# Interaction 1

How hard something 'hits' or 'impacts'.

Discipline
Materials

Forces is the cause of motion changes. But what is the cause of forces?

It turns out that interaction is the answer. No forces exist without something else having applied that force. Without two two things interacting.

This discipline answers questions like ‘how hard does it feel to get hit by something?’ and ‘why is it tougher to catch than to carry something’?

So far, the disciplines have dealt with things and objects themselves. But this Interaction discipline finally deals with what things and objects can do to each other.

1st out of 2 courses in the Interaction discipline.
Meet the other participants in the Interaction classroom.

New properties
Momentum $$\vec p$$
Impulse $$\vec J$$
Angular momentum $$\vec L$$
Precession speed $$\omega_\text{precession}$$

New mechanisms
Definition $$\vec p=m\vec v$$
$$\vec L=I\vec \omega$$ $$\vec L = \vec r \times \vec p$$
$$\vec J=\vec F\Delta t$$ $$\vec J=\Delta \vec p$$
Newton's 3rd law $$\vec F_\text{A on B}=-\vec F_\text{B on A}$$
"Speed difference" conservation $$\Delta v_\text{before}= -\Delta v_\text{after}$$
(Angular) momentum conservation $$\sum \vec p_\text{before}= \sum \vec p_\text{after}$$
$$\sum \vec L_\text{before}= \sum \vec L_\text{after}$$

New phenomena
Elastic & inelastic collisions
Mechanical pain $$\Delta \vec a_\text{body parts}$$
Precession $$\omega_\text{precession}=\frac{wr}{L_\text{spin}}$$
Rotational stability (gyroscope)

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