Monthly Archives: June 2015

Fun with Inertia!

You wouldn’t want to get hit in the chest with a giant hammer…but what if you were to lie down, hold a heavy cinderblock on your chest, and have your opponent smash the block instead? Hm, actually maybe that sounds even worse!


Thanks to a law of physics, the cinderblock actually protects you! Bill Nye taught us that Inertia is a property of matter. It is also one of Newton’s laws and tells us that an object at rest remains at rest and an object in motion remains in motion, unless an outside force interferes. The more mass an object has, the more inertia it has, and the more it resists changes in speed and direction.

The cinderblock is heavy, so it has a lot of inertia compared to the sledgehammer. When the two collide, the block’s motion isn’t affected very much. This is a bit like the collision between a pebble and the windshield of a moving car. The pebble’s flight path changes dramatically, but the car’s motion is practically unaffected! The brave experimenters you see here aren’t crushed because the cinderblock just move very much. In addition, the brick has a larger surface than the hammer, which spreads out the impact.


Inertia is also at play when we pull a sheet of paper out from under a tennis ball. The tennis ball remains at rest until an outside force interferes. When we pull the paper away, it exerts a frictional force, dragging the ball sideways. If we pull fast enough, that force only exists for a fraction of a second– not long enough to move the ball much. The only force remaining is gravity, so the ball falls straight down into the cup. Try it yourself!

so close

If done properly, all of the balls fall into the cups because the slight force from the stands being knocked out from underneath them doesn’t push enough to the side for them to miss the cup. So close!

This experiment can be scaled up to create the classic pull-the-tablecloth-out-from-under-the-dishes trick. Minimize friction with a fast pull, smooth cloth, and level surface for the most impressive effect!

DIY Giant Air Bazooka!

This is one project that we had a ton of fun with, and it’s something you can do at home! Here is how we made our giant air cannon.

  • Lets start with the materials you will need.
  • Help from an adult. They will need something sharp.
  • A trash can or bucket, and permission to ruin it
  • Plastic sheet–such as a shower curtain, or plastic trash bags
  • Tape or bungee cords, or both!


This can be done with almost any size of bucket or trashcan. Larger containers will be more dramatic. Once you have made your selection, make sure the plastic sheets that you have are big enough to cover the mouth of the vessel. then…

Start by cutting a hole in the bottom right in the center. The perfect size for this depends on the size of the trashcan, with smaller ones needing a smaller hole. You are aiming for something between 2 and 5 inches across. Get an adult to help with this. Depending on the material of the trashcan, a knife, box cutter, or sawzall might be the best tool for the job.

IMG_2601Turn it over so the newly cut hole is on the floor. Stretch your plastic material over the large mouth of it, and attach it    with bungee cords. Ours still slipped a bit so we added some tape.


Take aim! The bottom of the trashcan is now the muzzle of your giant air cannon! Fire it at your friends, or knock over stacks of cups by lightly smacking the plastic covering the opening! Have fun!


One interesting thing: Sometimes when you fire at something and miss by a little bit, it will get pulled backwards! This is because the air that you shot out is interacting with the air molecules around the room. Below is a cool animation of how the air gets stirred up when it goes through the room

Animation of how the air molecules interact when a pocket of air is shot through stationary air. Credit: William J. Beaty

Animation of how the air molecules interact when a pocket of air is shot through stationary air. Credit: William J. Beaty

If you can get some fog into the can, it will blow smoke rings! Looking at the animation above, you can see why the smoke rings form. The smoke is in the swirling red bits above. In two dimensions, this just looks like a couple of circles, but that is the cross section of a ring. As the smoke passes through the air, it spreads out a bit, leading to the rings you see below!


Soap in the Microwave Experiment

Woah! What just happened? To try and understand it, lets take a closer look at the Microwave. These are often thought of as magic food reheating boxes, but they are actually quite interesting!

Microwave ovens heat food by bombarding it with electromagnetic radiation, also known as light. Unlike the light most of us think of when we use that word, microwave light is invisible. All light travels in the form of waves, and these particular waves are stretched out too much for the human eye to detect.

EM Wave

This animation shows how an electomagnetic wave travels. It is actually two interconnecting waves, one electric and one magnetic. Source Dr. Hans Fuchs, Georg-August-Universität Göttingen

Eyes are great for detecting light, but they can only detect certain wavelengths of it. Most light is actually outside the realm of the human eye! Scientists often build telescopes to look at these other kinds of light like ultraviolet, x-ray, infrared, and even microwaves to see what it is that we are missing when looking at the sky with just our eyes.


Different types of light make up the electromagnetic spectrum and are separated by their wavelength. The visible spectrum makes up a tiny portion of it. Image from NASA

Microwaves are also the perfect length for transferring heat energy to food. As the electromagnetic waves move through it, they bend the polar molecules (molecules that have positively charged and negatively charged ends)! As the molecules wiggle in microwaves, they bump into one another and speed each other up. Temperature is really just a way to measure how fast the molecules in something are moving, so as the molecules wiggle faster and faster, the food heats up! Just as importantly, they also heat up any pockets of gas that are trapped inside of whatever we’re trying to warm.

Ivory soap contains a huge number of microscopic air bubbles. When the microwave oven begins heating, two important things happen: the soap itself softens and melts, and the air trapped inside the soap expand.

Soap Expand

This is due to Charles’s Law, discussed here. Ultimately, the tiny air pockets in the Ivory soap grow into a giant froth of bubbles. You may have seen this process before… it’s the same science that causes popcorn to morph from small, dense kernels into fluffy, bite-sized chunks!


Popcorn kernel popping. Source 9gag.

So, have we fundamentally changed the soap? Not much! We’ve just made it take up more space. If you try this experiment at home, you’ll find that the solid suds still work. Be sure to let your creation cool for a few minutes before you touch it.


Cloud in a Bottle

Let’s take a look at the science of clouds!


Pockets of warm air near the Earth’s surface naturally float up through the atmosphere like hot air balloons. As an air pocket rises, it expands and cools down. This causes the water molecules within it to group together, creating droplets large enough to see. When a lot of these droplets gather in one place, they form a cloud. We usually observe clouds on a very large scale, but we can also create a cloud in a bottle by imitating the pressure & temperature differences found in nature.


Diagram of how a cloud forms. Credit: North Carolina State University

This experiment begins with a small amount of rubbing alcohol in a 2-liter bottle. There are several ways to do this demonstration. We use rubbing alcohol because it evaporates at a lower temperature than water, giving us a more impressive tabletop cloud!


First, we pressurize the bottle using an air pump.  Next, we release the pressure, causing the air inside the bottle to expand and cool (much like a pocket of warm air does as it rises through Earth’s atmosphere).  Just as cooling, expanding air causes water vapor to condense in nature, the cooling, expanding air in the bottle allows the rubbing alcohol to condense into a visible cloud of droplets.

fire light

This cloud looks very similar to the ones you see in the sky, but there are some important differences, as you can clearly see.


Vortex Table: The Trampoline of Science!

Gravity is everywhere!  Anything that has mass exerts a gravitational force, including you and me!  Why can we not feel that force?  We are in the presence of a massive object that pulls on us more than we could ever pull on each other, and leaves our personal gravitational pull negligible.  That’s right, I am talking about the Earth!  In the absence of a large object with mass however, even tiny specs of dust and atoms of gas can feel a force pulling them together.

What does gravity have to do with a stretchy vortex table?  According to Einstein’s Theory of Relativity, the universe is not actually flat, but a stretchable fabric called spacetime.  Gravity, in this theory, can be represented as curvature in the fabric of spacetime.  In the diagram below, the Earth is causing the 2-Dimensional grid to stretch into the 3rd Dimension. In our 3-Dimensional universe, we can only imagine how gravity would stretch spacetime into the next dimension: the 4th Dimension!


A simulation of what the curvature of spacetime might look like. Credit: Wikipedia, Johnstone

Our Vortex Table is a great model for how gravity affects spacetime.  Any mass in the center of the table causes a small curvature in the fabric, which automatically attracts any other object placed on the table.  It is easy to see that the heavier the mass in the center, the greater the downward slope of the table, and therefore, the greater the gravitational pull on other objects.


Objects with different masses can bend spacetime in different ways. This view from the bottom of the table looks very similar to the simulation above.

The formation of stars is caused entirely by the gravitational force of attraction and can be modeled on the Vortex Table.  Clouds of dust and gas in the universe, called nebulae, are where stars are formed. The gas in a nebula is pulled together by the force of gravity to form a dense patch of gas called a protostar, which attracts even more gas and dust to become a full star.


The life cycle of a star, as illustrated on the back of an AstroCamp sweatshirt!

The formation of a star and its Solar System is not the only thing we can model on our awesome stretchy table.  When an extremely massive star dies, it becomes a black hole.  A black hole is a very dense and very massive, causing a huge curvature in spacetime.  Black holes have so much gravity that light can’t escape, so we have difficulty detecting them directly. To find black holes, astronomers look for the stuff around it, such as stars orbiting something invisible, or material that is getting accelerated to very high speeds.


The vortex table lives up to its name.

Simulating a black hole with fabric is difficult…Imagine the vortex table stretched so far down that the hole goes down into infinity!  The resulting slope can affect stars and planets more than 50,000 lightyears away, about the radius of our galaxy. Scientists theorize that there is a supermassive black hole at the center of every galaxy, which is what holds everything together.  Look familiar?

Galaxy Comparison

The image on the left is from the vortex table. The one on the right is a spiral galaxy called M74. Photo credit: NASA


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