Physical AI Lab // Episode 2.1 // Quiz
A grasping scene in MuJoCo
From a page of XML to a physical world that moves
01
You put the cup into the MJCF but forget to give it a free joint. When you press run, what does the cup do?
It still falls, it just can't rotate
It stays where it is, fixed to its parent body
It passes through the table and keeps falling
The simulation stops with a missing-joint error
02
After the cup lands on the table it doesn't settle: its height keeps growing until it flies out of view. What is the most likely cause?
No mass was given, so MuJoCo set it to zero
The timestep is too large, so it diverged
Parent and child bodies don't collide by default
The default gravity points upwards
03
You only want the forces in the current state, without moving simulation time forward. Which function do you call?
mj_forward
mj_step
mj_resetData
Set the timestep to zero, then mj_step
04
The MJCF gives the cup no mass. How does MuJoCo get one?
It uses a default mass of 1 kilogram
Volume times the default density, 1 (1 gram per cubic centimetre)
Volume times the default density, 1000 (water)
It uses zero, so gravity doesn't act on the cup
05
The fingers close and the base lifts. The finger-position curve reaches its target, but the cup-height curve stays at table level. What is the most likely cause?
The control range is too small, so the fingers never closed
Offscreen rendering failed, so the cup wasn't drawn
Parent and child don't collide, so the fingers pass through
Too little friction, so the flat fingers slip
Check