Class content > Newton's Laws > Physical content of Newtons Laws

Prerequisites

• Newton's Laws
• Object egotism
• Inertia

In almost all cases in our experience, a change in an object's velocity is associated with it's interaction with some other object.  To get a block on the table moving, someone has to hit it with a hammer. To get a box sitting on the floor moving, somebody needs to push it.  To stop a speeding baseball someone has to catch it, or it has to hit a wall, or roll on the grass, or.... Note that the change in our object's velocity doesn't have to be caused by an animate object exerting will or intent!  A ball hitting a wall changes the direction and magnitude of its velocity (vector) because it hits the wall.

[The one example we have where it is hard to grasp what object may cause the changes of velocity is when things fall, or roll down a slope, or tip over. We give the cause of those phenomena a name -- gravity. We'll see later on that this phenomenon of gravity fits in perfectly with the structure that we are building:  Gravity is not simply a word that summarizes the idea "given a chance, anything will fall", but it also has an object associated with it:  The object associated with gravity is the earth.  But let's save this discussion for later.]

This suggests that when we see an object's velocity changing, we should look for another object that is causing that change.  The fundamental conceptual idea that we will make a part of the language of Newton's theory of motion is the following:

Every change in velocity an object experiences is caused by the object interacting with some other object.

So we now have three ideas that tell us how to build models of motion:

1. object egotism -- Our models will talk about objects.
2. inertia -- The critical aspect of the motion we need to pay attention to is the change of velocity - in other words acceleration.
3. interactions --  In order to find what is responsible for an object's change of velocity, we need to look for interactions with other objects.

Technical term alert! We'll call what one object does to another in order to change its velocity a force.  This is an unfortunate choice, since it is a word that is common in our everyday speech and it has lots of different meanings. Your mother forced you to eat your mashed potatoes before you could have dessert. Your brother joined the Air Force.  He was a force to be reckoned with. The law came into force on July 1st.  None of these meanings are what we are talking about here.

We will use the term force in its technical sense in physics to mean what one object does to another when they interact and try to change each other's velocities.  I say "try" since there may be other objects interacting with the object we are looking at and multiple forces may wind up cancelling.

Since forces are always interaction, and since they have both direction and magnitude, we will label them carefully specifying both the object we are considering (object A) and the object that is causing the force on object A (object B).  We will use the notation

(read: F of B acting on A) to describe a force that object B exerts on object A and that could potentially change the motion of object A.

Follow-on

• Superposition 
  

Joe Redish 9/13/11

Wolfgang Losert 9/11 2012