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Newton's Laws.

  1. Oct 15, 2006 #1
    Ok, I have some troubles understanding the 3 laws of Newton.

    1st law: So if the net force exerted on an object is zero, the object continues in its original state of motion.

    The net force is the vector sum of all the external forces right? For example a book lying in the table. The net force would be 0 because the book is exerting its weight on the table; therefore the table is pushing back on the book with the same weight.

    So what if you pushed the book making it move. Would the Net force be equal to 0?? Net forces don't include the force you are acting upon it right?

    So if the net forces weren't zero. Then would the book just fall through the table? I don't understand this part. Please explain thanks!!
  2. jcsd
  3. Oct 15, 2006 #2
    I've just read the physics classroom guides on Newton's first law.. However, still I'm confused about something.

    Ok, what if an object has a force of 2 m/s, east exerted on it on a frictionless surface. The law of inertia keeps it in the state of motion. The net force however is not equal right? There force making the forces unbalanced?? no friction and there is a vector of 2m/s eastward. o.0

    But here is what is says:

    The force of gravity pulling downwards and the force of the table pushing upwards on the book are of equal magnitude and in opposite directions. These two forces balance each other. However, there is no force present to balance the force of friction. As the book moves to the right, friction acts to the left to slow the book down. This is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and it subsequently accelerates. Unbalanced forces cause accelerations. In this case, since the unbalanced force is directed opposite to the object's motion, it will cause a deceleration (a slowing down) of the object.

    To determine if the forces acting upon an object are balanced or unbalanced, an analysis must first be conducted to determine which forces are acting upon the object and in what direction. If two individual forces acting on an object are of equal magnitude and opposite direction, then these forces are said to be balanced. An object is said to be "acted upon by an unbalanced force" only when there is an individual force acting on the object which is not balanced by another force of equal magnitude and in the opposite direction . Such analyses are discussed in Lesson 2 of this unit and applied in Lesson 3.
  4. Oct 15, 2006 #3
    basically action is equal to reation . So if you add your own force in addition to the force ( weight ) of the book , then the table reacts to the sum of both these forces which ultimately makes the net force zero i.e
    Action = reaction
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