Rainbow Sample Lab

Red Section

Lab 1: Crummy Marble

You will need:

safety glasses                            3 colored tape dots
1 megamarble                           3 tape dots of a second color
marble roller                             masking tape
extension tube                           tape measure

Okay, no dilly-dallying. Let’s get right to it. Put on the silly-looking glasses from the kit. We always do that before doing an experiment, no matter how safe the experiment seems. We don’t want to get our eyeballs poked out, now do we? Never start an experiment without silly-looking glasses. It’s one of the rules of science.

Pick one of the megamarbles out of your kit, and roll it across the floor. Come on, I know it isn’t very exciting, but believe it or not you can learn something from this. Are you done? Okay, now wasn’t that fun? (Not really.) Now let me ask you a simple question. When you rolled the marble, you pushed it with your hand. That is, you applied force on it with your hand. But the force was finished when the marble left your hand. Then why did the marble keep going? I’m going to write a scrambled word below, and you see if you can figure out what it is.

ETINAIR            _______________________________

This word is the property of matter that makes it continue to do whatever it is doing: If it is resting, it tends to keep resting; but if it’s moving, it tends to keep moving.

If you had pushed harder on the marble, what would have happened? The marble would have gone faster and farther, right? Why? Because God made it that way, that’s why. It’s not because of Newton’s laws of motion. Marbles rolled faster with a harder push even before Newton was born. But Newton came up with a way of explaining the faster roll that happens to be true. He said that the harder you push a given mass, the faster it travels. In a perfect vacuum with no friction or gravity, the

REFCO                                                                                   is equal to the

SAMS                                                                                     of the object times its

CETNCROLAAEI                                                                  .

In the form of an equation we write:

F = m ž a

The dot means “times.” You are probably used to writing “x” instead; but get used to the dot in physics. You’ll learn why later. Well now, you’ve learned something already, and all you did was roll a crummy marble across the floor!

The next thing you should do is hold the marble above a carpeted floor. (Or if you don’t have a carpeted floor, lay out a blanket, and do the experiment there.) Hold the marble above the carpet by about 1 decimeter. (If you don’t know what a decimeter is, look at the explanation at the end of the lesson. From now on we’ll call it a dm to save space.) Notice that I said “about.” It doesn’t matter if it’s 2 dm or less than a dm, but something like a dm. Drop the marble onto the carpet, and notice how much noise it makes. Now drop the marble from a height of 1 meter (m) above the floor, and notice how much noise it makes. Did it make more noise or less noise? It made more noise. This is because the marble was traveling faster by the time it hit the carpet.

Gravity does not stop pulling when the marble leaves your hand. You let go, and gravity pulls it toward the earth. The longer the gravity pulls on the moving marble, the faster it travels. So, the longer the marble has to fall, the faster it will be moving when it hits the carpet.

This is how acceleration works. It is the increase in velocity over time. When you get in a car and step on the accelerator, the car increases its velocity—that is—it accelerates. The longer you hold the accelerator down, the faster the car goes. Now, the rate of acceleration may be the same, but the velocity increases. Imagine getting in the car, starting it up, putting it in “drive,” and pushing the gas pedal (also called the “accelerator”) all the way down to the floor. Just as the car begins to move, you look at the speedometer and see that your velocity is 5 miles per hour. Does this 5-mile-per-hour value tell you whether you are speeding up or slowing down? No. It only tells you how fast you are going at that moment. This is the importance of knowing acceleration. It doesn’t tell you how fast you are moving at that moment, but it tells you whether you are speeding up (accelerating) or slowing down (decelerating). Unless you have a slow car, if you have the accelerator pushed all the way to the floor, you will be accelerating.

Acceleration from the pull of gravity is constant. If you could ignore the effects of friction, every object would increase in velocity at exactly the same rate as it falls toward the earth. But the higher you are when you drop the object, the greater will be its velocity when it hits the earth.

Let’s illustrate this another way. Please get the super-whiz-bang marble roller from the kit. It’s the plastic gizmo that looks like a pipe elbow stuck to a piece of plastic. It also has an extension tube with it. Get on the carpeted floor. Refer to the Diagram while setting up your experiment as follows:

1.         Get a piece of masking tape out of the kit, and tape it to the carpet. Set the marble roller so that the tape is right up against the place where the marble comes out, being sure there is plenty of rolling room in front of the marble roller.

2.         First, use the marble roller without the extension tube.

3.         Hold the marble on the rim of the roller, and nudge it off of the rim as shown in the Diagram. The reason for doing this is that you can repeat your technique over and over again by doing it exactly the same way. That’s really important in an experiment.

4.         Lift the marble from the floor where it stops, and put a colored tape dot in that exact spot.

5.         Repeat this two times, so that you have three dots of the same color on the carpet in places where the marble has ended up.

6.         Now place the extension tube on the marble roller. Replace the marble roller at the edge of the masking tape so that it is in the same position that it was in before. Make three rolls using the same technique as before, but use tape dots of a second color to mark the spots where the marble ends up.

7.         Using your tape measure, measure the distance from the edge of the marble roller (the edge of the masking tape) to the center of each colored dot, and record the distance in the Table below. For example, I pretended that I rolled the marble from the roller without the extension, and on the first roll it traveled 35 cm. Now, you finish filling in the Table with your marble rolls, ignoring the number I put in it. (Just write your first number next to mine, and cross mine out.) Calculate the average distance for each set of marble rolls (that is, add the three distances together, and divide by three), and place the averages in the Table.

Table: Distances of marble rolls with and without extension tubes.

Look at the pattern of tape dots on the floor. Is it clear that the distances traveled using the extension tube were farther than the distances traveled without it? Why would the distances traveled be farther? Because the marble had a longer time to accelerate as gravity pulled it toward the earth. The faster-traveling marble travels farther under its inertia.

Now that you’re a physicist, you’ll never look at a dropped marble, a thrown baseball or a bouncing jacks ball quite the same. Please carefully pick up all of the parts, and return them to your science kit.

Explanations:

There are 10 cm in 1 dm.

There are 10 dm in 1 m.

There are 100 cm in 1 m.