How things move

What is the "natural" way that things move?

"Common sense" physics I

Falling objects seem to drop effortlessly with no push needed to keep them moving.


"Common sense" physics II

But without some sort of "push", objects moving on earth's surface soon come to a halt.


Aristotle Teaching

Aristotle (384-322 BC) distinguished 3 types of motion.

  • Natural motion [vertical]
  • Violent motion [horizontal]
  • Celestial motion [in eternal circles]

Natural motion was typically vertical (though it also include water going downhill). It is most prominently illustrated by objects (mostly made of earth) falling towards their 'natural home' (Earth). But Aristotle also recognized the idea of hot air rising and flames leaping upward: The natural 'home' of fire and air is not earth.

Violent motion. It seems that effort is required for horizontal motion, whether it be dragging a load or throwing a stone. Aristotle viewed this effort as arising because of the difficulty in getting objects to do something other than what was natural. (See 'natural motion' above!)

Celestial motion: The stars appeared to move in circles in the firmament without external assistance, and yet not towards the earth. This is not in conflict with "violent" motion which requires continued effort: Because different rules apply in the earthly and heavenly realms.

Motion might depend on...

  • gravity
  • starting speed
  • friction
  • air resistance
  • weight/mass
  • size
  • direction (horizontal/vertical)
  • incline
  • texture of object
  • etc., etc.

Amount of earth (weight): Aristotle

How to disentangle the effect of each of these?!?!

Problems with natural motion

How to make sense of the paper vs paper drop??

One way to bring together natural and violent motion is to say

"Nothing moves without a force".

...force of pushing on cart, or

...force of gravity acting on a weight.

So, let us consider the idea that falling motion depends on the weight of an object.

[Writing: Galileo's argument from De Motu]

Galileo's Law of falling

Galileo concluded that...

  • *if* you could get rid of air resistance / friction,
  • objects all fall the same way, no matter their weight.

[Apollo clip]

Another problem with violent motion

Roll the bowling ball

Arrow of ArtemisWhat keeps an arrow moving after the bow gives it an initial push??

Other objects, such as bowling balls, seem to move for a long time before they slow much.

Maple City Bowl

Galileo and friction

If you start a ball rolling going up an incline it slows down.
slowing down

If you start a ball rolling down an incline it speeds up.

So, what behavior is "half way" between slowing down and speeding up?

The tendency of objects to keep moving seems more fundamental than the tendency of objects to slow down.

The law of Inertia

A body that is subject to no external influences* will keep moving, if it was moving to begin with, in a straight line with unchanging speed. Or, if it was at rest, such a body will stay at rest.

* External influences, also called external forces, include things like air resistance, sliding friction, whacks with hammers, and also gravity.

Just as it was a huge intellectual leap to imagine motion without "friction", it was also a leap to imagine a situation without gravity.

Things tend to stay at rest...

Broken glass

What causes motion?

Asteroid Gaspra[Asteroid Gaspra]

Why do asteroids and anything else move through space without slowing down?

We might say 'Because of Inertia' but this really just means "We don't know".

Suggested Exercises

Conceptual Exercises in Chapter 3: 1, 3, 4, 6, 7