What is force?

What is force?

Prerequisites Required: None

Today, we are going to talk about force.

Figure 1: Dark Vader choking Admiral Motti with the force.

Nope. Wrong kind of force. Instead, we are interested in physicists' and engineers' definition of force.

The motivation for defining force comes from causality, the idea that events build upon each other in a cascade that causes objects to evolve. For a process to be causal, one event leads to the next which leads to the next, and so on. A fundamental expectation of our universe is that the properties of objects are casual. For example:

  • A paper is blank until a person writing on it with a pencil causes markings to appear on it.
  • A steak is raw until it is caused to be cooked by being placed onto a fire.
  • An object sits on the table until it is caused to be moved onto the floor by a cat.

This last example is of high relevance.

We make the following unprovable yet apparently obvious statement about the nature of the universe:

The position of objects is causal.

This fundamental observation was packaged by Isaac Newton in his First Law of Motion:

"Every body must persist in its state of rest or of moving uniformly in a straight direction, except in so far as it is forced to change that state by impressed forces."- Isaac Newton (translated from native latin)

This should be intuitive. If objects are not moving (at rest), they remain not moving and some cause is required to get them moving. By the same logic, if an object is moving at constant speed, it remains moving at constant speed and some cause is required to bring it to a stop.

To bring a moving object to a stop is to change the objects speed, so any sort of change in speed also requires a cause.

Newton referred to any cause that changes the velocity of an object a force.

Whenever the speed of an object changes, it is accelerating. Therefore, any object's acceleration is caused by a force.

Consider the below experiment where a person pulls a block and a spring.

Figure 2: A person pulling on a spring and PLA mass.

The hand pulls on the spring, which transmits the force to the block, which causes it to accelerate. There is something else we should keeny observe from this experiement. Due to the force applied by the hand, the spring deforms!

Forces cause objects to accelerate and deform.