We’ve been discussing motion in terms of speed, velocity and acceleration. But what is it that allows an object to be in motion in the first place? Force!
In order for an object to move or stop, an external force must first act upon it. Without some force to get an object moving, the object would just sit in place, forever.
What is force?
Force: A push or pull on an object.
All forces have both size and direction.
Forces are measured in a unit called the Newton.
Forces are what cause changes in acceleration, motion, direction, etc. All movement is caused by force.
Forces are not always easy to detect, however.
We will discuss two main categories of forces:
Balance of Force
In order for a force acting on an object to cause a change in motion, that force must be greater than 0N.
When the total force acting on an object is not 0N, it is an unbalanced force. In these cases the total motion of an object will change.
When the total force acting on an object is 0N, it is a balanced force. In these cases, the overall motion of an object will not change.
For example:
In the next image, a book is at rest on a table.
All forces are balanced, and no change in motion occurs.
If I shove the book right, forces will be unbalanced, and the book will change motion.
If no other force acted on the book, it would continue to move forever to the right.
Friction applies a leftward, unbalanced force to the book, which eventually changes its motion and stops the book.
Net Force
More than one force may (will, if you are on earth) act on an object at any give time.
This makes it useful to understand the effect of net force on an object.
Net Force: the vector sum of all forces which are acting on an object.
Rules for calculating net force in 1-dimension are fairly simple:
Add forces in the same direction.
Subtract forces in opposite directions.
The final change in motion always occurs in the direction of the largest vector.
