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Understanding The Second Law of Motion

By Kathy Foust

This brief study guide makes it easy to understand Newton's Second Law of Motion. Use it in conjunction with your teacher's motion lesson plans to develop a better understanding of this scientific concept.

Force, Mass and Acceleration

Newton's Second Law of Motion: Force = Mass x Acceleration

Let's take a look at these three terms:

  • Force:
    A vector quantity capable of producing motion or change in motion. In other words, force itself is at times a type of motion or potential motion. For instance, gravity does not move, but it is a type of force because it can create motion. Consider a dropped ball. The pull of the force of gravity brings the ball downward.
  • Mass:
    The amount of matter in any given object. Every object has mass and responds to gravity. We discuss mass instead of weight because weight is relative to the gravity of the Earth whereas mass is unchanging, whether it be on the moon or on the Earth.
  • Acceleration:
    is created by any change in speed, velocity or both. Acceleration and force are directly proportionate to each other. If the acceleration doubles, so does the force.

Examples of Newton's Second Law of Motion

If Newton's Second Law of Acceleration states that Force = Mass x Acceleration, then we know we can find any one of the three units if we have the other two.

We can also see that If the mass or acceleration changes, so does the force.

Some examples of this law of motion at work are below.

  • If a 50 lb child pushes on an 80 lb box, the force would only be half that it would be if a 100 lb child pushed on an 80 lb box.
  • If the same 100 lb man pushed on a 160 lb box, the acceleration would be half that it would be if pushing on the 80 lb box.
  • Try pulling a wagon that weighs slightly less than you do. It will move, but the acceleration is limited because the force limited.
  • Now have someone help you pull the same wagon. Because the mass behind the force increases, so does the acceleration.
  • If you were to double the weight of the wagon, you would decrease by half the acceleration of the wagon because the mass has doubled.

Use this study guide and some motion experiments at home to help you to better understand and use the Second Law of Motion.