Rhett Allain writes the popular Dot Physics blog for Wired and in this book he takes a longer, smarter look at his most popular topics, from the purely fun (How much bubble wrap would you need to safely jump off a 6th floor building?) to the completely mind-boggling (Why does a mirror reverse left to light, but not top to bottom?).
Allain is a physics professor and he's become very good at finding physics questions buried in the latest movie, video game, viral video or news hook. Each chapter will not only cover interesting subjects like lightsabers and McDonald's drive-thrus, but use those questions to teach readers basic physics concepts. He'll explore questions like: Is Angry Birds using real physics? Does a heavier truck make a better snow plow? What if everyone on earth jumped at the same I time? How many dollar bills would it take to stack them to the moon? Often the way he gets to the answer is more interesting than the answer itself.
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How much does R2-D2 weigh if he can fly?
Let me start with a quiz question.
In Star Wars Episode II: Attack of the Clones, R2-D2 shows that he can fly. What is wrong with a flying R2-D2?
a) Nothing. This is the way that George Lucas would have wanted it in the original Star Wars but he couldn’t digitally render a flying R2 with a Commodore 64*.
b) If R2 can fly, why didn’t he do this in the original trilogy?
c) He wouldn’t fly that way.
d) Droids shouldn’t be allowed to fly.
And here you see that this is really a trick answer for choice a. Why? Because the Commodore 64 didn’t come out until 1982 and Star Wars (it was just called Star Wars then) came out in 1977. So, choice a) can’t possibly be correct.
The correct answer is c). He wouldn’t fly that way. But how does he fly? If you watch the movie carefully, you will see that while R2 is flying (all his close friends just call him R2 and not his full name of R2-D2), he looks like this.
This is a flying R2 while he is moving at a constant speed. You might think: well, what is wrong with this? It looks perfectly fine, right? I think this is the real issue right here. R2 is shown to fly the way people think about forces and motion, so there is no real problem.
Ah, but now we are talking about forces and motion. Let me start with two experts in this field: Aristotle and Isaac Newton.
According to Aristotle, what does a constant force do to an object? For him, constant force means constant motion. Honestly, to most people this idea just makes sense. It is really easy to agree with Aristotle, even if he is speaking in Greek. Doesn’t this idea of forces always seem to work? If I push a book on a table, the book moves. If I push harder, it moves faster. If I stop pushing the book, the book stops. Seems simple.
For Aristotle, R2-D2 flies as he should. If he wants to fly at a constant speed, he needs to angle these thrusters back a little. This way, part of the thrust pushes down to keep him up and part pushes back to move him forward.
Now, what about Newton? Well, it’s not just Newton that came up with a better idea about force and motion. It’s just that people call them “Newton’s Laws” of motion. The Newtonian idea of force and motion is that forces CHANGE the motion of an object. Change is the key word here. If you have a constant force on an object, it would constantly change its motion. This means that it could keep increasing in speed with a constant force.
Consider the following example. A bowling ball is sitting on a smooth bowling alley. If you give it a little push, it will start to move. It will keep rolling for a while, but will eventually stop. This is because there is another small force on the bowling ball that most people tend to forget about: friction. So after the push, there is just one force acting on the ball to make it slow down. However, if you ran along behind it, you could keep giving it little pushes to keep it going the same speed, balancing the frictional force. If you used a really long stick to keep applying a continuous level of force that was greater than the frictional force, it would go fast and faster.
Suppose there was a way to remove ALL forces from a moving object. For such an object, the speed would stay constant. I know this is hard to image because everything we see on this Earth has some type of frictional force on it.
Okay, so if you look at the way R2 flies, it seems to agree with Aristotle and probably 90% of the human population. But, is there any way to make this flying R2-D2 agree with the Newtonian ideas of force and motion? In other words, in what situation would this type of flying agree with Newtonian physics?
Air resistance is our best bet. Suppose there is non-negligible air resistance force on the flying R2. In this case, he would have to have his thrusters angled back to balance the horizontal air resistance. We can actually use this to figure out how much R2 weighs. If we know what the air resistance and his speed is, we can figure out how much thrust he is putting out. If we know how much thrust he putting out, we can figure out how much he must weigh for that thrust to lift him.
Table of ContentsHow Fast Do Tweet Waves vs. Seismic Waves Travel?
How Much Would R2-D2 Weigh If He Could Fly?
How Realistic Is the Physics in Angry Birds?
Big Space Ships With Even Bigger Thrusters
Jumping Off a Building With Bubble Wrap
Is It Better to Crash Into Another Car, or a Brick Wall?
Can a Human Out-Pull a Truck?
Would the Hulk Break the Road Whenever He Jumped?
Can Humans Fly Like Birds, With Wings?
Can You Fall Faster Than the Speed of Sound?
Big Hail Is Bad
Why Do Astronauts Feel Weightless?
How Can a Broom Balance On Its Brushes?
Can You Cook a Turkey By Dropping It?
How Much Energy Does It Take to Get a Candy Bar in Orbit?
Building and Supplying a Death Star With the ATV
The Long Jump: Gravity and Air
Linear Regression and the Long Jump
Power and Olympic Swimmers
Scoring the Decathlon
Diving and the Moment of Inertia
Could Cyclists Cheat With Hidden Batteries?
What is Arnold Schwarzenegger Made Of?
What Does Pi Have to do With Gravity?
Can Airplanes Save Fuel by Using iPads?
How Many Zombies Could You Drive Through?
How Much Water Would It Take to Move a Car?
Could Adam Savage Have Accidentally Killed Himself in the Swimming Car Myth?
A Scale at the Bottom of a Pool
How Much Ice Do You Need to Cool Your Beer?
Can Ice Cream Get Cold Enough to be Zero Calories?
The Power Source for a Light Saber
Can You Charge Your Phone by Typing?
Why Do Mirrors Reverse Left and Right, But Not Up and Down?
Can a Building Be a Sun-Death Ray?
The Physics of Thor’s Hammer
A Banana Powered Generator
How Many Dollar Bills Would it Take to Stack Them to the Moon?
Could You Build a Scale LEGO Model of the Death Star?
Would You Rather Fight a Horse-Sized Duck or 100 Duck-Sized Horses?
NASA Space Games
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