Tag Archives: Inertia

From Apples to Inertia

One day, Isaac Newton was sitting underneath a tree, and an apple falls and bonks him on the head. In a stroke of genius and coincidence, Isaac comes up with the theory of gravity and the rest is history…or so the story goes. This simple anecdote actually does a disservice to just how much of a contribution Isaac Newton made to the core of science. While there are many subjects to pick from, today, we are going to focus on the first of Newton’s Three Laws of Motion.

 

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As many of us learned from another famous scientist, Bill Nye, inertia is a property of matter. This is also Newton’s First Law of Motion, and it is actually a very simple concept: an object at rest or in motion will remain at rest or in motion unless acted upon unless something makes it move or stop. So why did it take someone like Isaac Newton–the guy who invented Calculus just to help him understand gravity–to come up with it for us?

The main reason is that in our experience, the law seems wrong. If you throw a ball, it doesn’t keep going forever. It hits the ground and stops rolling after a bit. If you start walking, you don’t just glide on forever. This does not break Newton’s first law though, and that is because there are little forces like the air and the friction of rolling a ball that slow them down. These outside forces are exactly what the law is talking about! If instead you were to throw a ball out in space, it would keep on going forever until it hit something or got pulled in by gravity!

In these demonstrations, we are showing that an object at rest remains at rest. The little metal hex nuts sitting on top of the orange ring are stationary directly above the small opening of the bottle. When the ring is pulled out, the hex nuts do not move horizontally, and instead fall straight down due to gravity!

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This idea can be expanded to the table cloth magic trick that you may have seen, but  you have to get it just right!

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Too see more cool demonstrations with inertia, click here!

Rockets & Newton’s Laws in 3 Gifs!

RocketLaunchRockets blast off from earth with a rumble and cloud of smoke on their way through the atmosphere to the vacuum of space beyond. This is something that is accepted today, but it hasn’t always been that way. Doctor Robert Goddard is known as the father of modern rocketry, but when he first postulated the current method of rocket propulsion in space, he was ridiculed. Even the New York Times published an article on the preposterousness of his ideas. Although rockets had been around for centuries, trying to use them in space was seen as ridiculous at the time.

The tools for understanding the basis of how a rocket works had been developed by Sir Isaac Newton hundreds of years earlier. Using Newton’s Laws of Motion, most of rocketry can be understood with relative ease.

His first law, often referred to as the law of inertia, states that an object in motion or at rest will remain in motion or at rest, respectively, unless acted upon by an outside force. This means that a rocket in space can turn its engines off and not slow down or even turn– unless an outside force, like gravity, acts on it. The bottle rockets in the video run out of fuel after a tiny fraction of a second, but they continue traveling upward until Earth’s gravity overcomes their momentum.

TennisBallsThis variation on the classic “pull out the tablecloth” trick is a great example of Newton’s first law. Nothing pushes or pulls the tennis balls sideways, so inertia mandates that they don’t move horizontally. For more cool inertia demonstrations, check out our blog post here.

Newton’s second law is most easily understood as an equation.

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FootOfScienceThe equation says that if you apply the same force to objects of two different masses, the lighter one will accelerate more than the heavier one. This shouldn’t be a surprise. If you kick a car with all your might, and then go kick a soccer ball with the same force, you will see (and feel) the difference! It’s easier to move or stop a light object than a heavy one.

The third and final law is what we like to call the law of interaction: for every action, there is an equal and opposite reaction. When a person stands, they push down on the ground, and the ground presses up equally on them. Imagine being on a frozen, icy lake, so slippery that you can’t walk. Normally, when you take a step, you push backwards against the ground, which in turn propels you forwards– an equal and opposite reaction. On an icy surface, you don’t have enough traction to push and move ahead this way. So, are you stuck? Not quite! Imagine standing on the ice and throwing a heavy rock. The rock exerts an equal and opposite force on you, pushing you in the other direction! Thinking this way makes a rocket easy to understand. Just like you can move across a slippery surface by throwing a heavy object away from you, a rocket can travel through the vacuum of space by shooting fire in the direction opposite its desired flight path.

WhiteboardBalloonA rocket doesn’t have to push against the air. It just throws fire out behind it, and the equal and opposite force thrusts it forward. As you can see, the basic idea behind rocket propulsion isn’t so complex after all.

So, what makes rocket science difficult in practice? Well, a rocket is basically a gigantic cylinder of explosive fuel with a nozzle to direct the explosion. The whole system moves at a very high speed. Without the right knowledge to control it, that’s a recipe for disaster! A rocket is also constantly launching its fiery fuel out behind it, so its mass is always changing. This makes calculations less than straightforward. As is the case in many fields, the core concepts in rocketry are simple. Complications arise when applying these ideas in real-world situations that require a lot of precision.

Shuttle

 

Blast-Off for the first space shuttle in 1981 from Kennedy Space Center. Credit NASA

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.

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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!

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