, for all intents and purposes we can pretend they are, as they are certainly more perfectly round than a billiard ball. The Earth actually bulges slightly in the middle from the tug of the Moon’s gravity, like a tectonic high tide.
We know that everything with mass exerts gravity. Even the coffee cup currently next to me is pulling me toward it, and I’m pulling it toward me, however infinitesimally imperceptible that pull may be. Actually, that tug might be because I need coffee, but you get the idea. What most people don’t realize is that objects like the Earth and Moon don’t have evenly distributed mass, and likewise don’t have completely even gravity.
Everywhere on the Moon that there’s slightly denser, heavier rock, there’s slightly more gravity exerted above that spot. The GRAIL mission mapped the Moon’s blips and bulges in the greatest detail ever, giving us this abstract-art-like map:
If you want to read more about Earth’s lumpy gravity, check out this post by Phil Plait.
Can plants grow leaves up, roots down … in space?
Some new space science is helping to answer that. A plant experiment recently done on the International Space Station showed that plants do not need gravity in order to grow normal root patterns and send their leaves up toward the light. They grow more slowly, but as long as they have a light source above them, they are able to orient their direction of growth just fine (contrary to previous research).
We’ve all seen a houseplant grow toward the light, right? This is a phenomenon called positive phototropism. The microgravity experiment showed that when a plant senses light, it not only grows toward it but sends its roots the other way. The root effect is called negative phototropism, and it seems to be enough to get a normal looking plant in space.
Of course it’s not that simple, right? Nope. On Earth, it turns out that gravity does help, and plant roots have these dense little “molecular weights” that are pulled down by gravity and help a new seed orient the roots downward. They’re really cool. So it looks like, for now, that Earth plants use a combination of gravity and light to orient upleaves from downroots, and space plants can do almost as gooda job with light alone.
Bring on the space gardens! Whole Foods Lunar Base by 2020!! Would that count as organic?
(if you want to dig deep into the space plant biology, here’s the original paper)
Let’s put 2,000 ping pong balls and 30 teachers inside the vomit comet and see what happens!
Best science done with ping pong balls since these guys put 1,500 of them in a trashcan with a liquid nitrogen bomb.
NASA’s Grail Paints the Moon Abstract
It looks like some sort of modern art piece, but this actually represents the most high-resolution gravity map of the Moon ever created. Wait, scratch that. The highest resolution gravity map of anything in space, including Earth. The Earth, Moon and other celestial bodies look round, but they (and their gravity) aren’t perfectly even and spherical.
The GRAIL mission uses a pair of satellites orbiting the Moon who sense the slight bumps and blips in gravity by watching how their position relative to one another changes. We’ve looked at how that works before. The colors in the photo represent tiny fluctuations in the Moon’s gravity, just like we see on Earth.
Common Physics Misconceptions
Isn’t it time that we stop teaching our kids that the Earth is flat? Sure, we can’t exactly jump into special relativity in 8th grade science classes, but surely we can bring physics education into the 20th, or maybe 21st century?
Falling For It
So are you ready for the answer to the earlier pop quiz about the falling ladder-chains? The video spoils the ending, but here’s what’s up:
It’s been well-known since Newton’s time, and likely long before, that two objects dropped simultaneously from the same height will hit the ground at the same time regardless of their mass.
Go ahead, try it. Take something heavy that you don’t care about, like a first-generation iPhone, and drop it along with a wadded-up ball of paper. They hit the ground at pretty much the same time. If you were in a vacuum, it would be exactly the same time, but you’d be dead, so you wouldn’t care. See, if you take away the effect of air resistance, the force of gravity demands that objects accelerate toward the Earth, or fall, at the same rate no matter what their mass. Sal Khan explains this all pretty well here.
You can actually do this with a gun, too (although I don’t recommend it). Fire a bullet horizontally and drop one out of your hand from the same height and they’ll hit the ground at the same time. It’s true. Because of vector reasons.
So how about those falling ladders? Everything you know from physics class tells you that they should fall at the same rate, no matter if there’s a table underneath one or open space. Guess what? You’re wrong. The ladder that hits the table falls faster.
Here’s why: You’ll notice that the rungs of the ladder are offset, with short and long segments linking each rung. When the lower end of one rung hits the table, it actually pulls down on the end that hasn’t hit (thanks to angular momentum) and accelerates it down to the table. It’s just like how if you hold one end of a ruler on a table and pull up on the other, it whips down like a guillotine.
Rung after rung, the lagging end pulls down on the short segments of rope, accelerating the ladder and speeding up the fall. Here’s the whole saga:
Don’t worry, your physics teachers didn’t lie to you. They just didn’t tell you about this. Want more? You can check out a whole research paper and more videos here.
(Thanks to Radiolab’s Jad Abumrad for pointing this out on Twitter this morning)
What is a Gnome Doing at the Large Hadron Collider?
Meet Kern, the globe-traveling gnome. Here he is in one of the LHC’s tunnels. Why? Are gnomes the secret to unlocking neutrinos?
Kern is a project of Kern Precision Scales, a company that makes … you guessed it: Precision scales. See, gravity is slightly different at different places on the Earth’s surface, which is where lumpy-Earth maps like this come from. That means you’d weigh slightly more or less at different spots on Earth.
Kern the Gnome gets shipped all over the Earth where he is unpacked, weighed, photographed and finally sent to his next destination. He was heaviest at the South Pole, weighing 309.82 grams compared to Geneva’s 307.65 grams.