Force & Motion

Once upon a time in a land far, far away… …Aristotle (384 – 322 BC) proposed the idea that the “natural” state of any earthly object was to be at rest. Trees didn’t move on their own, boulders rolled down hills and then stopped on their own, spears fell to the ground after traveling for a while: Everything tended to come to a state of rest.

Elevate one end of the plastic ramp 1 – 2 feet, place the block near the top of it, and let it go…now (before you click). Did it slow down and stop once it got to the table? If it didn’t, run away as fast as you can!!! Aristotle would say that the block wanted to be at rest, so it stopped…all by itself. Experiment #1:

He reasoned that all earthly objects were made of the earth to some degree; and so, they all wanted to be part of the earth again. Therefore, their natural state was to be at rest – one with the earth. At the time this idea made a lot of sense. In fact, this idea dominated thought for almost 1500 years. I told you it was a quality idea!

This time, place the wooden sphere at the top of the ramp and let it go…before you click the mouse (just like last time). Did it stop like the block did? Aristotle said it would. Since it is made of the same material as the block, it would have the same tendency to come to rest. So, I’d say that Aristotle has a little problem… his theory no work so good, eh? Experiment #2:

This is exactly what Galileo Galilei (1564 – 1642) noticed about nature, and he went about overturning Aristotle’s work. I don’t think so, Scooter. Galileo didn’t buy the “from the earth, to the earth” theory of motion that Aristotle championed because it didn’t explain why the block would stop but the sphere would keep rolling.

Galileo set up an experiment in which he rolled spheres down a track that was also inclined at the opposite end, as shown below. He noticed that the spheres came to rest on the opposite slope at approximately the same height they started on the first slope. The smoother he made the tracks and spheres the closer they got to that height.

As he made the second half of the track a gentler slope, the spheres still rolled to the same height, WHICH MEANT THEY ROLLED FURTHER BEFORE COMING TO REST. He extrapolated these results to consider a track that didn’t end in a slope, but rather remained flat to infinity. This sphere should never stop! (since it would never get back to its original height) To infinity and beyond!

He concluded from all of this that a force must be responsible for slowing objects down as they moved across the ground. THEREFORE, THE NATURAL STATE OF MOTION OF AN OBJECT IS TO DO WHATEVER IT IS DOING AT THE MOMENT! Specifically, an object at rest would remain at rest while an object placed into motion would remain in motion. (You’ve heard this before, I know it.)

Galileo explained this predisposition of an object to maintain its current state of motion by instilling in all objects a certain physical property called… INERTIAINERTIA Basically, this INERTIA enabled the object to resist any change in its current state of motion. The amount of inertia an object contained was related to its mass – the greater the mass, the more inertia the object had.

So, if you want to move a box out of your way you have to push it. If the box is heavier, you have to push it harder to make it move because it has more inertia – more resistance to moving. But there are a couple problems with that inertia thing: Number 1: Inanimate objects have been given the ability to assess their current situation and to decide what they are or are not going to do. (In other words, we have personified them – which you can only do in literature.)

Number 2: An object’s inertia is not a consistent property. It exists in certain situations but not in others. It’s as if a banana is only yellow when it is sitting on the table but not when you push it across the table. To illustrate this problem, take the textbook that is sitting on the table and make it move slowly across the table. Did you do it? (Git-R-Done!) You had to apply a force to it, didn’t you? Galileo would have said that you had to overcome the book’s inertia in order to get it to move. Its inertia was trying to keep it at rest.

Now align the dowel rods parallel to one another, place the book on them and try to make it move. You didn’t have to push as hard, did you? So, where did the book’s inertia go? It was there when the book was on the table, but now it’s gone (or seriously diminished). But how could that be? You didn’t change its mass or shape or orientation; you just placed it on the dowel rods.

Do you see the problem with the concept of inertia now? It’s kind of hard to be consistent with it. Sometimes it exists, and sometimes it doesn’t.

I’ll tell you this much: it is NOT a force. In fact, if you truly comprehend the ideas presented next, you will see that INERTIA is an unnecessary concept. So what is INERTIA exactly? The object you are pushing against is not fighting you or pushing on itself to stay in its current state of motion. INERTIA is just the idea that an object will not change its motion for no reason. Something must make its motion change.

This man’s name is Isaac Newton. He was born on Christmas Day, 1642, and died in Isaac Newton took hold of Galileo’s idea of motion and adjusted it in a very important way. Newton realized that when you tried to slide a heavy crate across a floor, the crate wasn’t trying to keep itself from moving, SOME OTHER OBJECT (in this case, the floor itself) WAS ALSO PUSHING ON THE CRATE, and the two of you were canceling each other out. By the way, your physics teacher was also born on Christmas Day. Coincidence?...or DESTINY?

Newton concluded that when all of the pulls and pushes acting on an object were cancelled out, the object would remain in its “natural” state: moving in a STRAIGHT LINE at a CONSTANT SPEED In other words, an object would maintain a And instead of the property of inertia causing this “tendency to remain as it was”, Isaac utilized the idea of BALANCED OR UNBALANCED FORCES. or remaining at rest (which is a constant speed of zero). CONSTANT VELOCITY unless a push or a pull acted on it to change that velocity.

BALANCED FORCES would be those that cancel each other out. If you push on a box with a force of 10 pounds directed to the EAST… 10 lb. While I push on it with a force of 10 pounds directed to the WEST… 10 lb. The forces will add together to produce a total force of zero pounds. The forces acting on the box will be BALANCED and the crate will remain in its current state (at rest). F total = (+ 10 lb.) + (- 10 lb.) = 0 lb.

So if you push the textbook so it slides away from you a little bit… …go ahead, do it… {I said, “Push the textbook!”} …the book slides a bit and comes to rest, but not because it “wants” to stop. It stops because a frictional force is acting on it, pushing on it opposite its direction of motion.

If we could get rid of the friction, then once you put it into motion by pushing it… …it should continue at constant velocity forever! (which is a very long time) We can do that – kind of – if we use the balloon puck that is sitting on your desk.

Experiment #3: Detach the balloon-stopper from the puck and inflate the balloon; reattach it to the puck. Air escaping through a tiny hole in the bottom of the puck will lift the puck off the table – reducing friction to a negligible level. Before all the air runs out, give the puck a small push. It didn’t slow down and stop, did it? It continued across the table at a (fairly) constant velocity, didn’t it? In order to stop the puck, you have to apply an unbalanced force to it, don’t you?

This first principle of motion: “An object will maintain a constant velocity unless acted upon by an unbalanced force.” is known as

You may now go back and take notes on any “main ideas” within the presentation, if you wish. (Although, it’s mostly just background information for the upcoming chapter.) When finished, please log off the computer, return to your seat, and read sections 1 & 2 of chapter 4 in the text.