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Intertia and newton's law

  1. Oct 17, 2007 #1
    I'm taking my first Physics course (it's a foreign language), and in English class we were given a piece of paper on Skating taken from an English Physics textbook. We are having problems with some expressions, such as Inertia. What the heck is inertia? Is it a law? Or an expression? (I checked in my dictionary, found nothing useful)

    Taken from textbook:
    Then on the following page, they bring up Newton's First law.
    Some of my mates said that Inertia and Newton's First law is the same thing. Are they right? How the can it be the same thing? Newton's Law is a law, while inertia is ... well, something else.
    Last edited: Oct 17, 2007
  2. jcsd
  3. Oct 17, 2007 #2

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    Inertia is the property of any object to maintain a constant velocity, i.e., constant speed in the same direction unless acted upon by an external force. So, you see. written like this it becomes the same as Newton's first law. This velocity can be zero, and then it is called the inertia of rest.

    But, in addition, Newton's first law defines force as that agent which tends to disturb the body from its state of uniform motion.

    Newton's second law tells you how to measure the force.
    Last edited: Oct 17, 2007
  4. Oct 17, 2007 #3
    Inertia is the word describing the property for an object to resist against a motion. Thus you have to push the object with a strong enough force if you want to move it. As long as you don't, as long as the object resists (due to its inertia; but sometimes also because of the frictions), it does not move. It stays at rest.
  5. Oct 17, 2007 #4

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    If you push an object at rest with any force, however small, it will move, with however samll acceleration. There is nothing like pushing it with a strong enough force. The resultant of all forces, including friction, has to be non-zero.
  6. Oct 17, 2007 #5


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    Is that only saying that a Force is required to change a body's position?
  7. Oct 17, 2007 #6
    OK, I'm not sure i got everything (Remember, english is not my mummy-tongue) I get Newtons law, that's all right. I just dont get if Inertia and his law is the same thing. If it really is the same thing, why have two laws that explain precisely the same thing.(jeez hwo many times did I use "thing"?)

    I dont get it.

    EDIT: To me more precise. I DO understand this sentence: A body in motion tends to remain in motion, a body at rest tends to remain at rest. and I do indeed understand this sentence: An object that is not subject to any outside forces moves at a constant velocity, covering equal distances in equal times along a straight line path. . What I need to know if they are the same thing.

    To me, IMO, it doesnt seem to be, as Inertia does not take velocity, acceleration and position etc in consideration, whilst Newtons law does. (?)


    Also what confuses me is that who the heck came up with the concept? Galileo or Newton? Didnt galileo claim (quoting from textbook)
    someone is stationary, the person in question will remain stationary, the same principle applies to if someone is moving (he/she will continue moving) in a steady pace and in the very same direction.

    Kind of sounds like Newtons law.

    PPS: What is COASTING? It says "coasting is one of the most basic concepts in physics" ... but nothing else
    Last edited: Oct 17, 2007
  8. Oct 18, 2007 #7

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    Inertia is the property of the body to maintain its state of rest or of uniform motion in a st line. (As I had said, this means const velo.)

    Nweton's first law says that this state of rest or motion can be changed by something called a force.

    What more is confusing?
  9. Oct 18, 2007 #8
    To "coast" is to move with a constant velocity without being slowed by a resisting force. An ice skater "coasts" along the ice. It's not a term that is used in a very technical way. Sometimes we say someone is "coasting along" if they are moving through something without effort.

    Inertia is an abstract concept. It does not refer to any measurable property, and you can't place a number on it. It's just a "tendency." Newton's First Law is called the "law of inertia."
  10. Oct 18, 2007 #9
    This is partially not exact. Since force = inertial mass x acceleration, the inertial mass is the number that you place on the inertia of the concerned body. You state that for any given (and non zero force) the greater is the inertia, the smaller is the acceleration resulting from the applied force. Perhaps is it the tendency you are speaking about.
  11. Oct 18, 2007 #10


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    Galileo gave the inertia law, Newton finished the story...
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