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