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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)

Click for the questions that go with this reading
2nd Law

Newton's 2nd law says:



  • F= force; measured in Newtons, [N]. Newtons are abbreviated with a capital "N." Not a lower case. Note too that the force is a capital "F."
  • m= mass; measured in kilograms [kg]
  • a = acceleration [m/s2].

In other words, an UNBALANCED force ,F, gives you an acceleration. Whenever you have a change in speed or direction, it is because of an unbalanced for and Newton's 2nd law.

When watching a video, if a body changes its velocity, then this is due to Newton's seconds law. In the video below. The brain's velocity changes because the skull applies an outside force to the brain. This changes the brain's velocity from left to right. Recall that an acceleration is how quickly the velocity changes.

Note: Typically physicists and engineers do not use the term unbalanced force, Instead they use the phrase "net force." The net force is the resultant force when all the forces are added together. This acceleration is called the net acceleration. It is the same acceleration that is used in the kinematics chapter.

Example #1
A 1250 kg car decelerates from 12.0 m/s to 8.00 m/s in 25.5 m by air resistance. What was magnitude of the net force slowing the car down?

Example #2

Describe how the 2nd law is demonstrated in the animation below.



A Special Application of the 2nd Law


Every force is measured in Newtons. When you are near the surface of the Earth the force of gravity is written as

w = mg


  • w = weight (Pull of the gravity) measured in Newtons, [ N ] (lower case "w.")
  • m = mass of the object measured in kilograms [kg]
  • g = acceleration due to gravity measured as 9.80 m/s2.

You only feel your weight when you are standing on something solid, like the ground. This is because the ground is pushing up on you.

Weight depends on the pull of gravity. If you traveled to different places in the solar system, the acceleration due to gravity would change. This means your weight would change. But your mass would remain unchanged.




To be weightless means to make "g" equal to zero. This only happens very far away from the Earth. ...Somewhere in outer space. If you jumped off a tall platform or building, you are not weightless. You may feel weightless, but you are not weightless. The pull of gravity is still pulling you down to the Earth with an acceleration of 9.80 m/s2.

Choose the best answer. It does not matter how many questions you miss. What matters is that you learn from any mistakes.


by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)