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Questions About Newton's Laws

  1. Dec 26, 2014 #1
    Q1: Are objects at rest or are they in a uniform motion along a straight line in the world and in the universe?
    Q2: Why does an object with an important inertia not move if I apply to it suddenly a big force (accelerate it rapidly)? And on the other hand, the same object does move with a small force applied to it (small acceleration)?

    Video done by Julius Sumner Miller to visualise Q1 and Q2:

    I know that forces and Newton's laws seems easy to understand, and I actually understand the basics and I memorize what every law does say, but, suddenly many questions (like the above ones) pop up into my mind and I find myself blocked there, because I have a natural tendency to understand everything from the beginning and not to go on to the next level unless I'm feeling that I've mastered the current one.

  2. jcsd
  3. Dec 26, 2014 #2


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    Watch the video again, and observe what happens when the "sudden big force" is applied. Something does happen. Identify that "something," and you will understand.
  4. Dec 26, 2014 #3


    Staff: Mentor

    For Q1 it doesn't matter. You can perform the analysis in a reference frame where a given object is at rest or in a reference frame where that object is moving in a straight line at constant velocity. Either way you will get the same answer for any measurable outcome, even though the intermediate quantities may differ. This is Galileo's principle of relativity.

    For Q2, the change in momentum is related to a quantity called impulse, which is force times time. A small force applied for a long time can easily have a greater impulse than a large force applied for a very short time.
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