Properties, loads and motion

Properties (right panel) edits whatever is selected. With nothing selected it shows the scene: simulation, ropes, gravity, air, grid and snapping, overlays, export and energy. Every number takes SI units and can be typed; hover a row for an explanation.

#A body

• Mass & inertia: mass in kg (or density), and the moment of inertia, from the shape by default. Type your own for a hoop (Ring (m r²) for a circle), a flywheel with a heavy rim, or any body whose mass is not spread evenly.
• Material and contact: a material (Default, Wood, Steel, Rubber, Ice, Mud, or one of your own) sets static, kinetic and rolling friction and the bounciness (restitution); each can be overridden. Pair friction sets the friction of one pair of bodies only (a frictionless track for a cart while the disk on it keeps rolling).
• Body type: Dynamic; Static (fixed); Pinned axle (turns on its centre, with an optional motor); or Driven, moved by formulas (below).
• Shape: sizes, corner radius, a circle's hole.
• Air resistance: none, from the shape (needs air in the scene), F = −k v², or F = −b v. Properties shows the drag now and the terminal speed. From the shape, a spinning ball is also pushed across its path, towards ω × v (the Magnus effect: backspin lifts, topspin dips; Magnus lift k), and its spin slows by the friction of its surface (Skin friction C_f); a box or polygon's spin is slowed by the air on its faces. Turns into its flight makes a box or polygon turn its nose (its own x axis, as it was made) into its flight, as fins or an arrow's feathers do (bullets from the Shoot tool have it); in a vacuum nothing turns a flying body.
• Liquid (a pool from the Liquid tool): the liquid and its density, and its Wave damping.
• Damping: a slow-down that is not air resistance, for quick Sandbox scenes.

#Push

A body that can move has a Push section: choose the kind — an impulse (N·s: J = m Δv, at once), a change of velocity, a new velocity (m/s), or a force for a time (N, for so many seconds once the scene runs) — its x and y (y up) and a turning part (counter-clockwise), and press Apply. Paused, it takes effect when you press Play; running, at once. A body on a fixed axle only changes its spin.

#Your own materials

Properties → Material → New material… makes a material of your own; Edit materials… opens the list of the scene's materials. A material has:

• a name, static μs, kinetic μk and rolling c_rr friction, and restitution e;
• a density in kg/m² (the mass of one square metre of the 2D shape): a body that takes the material gets mass = density × area; 0 leaves its mass alone. A mass typed in afterwards stays;
• a figure look: a fill colour and a hatch (lines, cross-hatch or dots), drawn on screen and in exported figures.

Two bodies of the same material use its coefficients. Against any other material — a preset or another of yours — the pair uses the geometric mean of the two materials' own coefficients (a preset's own value is the one against itself: Steel on Steel 0.7). That is a convention, not a law: the presets' own table differs from it by 36 % on average. Where you have measured values for a pair (your material on steel, say), add a measured pair and it is used instead.

Test Scene opens a tab that shows the material's numbers: a block on a ramp just below atan μs (it holds), one just above (it slides at g(sin θ − μk cos θ), written beside it) and a ball dropped 1 m (it rebounds to e² h). Save to My Assets keeps the material; in My Assets, Add to Scene brings it into any scene (and gives it to the selected body). A scene file and a part keep the materials their bodies use.

#Loads and motion over time

A body can carry formulas of time t (Properties → Loads & motion over time):

• Force x(t), Force y(t) in N and Torque τ(t) in N·m act on a dynamic body.
• Move x(t), y(t) in m and Rotate θ(t) in ° move a Driven body (a shaken support, a lift, a cam).

5*sin(2*pi*f*t)          a force at frequency f
if(t < 1, 10, 0)         10 N for one second
0.02*sin(2*π*f*t)        a support shaken by 2 cm

Names you can use: t, pi (or π), g, e, the masses (m1 for a body named m₁) and constants you add to the list (with units; a parameter sweep can vary them). A formula the body type ignores is shown struck through with the reason, and kept. The formula language is the same as the table's: see Graphs and tables.

Keyframes… writes the formula by clicking instead of typing: set values at chosen times and how each one changes to the next (linear, ease, exponential, step), see the curve, and apply. The result is one piecewise formula, if(t < 1, …, …), in the row. Editing that formula by hand later is fine; the keyframe editor tells you when the formula no longer matches the keys.

#A joint

A pin's motor (speed and torque limit), torsional spring and damper, friction, angle limits and break force; a slider's limits and motor; a spring's k, c and rest length; a cable's length, slack, route over its pulleys, tension and mass per length. Read-outs show the joint's force (reaction) and, for a pin, its angle and speed.

#The scene

• Simulation: time step (1/60 s Sandbox, 1/240 s Research), substeps, solver iterations, exact joints, exact stacking, sleeping bodies (Sandbox), time scale, and Drag: a body dragged while the simulation runs either follows the cursor closely (the default) or pushes gently, like a wall moving at the cursor's speed.
• Gravity: x and y in m/s², or a preset: Earth, the Moon, Mars, the Sun and 20 more surface gravities. Gravitation G lets bodies with gravitates attract each other (orbits).
• Air: the atmosphere (vacuum, sea level, altitudes, Mars, Titan, Venus), its density and wind.
• Energy: the zero of potential energy (the origin, a height, the ground or a body), used by every energy graph and column.
• Ropes (lower down): a Rope's internal friction, which damps its wiggles.
• Credits: the items this scene took from the marketplace, with their authors and licences.