# Full non-video equivalent

All teaching is available without sound, motion or video. Numbers and geometry are original simulation examples, not hardware settings.

## 00:00 — Reach it. Still drop it.

**Visual mechanism:** Two parallel jaws close around a block, rise once and lose the block onto a support. Reach and clearance labels remain distinct from holding.

**Spoken explanation:** Why does a robot drop an object it can reach? Reaching the object, getting there without a collision, and holding its weight are three different tests.

## 00:10 — Can the joints get there?

**Visual mechanism:** A two-link planar arm extends to a one-metre radius and retracts to a reachable pose. A red target outside maximum reach remains unreachable. The freely rotating position workspace has an inner radius too.

**Spoken explanation:** Start with reach. These two links are sixty and forty centimetres long. Their tip cannot reach beyond one metre. Inside that boundary, inverse kinematics finds candidate joint angles. But real joint limits and the required gripper orientation can still rule a candidate out.

## 00:31 — Check the whole motion.

**Visual mechanism:** The direct approach stops at a detected collision. An explicitly labelled alternative route moves the entire arm around the obstacle. The example checks simplified link capsules and gripper geometry, not an attached object.

**Spoken explanation:** Now watch the approach. The destination is reachable, but the direct move hits an obstacle. A clear endpoint is not a clear path. Check the arm links and the gripper along the entire approach, then check the retreat with the object attached.

## 00:49 — Touching is not holding.

**Visual mechanism:** A contact-force schematic shows inward normal forces, upward friction and downward weight. Friction capacity is labelled as a bound, not a constant applied force.

**Spoken explanation:** At contact, each pad pushes inward. Friction opposes the object's downward slip. In this simplified model, each pad can supply at most mu times N in upward friction. Mu is the friction coefficient. N is the inward force from one pad.

## 01:09 — Capacity versus weight.

**Visual mechanism:** For a 0.5 kg block, the comparison changes from 4 N to 6 N per pad, with mu=0.5. Total capacities are 4 N and 6 N against 4.9 N weight. The first trial slips; the second passes the static vertical test. Values are not hardware settings.

**Spoken explanation:** Our simulated object has a mass of half a kilogram, so its weight is about four point nine newtons. With mu equal to one half and four newtons from each pad, total friction capacity is four newtons. It slips. At six newtons per pad, capacity is six newtons. The static weight test passes.

## 01:31 — More squeeze has a limit.

**Visual mechanism:** An invented 5 N-per-pad damage limit rejects the 6 N squeeze. The block rests on a support and the jaws are open. The narration names torque, uncertainty and actuator limits.

**Spoken explanation:** That is not a safety certificate. A fragile object might be damaged by the extra squeeze. Real grasps must also handle torque, acceleration, uncertain friction, and actuator limits. Two ideal point contacts do not guarantee full three dimensional force closure.

## 01:51 — Which test is missing?

**Visual mechanism:** A slipped object rests on a support. Learners choose reach, clearance or contact and consider whether more squeeze is appropriate. A three-second quiet retrieval pause precedes the answer.

**Spoken explanation:** Your turn. A planner finds joint angles and a collision free approach. The object still slips. Which test was missing: reach, clearance, or contact forces? And should the robot simply keep increasing its squeeze?

## 02:09 — Reach. Clear. Hold.

**Visual mechanism:** The three evidence checks illuminate in order. A block moves onto a support to illustrate revising the task instead of escalating force. The narration invites the simulation and quiz.

**Spoken explanation:** Contact forces. Compare available friction with the load, and respect damage and actuator limits. If a safe force is not enough, change the grasp, support the object, or stop for help. Try the simulation and quiz in the Humanoid Hub AI Robotics course.