# What are the meanings of highlighted term?

1. Oct 29, 2010

### Hyperspace2

An interesting sideline about Galilean relativity is the following. Up to that time the perennial question was, what kept a body moving? Galileo realized that this was the wrong question, since uniform motion in a straight line is not an absolute concept. The right question is, what keeps a body from moving uniformly in a straight line? The answer to that is forces'' (which are defined by these statements). This illustrates a big problem in physics, we have at our disposal all the answers (Nature is before us), but only when the right questions are asked the regularity of the answers before us becomes apparent. Einstein was able to ask a different set of questions and this lead to perhaps the most beautiful insights into the workings of Nature that have been obtained.

This statements are from http://physics.ucr.edu/~wudka/Physics7/Notes_www/node47.html

2. Oct 30, 2010

### zhermes

What are you looking for here?

3. Oct 30, 2010

### Hyperspace2

I just couldn't grab meaning of the highlighted sentence.

4. Oct 30, 2010

### D H

Staff Emeritus
What kept a body moving was the wrong question to ask. Yet that was the question that was asked between Aristotle's time and Galileo. The reason this is the wrong question is because it erroneously assumes that the natural state of an object is to be at rest.

That is the way Aristotelian physics saw the world as working. Suppose you need to push a heavy block from one side of the room to another. You need to be constantly providing a motive force to the block to keep it moving. If you don't the block will stop moving very shortly after you stop pushing it. Philosophers debated endlessly on what this motive force, or impetus was.

Galileo turned this debate upside down. The answer to the question "what keeps an object moving" is "nothing". The natural state of some object is not at rest. The natural state is to keep on going the same way it was going. Instead of asking what made an object keep moving, Galileo said that something is needed to make an object's motion change.

5. Oct 30, 2010

### Hyperspace2

Thanks for the reply. I got it.