Newton introduced several concepts into physics in order to label the various parts of his equations. He did so without giving any specifics as to their fundamental nature. Labels such as force, inertia and energy were all defined in terms of each other.

Some of the labels are easy to understand in terms of what we can actually see, others less so.

Mass is an abstraction

In the case of Newton’s second law of motion, Newton stated that force is mass times acceleration:

F = m*a

While acceleration is easy to picture and imagine, force and mass are not.

Mass is not matter, but an attribute of matter that may refer to inertia or gravity, depending on context.

Force has many forms

Force is an even more ambiguous term. It can mean anything from tension and pressure to something that does work.

By bundling the concept of work up with pressures and tension, all sorts of confusions can arise, so I like to restrict my use of the word force in much the same way I limit my use of the word mass.

Ideally, the word force should only be used when there is a transfer of energy from one object to another. In all other cases, pressure, force-fields and tension should be used.

If we do this, the magnetic, electrical and gravitational forces would be relabeled as force-fields or pressures, because none of these forces do any actual transferring of energy.

Energy transfers inside force-fields

To illustrate, we can take the case of a wood block laying on the floor.

If I pick the block up, it is I who transfer energy to the block. Gravity merely provides a low pressure environment.

When I put the block onto a shelf, I’ve given the block some of my energy. This energy is stored in the block.

If the block slides off the shelf, gravity pulls it to the floor. However, gravity does not add any energy to the block. We get an acceleration, but the energy in the block remains the same. For accounting purposes, we can say that potential energy is converted into kinetic energy. But no actual transformation is going on.

It is only when the block hits the floor that it yields any energy to its environment. Only then is energy transported out of it, mostly in the form of sound and heat.

This same logic applies to the magnetic force and electric force as well. They do not add or subtract energy. The energy is always applied mechanically to the system, as explained in the chapter on motors and generators.

Adding energy to inertial matter

To change an object’s energy we always start by applying pressure. But if we are unable to change its energy, pressure is all we get.

Note that if we apply pressure over time to an accelerating body, and we express this mathematically, we end up with the formula for kinetic energy.

If we add pressure over distance inside a force-field like gravity, and this pushes an object higher, we get potential energy.

Newton’s second law of motion

The problem with the formula F = m*a is that it applies to all accelerations, regardless of whether energy is transferred or not. Force fields like gravity is treated as no different than mechanically applied force over distance. There’s a gravitational force, F = m*g, that adds no energy, and then there’s the mechanically applied force, F = m*a, that does apply energy.

While this is a stroke of genius under certain circumstances, it muddles the waters in many other cases.

To help our understanding of what’s going on, we shouldn’t use the same word for force that result in energy transfers, and pressures, tensions and force fields that don’t.

Conclusion

If we limit our use of the word force to mechanically applied energy transfers through motion, and use terms like pressure, tension and force-field in other cases, we avoid a lot of confusion.

Big ship, big inertia

By Wmeinhart – Foto wurde mit einem Panoramaprogramm aus drei Fotos zusammengesetzt, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=124261