Force

Goals

1. Describe a force.
2. Skillfully use forces in applications.

What?

A force is an interaction between two bodies that can be modeled as a push or pull acting at point in a direction aligned with the push/pull.

Examples

1. A person standing at ladder exerts a downward push. We would typically model this as a force which has a magnitude equal to the weight of the person. ![[person-on-ladder.png|200]].

2. Strong wind blowing a tall van might tip the van over. We typically model the push of the wind as a force. The magnitude of the force is easily calculated using the drag equation from fluid mechanics.

Facts

1. The SI unit of force is the Newton (N). Other common units are the pound-force (lbf).

2. Forces can be classified into two categories (cats):

   1. Surface forces. The two bodies must be touching. Most forces are surface forces.

   2. Body forces. This is a force that can act without the two bodies touching. Weight is a body force. A magnetic force and an electrostatic force are also body forces.

This classification is super-useful, especially for drawing free-body diagrams.

3. Forces are represented with vectors.

4. In mechanics, forces are often described in two ways:

   1. Action force: A force that tends to cause a body to move.

   2. Reaction force: A force that tends to cause a body to stay stationary.

This classification is used mostly in structures because building, bridges, and so on are not suppose to move. I don’t use these ideas, but many textbooks talk about reaction forces.