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Friday, January 15, 2016

5 of the Greatest Physics Demos From the MythBusters

Written by Rhett Allain

YES, THIS IS the final season of the MythBusters. The greatest part of the show is the fact Adam Savage and Jamie Hyneman aren’t scientists but they still do some awesome science. They show us that everyone can do science—the MythBusters simply have better building skills than most of us.

As a physics educator, I’ve been impressed by the excellent physics demos the MythBusters make. Some of these are classic examples straight from your physics textbook and some are surprising results that no one expected.

Let’s take a look at a few of my favorite examples from previous seasons.

Shoot a Bullet, Drop a Bullet

When you take introductory physics, just about every book says something like this:

When we are looking at projectile motion, the vertical and horizontal motions are independent. In fact, if you shot a bullet horizontally and dropped a bullet from the same height, the two bullets would hit the ground at exactly the same time.

The instructor may then proceed to show you, using some small balls at low speed instead of real bullets. But would it work with a pistol? As you can see above, the answer is “mostly yes”. In the high speed video, the dropped bullet hits just slightly ahead of the fired bullet—a time difference of just 39.6 milliseconds. Honestly, this is close enough for me.

But what if you include air resistance on both bullets? Do they still hit the ground at the same time? The answer is no. The dropped bullet will hit the ground first. Here is a more detailed explanation of why that happens. The short version is that the fired bullet has much more air resistance and this produces more vertical air drag than the dropped bullet.

A Real Lead Balloon

The band Led Zeppelin was possibly first named Lead Balloon because everyone knows a balloon made of lead would fall out of the sky. But it is possible to make a lead balloon float. But how does it work?

Whenever something is displacing either a fluid or a gas, there is a buoyancy force on that object pushing it up. This buoyancy force depends on the density of the fluid or gas and the volume of the displacement. For many objects—like a car or a human in air, this buoyancy force is too insignificant to matter. If you take a simple party balloon, it has a significant volume and very little weight so that it can float.

The trick to making a lead balloon float is size. Think of a balloon as consisting of two parts. First, there is the filling gas—almost always this is helium since it has a density lower than air. Second, there is the “skin” to contain that gas. For a party balloon, this skin is rubber or something, but the MythBusters use lead. Why does size matter? If you double the radius of your balloon, you increase the volume by a factor of 8 (radius cubed) but the area of the skin only increases by a factor of 4. So making a bigger balloon increases the lift (from the volume) faster than it increases the weight of the skin.

The MythBusters achieved this floating lead balloon by making it a cube with sides of 10 feet. Of course, you could probably even make it smaller and still float—here are my detailed calculations of the smallest possible floating lead balloon.

Car Crash Into a Wall vs. A Head to Head Crash

Another classic physics textbook problem goes something like this:

A car is traveling at 50 mph. Would it do more damage to crash into a brick (and unmoving) wall or a head on collision with a similar car also traveling at 50 mph?

Most students would say that a head-on collision is worse than crashing into a wall. The thinking is that there are two cars in the head-on collision so it should be twice as bad. If it is indeed twice as bad then it should be the same as one car traveling at 100 mph crashing into a wall. But it’s not.

Consider one car traveling at 50 mph crashing into a wall. The car goes from 50 mph to 0 mph over some time interval. This means the car has a change in momentum due to some external force (from the wall). Now switch to two cars crashing head on at 50 mph. Both cars still go from 50 mph to 0 mph so they each have the same change in momentum (but in opposite directions) requiring the same stopping force.

What about a car at 100 mph? Yes, this car would have double the momentum of a single 50 mph car. However, it would have 4 times the kinetic energy of a car at half the speed. As you can see from the MythBusters episode, crashing one car at 100 mph is way worse than a head-on collision between two 50 mph cars.

Yes, Humans Actually Landed on the Moon.

It is hard to believe that some people think it would be easier for hundreds of employees to keep their mouth shut about a fake moon landing than it would be to actually land on the moon—but there are those that think this. In this episode of MythBusters, Adam and Jamie tested theories about the moon landing being an elaborate hoax. The first deals focuses on a picture, shown above, of Buzz Aldrin in the shadow of the lunar lander. Conspiracy theorists argue that because Aldrin is in the shadow, he should be all but dark because the sun is the only light source. This is of course wrong because light also reflects off the surface of the moon and onto the astronaut.

What makes this episode so awesome is the attention to detail in the recreation of the Aldrin image. The MythBusters made a lunar lander and a replica moon surface to capture a model image using a single light source. The two images are fairly close to being identical.

I also like this myth because of it’s demonstration of the nature of science. Does this photo prove that humans landed on the moon? No. Science never proves anything to be true. However, it does say something significant. Suppose you have an idea (or I would say model) that this image could only be created with multiple light sources. The MythBusters experiment proves this idea is wrong—since they did in fact make a similar image with just one source. So, science can prove things wrong, but not right.

The Ultimate in Relative Velocity

What would happen if you shot a ball with a speed of 60 mph from the back a car traveling at 60 mph? Would the ball remain stationary? Yup—that’s exactly what the MythBusters did. That animation is so awesome to watch.

This is all about relative velocity. The speed of the ball with respect to the car is 60 mph to the right (negative) and the speed of the car with respect to the ground is 60 mph to the left (positive). This means that the speed of the ball with respect to the ground is 60 mph + (-60 mph) = 0 mph. The math doesn’t seem so complicated, but getting the relative speeds just right isn’t so simple.

Even More MythBusters Science

These are just five examples of MythBusters science; there are countless more. Perhaps I will have to share another five science explanations soon. Oh, and what about the final season? Will there be more great examples of science? I am certain there will be. It’s going to be great.

Click here to read more.