Playing With Inertia

The physicist Newton developed three laws that have become the cornerstone of modern physics.  The first of these is:

In the science experiment we had in July of last year (see past science experiments), we did an experiment to show the second half of this law.  Now, let's see how the first half of the law works.

You Will Need:

• A piece of clean notebook, or typing paper

• A small non-motorized toy car

• Six nickels

• A plastic cup

• A piece of smooth cardboard, or stiff paper about 4 inches square

• A clean, smooth table 

Doing the Experiments

There are three different experiments we are going to try.  Each of them shows the same principle, but they do it different ways.  After you do the experiments, you might try showing them to your friends.  They'll think you're doing some kind of trick.

1. The unmoving toy car:

   1. Put the piece of paper on the end of a table with a little bit of it hanging over the edge.  

   2. Place the car on it, pointed towards the end of the paper.  

   3. Grabbing the end of the paper firmly, pull it out from under the car as fast as you can.  

   4. What happens to the car?
      

2. The replaceable nickle:

   1. Stack up five of the nickels on the table.  

   2. Using your finger, flick the sixth nickle into the stack, like you are playing hockey with it.  Make sure you hit the nickle hard.

   3. Did your "hockey puck" replace the nickle on the bottom of the stack?
      

3. Getting the nickle in the cup:

   1. Place a plastic cup on the table.

   2. Place the cardboard, or stiff paper over the mouth of the cup.

   3. Place a nickle on the top of the cardboard, over the center of the cup.

   4. Flick the cardboard, like you did in the second experiment, with your finger.  Make sure that the back of your hand is vertical, perpendicular to the table top, otherwise you'll be pushing the cardboard the wrong way.

   5. What happens to the nickle?
      

LOOKING AT THE RESULTS

In the first experiment, with the car, the car should have stayed put.  If it didn't that means you didn't pull the paper out fast enough.  Try again.

In the second experiment, with the stack of nickels, the "hockey puck" should have replaced the nickle on the bottom of the stack.  This one is tricky, you might have to try it several times.

In the third experiment, with the cup, the nickle should have fallen into the cup.  If it didn't, you might check to be sure you were flicking the cardboard off straight.  If it was going up at all, it would push the nickle off to the side with the cardboard.

EXPLAINING HOW IT WORKS

In some of the old movies, they had a trick where the waiter would pull the tablecloth off of a table full of dishes, and the dishes would stay there.  The actor was doing the same thing you just were in these experiments, using Newton's first law.

We call the force that holds an object in place "inertia."  The mass of the object needs to receive enough kinetic energy (that's energy in motion) to cause it to start moving.  Everything has inertia, and the more massive an object is, the more inertia it has.  

When you ride a bike, the first push of the pedals is harder than the other pushes.  That's because on the first push, you have to overcome the inertia of the bike, and of your own mass.

There are four forces that affect how well the experiments worked.  They are: inertia of the object, gravity, friction, and the kinetic force that you apply.  We already talked about inertia, so let's look at the others.  

Since we are on the earth, we are dealing with the same amount of gravity that we are used to.  Gravity pulls everything towards the earth.  That's why you don't float off into the air when you walk.  All of the objects in the experiments are affected by gravity.  Gravity pulls the objects into each other, affecting the amount of friction between the different surfaces.  

Friction is the force that is caused by the contact of the two surfaces.  Because of the difference in these surfaces, which looks like little bumps under a microscope, it prevents objects from moving across each other. To move one object out from under another, you must overcome the friction between them.

The last force is the one you provide, the kinetic force.  When you pulled the paper out from under the car, you had to overcome the inertia of the paper, the friction between the paper and the table, and the friction between the paper and the car tires.  Since the paper has very little mass, it is easy to overcome its inertia.  The faster you pull the paper, the easier it is to overcome the friction.  In fact, if you didn't pull the paper fast enough, the experiment didn't work right.

Once you overcome the friction, the paper moved easily.  Because of the inertia of the car, it didn't want to move, and tried to stay right where it was.  The other experiments worked basically the same way.  Can you think of some other ways you can use inertia to produce "tricks" like these?