Newton's Laws

In late October, we started looking at Newton’s Laws.  This was initially a discussion about forces and how forces are analyzed in Classical Physics.  Newton’s three laws were focused on: Law #1 - objects stay at rest or at a constant velocity unless acted on by an unbalanced external force; Law #2 - an unbalanced sum of all forces acting on an object causes that object to accelerate in the direction of the resultant force; and Law #3 - for every force applied to an object, there is an equal and opposite force applied by the object. 

We then learned how to create Free Body Diagrams (FBDs) which are dots or boxes with arrows coming out of them showing all the possible forces acting on a given object.  Here is an example of a free body diagram for a box sliding across a floor that has friction:


F norm is the force exerted by the floor on the object.

F grav is the force exerted by the object on the floor. 

F frict is the force due to friction between the object and the floor (this force acts parallel to the floor).

F app is the force applied to the object. (This could be a push or a pull.  Regardless, it is always depicted by an arrow coming OUT of the figure).

It is very important for the students to learn how to create FBDs so we have spent a lot of class time doing these sketches so they can get an understanding of how forces act on an object.  

After this, we looked at masses on pulleys and other systems where the objects were connected to each other.  

Here are some examples of student work:

Free Body Diagrams (FBDs)


Using FBDs and Newton’s 2nd Law to Find the Acceleration of the Object


USING FBDs and Newton’s 2nd Law to Find Tension (T) in the Strings Connecting Weights on a Pulley


Ian Joseph
Grade Level: 10
Class: G9 Physics 1
Current Unit: Unit 2: Newton's Laws