Igmos r Us

Motion physics

The physics of motion


1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Galileo showed us this on the tower of pizza when dropping the cannon balls.

2. The relationship between an objects mass (m), its acceleration (a), and the applied force (F) is (F=ma).Acceleration and force are vectors. In this law the direction of the force vector is the as the direction of the acceleration vector.

This is the most powerful of Newton's three Laws, because it allows quantitative calculations of dynamics: how do velocities change when forces are applied. Notice the fundamental difference between Newton's 2nd Law and the dynamics of Aristotle: according to Newton, a force causes only a change in velocity (an acceleration); it does not maintain the velocity as Aristotle held.

3. For every action there is an equal and opposite reaction.

This law is exemplified by what happens if we step off a boat onto the bank of a lake: as we move in the direction of the shore, the boat tends to move in the opposite direction (leaving us facedown in the water, if we aren't careful!).


Free body diagrams

Free Body Diagrams

Hopefully now you have an idea of what a force is and what it isn't. What do you do with them? The useful thing to do with forces is to determine the total force acting on an object. At the beginning of the introductory physics course, you will likely look at cases where the total force is the zero vector. This is called equilibrium. Even if you are looking at cases where the forces don't add up to the zero vector (I say that instead of just "zero" to remind you that the total force is still a vector), you still need to determine the total force. Physicists like to represent forces on an object by drawing a free body diagram. This is simply a representation of an object and a graphical representation of all the forces acting on that object.

Simply put, in a free body diagram, all the forces acting on the given object are represented as arrows. Let me start with a simple case, a box sitting on a table.

There are only two forces acting on this box (essentially): the table pushing up on the box and the gravitational force of the Earth pulling down on the box. The free body diagram for this box would look like this:

Note that I have used proper vector notation on my force vectors. The force of the table pushing up on the box is labeled as N because these types of forces are called "normal forces" - maybe I will talk about that in more detail later. Another useful thing is to include the labels "table-box" and "Earth-box" to indicate that each force is an interaction between two objects. A final note on this first example is the length of the arrows representing the forces. They are the same length indicating that they are the same magnitude of force. Since these forces are the same magnitude, but different directions, the total force on this box is zero vector.