What is gravity?

Prerequisites Required: Basic Understanding of Forces

Gravity is the most familiar cause for acceleration in the human experience. Drop a ball and it falls to the ground. Isaac Newton observed this tendency for objects to accelerate toward the ground and built a physics empire that transformed the world.

Is gravity a force?

When considering the cause of gravity, there are two possible options for describing gravity, stemming from the dual nature of observer/observee (read this article for more on this dual nature):

1. An invisible and non-contact force called gravity acts between two objects (like the earth and a ball) and causes them to accelerate towards each other.

   • Dropped falling objects are pulled by the force of gravity which causes them to accelerate toward the ground.
   • Stationary observers on the earth's surface continue to be pulled by the force of gravity but the surface provides an equal and opposite force. Stationary observers are therefore not accelerating and are inertial reference frames.

2. The fabric of space and time itself is distorted so that objects appear to accelerate to the earth (similar to a ball moving backward in an accelerating car).

   • Dropped falling objects appear to be accelerating toward the earth because the earth's surface accelerates toward the objects. Dropped objects are therefore not accelerating and are inertial reference frames.
   • Stationary observers on the earth's surface have a force applied to them by the surface of the earth; they are being accelerated upward by the surface of the earth.

Isaac Newton felt that option 1 seemed far more likely than option 2; clearly the notion of distorting space and time itself is ridiculous, these are just imaginary coordinates we use to describe the relative positions of objects and duration of events in our universe. In the next few experiments, we will prove him wrong.

Consider the below experiment. We've got a couple of weights (aluminum and PLA) and a light spring.

1. We measure the length of the spring (194mm long unstretched).
2. We measure the lengh of the spring with both the aluminum and PLA blocks secured to the end (220mm).

We observe that when the spring is sitting on the ground in a horizontal orientation, the spring remains unstretched. When a force is applied to the spring by a hand, it deforms and moves.

We conclude that the spring sitting unmoving is inertial in the horizontal direction; there are no changes in deformation.

We also conclude that the spring being pulled by the hand is non-inertial in the horizontal direction; there are changes in deformation.