Tag Archives: gravity

What Would Happen if the Earth Stopped In Its Orbit?

earth space finger

Aatish Bhatia (modifying images under fair use)

As we celebrate the Earth completing another lap around the Sun, let’s take a moment to imagine what life would be like in a world without years – a world that somehow ceased to orbit its star. Admittedly, it’s a strange question, but its’s one that I’ve been obsessively wondering about lately. Not because it’s of any particular relevance, but simply because it’s amusing (at least to me) and fun to think about.

What would happen to us if a giant space finger were to gently stop the Earth in its orbit?

Nothing good.

Here, try it out for yourself. Press ‘start’ in the simulation below (created by Michael Dubson and the folks at Phet Interactive Simulations / University of Colorado). You should see a planet orbiting the Sun.

Now press ‘reset’, and drag the circle with the letter ‘v’ to shrink the planet’s speed . Then press ‘start’ again. What happens? (While you’re playing with this, you might enjoy trying out some of the different scenarios in the drop-down menus, and watching the gravitational ballet that ensues.)

If you slowed down the planet enough, you should see it crash into the Sun.

To see why, let’s first remember why things stay in orbit. Every child looking at the sky has at some point wondered, “why doesn’t the moon fall down?” The answer is beautifully simple, yet it took a mind as brilliant as Isaac Newton’s to work it out. (Perhaps a sign of genius is coming up with simple answers to children’s questions.)

Newtons’ response to the child’s question would have been – the moon does fall. It’s constantly falling. Being in orbit is a state of always falling, and always missing what you’re falling towards. In The Hitchiker’s Guide to the Galaxy, Douglas Adams describes the secret to flight. “The knack”, he writes, “lies in learning how to throw yourself at the ground and miss”. As it turns out, this is also a great description of what it means to orbit something.

newton cannonball

Isaac Newton / A Treatise of the System of the World (Public Domain)

Here’s how Newton explained it. Imagine a cannonball is fired from a height. If you fire the cannonball with more speed, it’ll travel further before it hits the ground. The faster the cannonball, the further it travels.

But wait – the Earth is round. That means that if you shoot the cannonball with enough speed, then by the time it would have hit the ground, it’s travelled far enough that the ground has curved away beneath it. So the cannonball continues to fall towards the ground, and the ground continues to curve away from it. It’s now in a state of perpetual free fall – the cannonball is in orbit!

(Newton’s idea is masterfully explained in this wonderful Radiolab segment.)

So the only thing that makes an orbit different from plain-old falling is having enough speed to miss the thing you’re falling towards. Think dropping a cannonball with zero speed versus shooting it into orbit. And for the same reason, if the Earth were robbed of all of its orbital speed, it would fall straight into the Sun. It would no longer have the speed it needs to miss the Sun.

How long would this ‘Earthfall’ take? Continue reading

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The Physics of Doing an Ollie on a Skateboard, or, the Science of Why I Can’t Skate

Skateboarding is hard.

When I was about 10, I broke my first skateboard by riding it into a ditch. A decade later, in college, I broke another skateboard within an hour of owning it (surely a record) in a short-lived attempt at doing an ollie. (Surprisingly, the store accepted a return on that board even though it was in two pieces.) Then I was gifted a really nice, high-quality skateboard. The first thing I did with it was ride it down a big hill, a valiant but ill-fated adventure which ended with me jumping off the skateboard, rolling down the grass, and arriving scraped up, deflated, and rather disoriented near the entrance to my college cafeteria. (In my defense, the wheels and ball-bearings on that skateboard had been pre-lubricated to minimize friction, and why would anyone do that, that’s just crazy.)

So believe me when I tell you that I am incredibly envious of skaters who can pull off tricks like this.

Now, I might not be able to skate to save my life, but I can do a little physics. So here’s a thought – maybe I can use physics to learn how to do an ollie. Here’s the plan. I’m going to open up the above video of skateboarder Adam Shomsky doing an ollie, filmed in glorious 1000 frames-per-second slow motion, and analyze it in the open source physics video analysis tool Tracker.

The first thing I did was track the motion of the front and back wheels (Tracker has a very convenient autotracker feature that can do this for you.)

ollie tracking optimized

One useful physics trick here is to track the center of mass of the skateboard, i.e. the average of the positions of the front and back wheels. Here is that curve overlapped in green.

ollie tracking with CM optimized

Now, if you were to do the same tracking exercise for a soccer ball that’s been kicked, you’d get a neat arc-like shape called a parabola. This is the characteristic shape you get when the only force influencing an object’s motion is gravity.*

But the green curve in the above gif — the motion of the center of mass of the skateboard — is nowhere close to being a parabola. It’s lumpy and weird. This means that gravity isn’t the only force affecting the skateboard. Unlike a soccer ball in mid-flight, a skateboard mid-ollie is being actively steered.

This is exactly what makes doing an ollie so hard. It’s not enough to get the skateboard up into the air – you also have to steer it while it’s in the air. Continue reading

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