MUJOCO · DEVICE PHYSICS

From CAD Body to Physics Body

Turn a geometric body into a physics body by giving it the right mass, then simulate on real MuJoCo until the spring-loaded part settles at the spec deflection (-0.10 m).

01Challenge

The Bench opens on a real MuJoCo device: a part hanging on a spring. Press Simulate: it settles, but at -0.040 m, not the spec -0.100 m. The geometry is fine; the PHYSICS is wrong, because the body's mass is only 0.2 kg. A CAD shape is just a shape until you give it mass. Set the right mass.

Predict firstYou drop your CAD part into a physics sim. What must be added that the CAD model didn't have?

02Model

A drawing doesn't fall. A shape has no weight until you tell the simulator how much mass it carries. That's the first thing that turns a CAD body into a physics body. Hang a mass on a spring and it stretches until the spring pulls up exactly as hard as gravity pulls down: stiffness times stretch equals m g. So the stretch is m·g divided by stiffness. Heavier body, more stretch. The spring here is 49 newtons per metre and the spec stretch is a tenth of a metre, so the mass has to be about 0.10 times 49 over 9.81, right around half a kilo.

▶ Live · scrub & hover — the mass settles exactly where spring pull equals weight — physics solves k·x = m·g live

03Guided practice

1. Find the mass. Equilibrium: stiffness·x = m·g, so m = |x|·stiffness / g = 0.10 · 49 / 9.81 ≈ 0.50 kg. 2. Edit the model. In the <geom ... mass="0.2"> line, change the mass to 0.5. 3. Simulate. The load now stretches the spring to -0.10 m and the autograder reads the settled deflection.

04Feedback

PASS WHEN PASS when the load settles (velocity ~0) at -0.10 m deflection (±0.006). Real MuJoCo statics on the device.

If your run fails, check:

Settled at -0.04 m, too little stretch. The body is too light; raise the geom mass (m = 0.10·49/9.81 ≈ 0.5 kg).
Settled past -0.10 m, too heavy; lower the mass toward 0.5 kg.
No mass / zero mass makes the body non-physical; give the geom a positive mass.

05Retrieve & space

Equilibrium recall: a mass on a spring settles where stiffness·x = ____ (m·g, the weight).
What turns a CAD shape into a physics body the simulator can drop? (mass + collision geometry).
If you double the mass on the same spring, the deflection ____ (doubles).

06Mastery & project

Set the geom mass so the spring-loaded part settles at -0.10 m on real MuJoCo. Proves you can give a CAD body the physical property (mass) that makes it simulate.

The physics-body habit feeds the M5 capstone, where your designed device must be massed and simulated to prove it works.

GO FURTHER · THE LIVE PLAYGROUND

Generate and simulate a robot in Forge ↗ — describe a body, get a MuJoCo sim, drive it

← A Mechanism That Moves Force Balance: Spring vs Gravity →

MUJOCO · DEVICE PHYSICS

From CAD Body to Physics Body

Edit the MJCF device model and simulate it on real MuJoCo physics, tuning the parameter so the part settles at the spec.

You read

the body's mass, the spring and gravity forces, and its deflection each step

You change

the mass, stiffness, and damping you set on the device

Fixed

the MuJoCo solver, the device, and the settle check