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Inertia

Page history last edited by Joe Redish 7 years, 8 months ago

Class content > Newtons Laws > Physical content of Newtons Laws

 

Prerequisites

 

As with many successful scientific theories, Newton's theory of motion helps us learn to see hidden causes and to bring a broader consistency to our understanding of how things move than we normally do from our everyday experience.

 

Now that we have decided (in our section on object egotism) to focus on an individual object, we have to decide what we should consider as its "natural" state of motion -- that state that one gets when there are no external influences acting on an object or, since we never have that situation, when all external influences acting on an object are balanced. (Whatever that means.  You will see that the theory is build up in a kind of circular or helical pattern, where we make certain unclear assumptions, then, as other concepts are sharpened, we come back and improve the original ones.) Once we've decided on what state needs no external influences to maintain it, we will be able to develop an understanding of hidden causes and create a coherent theory.

 

We might think -- from considering our block on the table example (in the Object egotism section), that the natural state of motion of an object is at rest.  But if we think about trying to stop a hard thrown fastball with our bare hands, we know there is something wrong with that.  The ball doesn't have anything acting on it at the instant it hits our hands, but it certainly seems as if coming to rest is something very unnatural for it!

 

Thought experiment 2: Bowling ball and a hammer

Let's consider an experiment that is the natural follow-on to our block-on-the-table experiment.  We saw that hitting the block with a hammer was not clarifying as to what was happening since something about the table was getting in the way.  Suppose we try to reduce the effect of the table by replacing it by an object that only connects to it at a very small point -- say a bowling ball rolling on the floor.  Now a rolling ball is still a pretty complex object.  What's happening when something rolls?  But at this point, let's not worry about that.  Let's make a simple model of the ball and treat it as if we are just paying attention to its position on the floor.


 

[We will deal with the complexity of rolling later, and we could imagine instead of a bowling ball rolling, the ball sliding down the bowling alley before it starts to roll, or we could decide that polishing the table to "reduce friction" would lead us to less of an effect from the table.  But that would assume that we already understood friction and at this point we are still trying to decide on the basic concept. This process of building a scientific theory is not a mathematical proof!  There's a lot of backing and forthing.]

 

If we hit the bowling ball with a hammer it starts to roll -- and keeps rolling until I hit it again.  Each time I hit it, it seems to go faster.  This suggests a different idea from "an unbalanced tap from an external object leads to a change in position" that we rejected.  Rather, it suggest we might try "an unbalanced tap from an external object leads to a change in velocity."  Hitting it in the same direction as it's moving speeds it up, while hitting it in the opposite direction slows it down.  Eventually, even if we don't hit it, it will slow down.  But remember that the first idea we decided to work with is object egotism -- if it's slowing down later, it's because of something that's happening later.  This suggests that for right now, we might try

 

All outside effects on an object being equal, the object maintains its velocity (including direction).  The velocity could be zero, which would mean the object is at rest.

 

This idea puts together our two intuitions about an object at rest -- to get it moving you need to do something to it -- and our sense of the speeding baseball -- to get it to stop you need to do something to it.  The idea that an unmolested object "keeps its velocity" is called inertia -- the tendency to maintain.

 

Careful!  We really do mean velocity here and not speed.  Think about the bowling ball moving in a straight line.  If we want to change the direction in which it's going we have to hit it on the side.  If we keep hitting it on the side we can change its direction a lot without significantly affecting its speed -- but it still takes being hit.

 

Follow-on

 

Joe Redish 9/13/11

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