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Newton's Laws of Motion

Shodor > CSERD > Resources > Applications > Newton's Laws of Motion


Newton's Laws of Motion

  1. An object in motion will remain in motion unless acted on by an external force.
  2. The acceleration of an object is directly proportional to the sum of the forces acting in the object, and inversely proportional to the mass of the object.
  3. For every force, there is an equal and opposite force.

Newton's first law states that unless there are forces present, an objects motion will keep moving the way it is currently moving. It will not change direction, and it will not slow down.

Discussion question - Does this make sense? Assume for a second that Newton was right. Gently toss a ball back and forth between yourself and another student, or between your two hands. Does this agree with Newton's first law? Slide a book gently across a table. Does it slow down? Does this agree with Newton's first law? If Newton was right, what can you say about these two examples?

Newton's second law states that more massive objects take more of a push to get moving, and that the harder you push, the more movement you will get. This can be written mathematically as

    a = F/m.

Sometimes this is rearranged and written as

    F = ma.

(Note that F and a are both vectors. Not only is the acceleration proportional to the net force, it is also in the same direction as the net force.)

Discussion question - The gravitational pull of the Earth is proportional to the product of the mass of the Earth and the mass of the object that is being pulled by the Earth. If the acceleration of an object pulled by that gravity is inversely proportional to the object's mass, what does that say about the acceleration due to gravity of any two objects of different mass near the Earth's surface?

Newton's third law states that when you push on something it pushes back.

Discussion question - Lean on a sturdy wall nearby. Note that when you lean on the wall, you are pushing on the wall. What would happen if the wall did not also push back on you?

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