A soft landing, together
- Move: hold ← or →. Tap ↑ to jump; near a ladder, hold ↑ to climb. Hold ↓ to climb down. Release to stop on the ladder.
- Grip: stand beside the cart and press the hand. Jump over it or use the walkway to switch sides.
- Push or pull: when holding a handle, pick Push, Pull or Off. Push goes away from you; pull comes toward you. The sign shows the force on the cart. Choosing a force starts the cart.
- Coast and brake: Off keeps existing velocity. Reverse force to slow down. Stop at the route’s marked dock and switch Off for one second. The shared camera pans and zooms out as you spread. Travel together: exceeding its readable range pauses the whole trial with an explicit checkpoint retry.
Local play on one device only. Around-table needs 900 px of width. Four players by keyboard require a numeric keypad; hardware key rollover varies. Tablet multi-touch, OS gestures and actual frame rate still need a physical-device test.
Numbers and shapes are always visible, with optional high-contrast names. No login, progress persistence or separate-device multiplayer inside the game.
OPTIONAL / EXPERIMENT BENCH
See what the forces are doing.
ΣF = ma · Right is positive · Acceleration 0.00 m/s² · Weight and normal force 98 N each
A checkpoint preserves position, velocity, clock, crew and run data.
Predict the next 2 seconds
Signed values: right +, left −. Nothing locks the controls.
The 16-second worked plan
Standard load: each player applies 5 N toward the dock for 2 s, Off for 11 s, 5 N opposite for 2 s, then Off for 1 s. Ends at the selected dock, stopped. Heavy cargo needs 10 N each. Reverse every force sign for the return direction.
The load scales with crew size: 5, 10, 15 or 20 kg. Heavy doubles it. No hidden changes to force, friction or dock size.
| t (s) | x (m) | v (m/s) | ΣF (N) | Measured a (m/s²) |
|---|
Model, limits & accessibility
The cart is frictionless. Balanced forces mean zero acceleration, not necessarily zero velocity. No hidden braking, drag or speed cap. Vertical weight and normal cancel; they act on the same body, so are not a third-law pair. Crew locomotion and gripping are control abstractions; their mass is not added.
Inelastic end-bumper collisions stop the cart and are flagged, not used as constant-force evidence. Sustained bumper contact adds a balancing contact force. Impacts are discrete impulses, not a resolved peak force; no impact-force arrow is invented. Delivery requires the cart center within ±0.25 m of the route’s dock, |v| ≤ 0.05 m/s and |net force| ≤ 0.5 N for 1 second. Delivering does not change its forces or motion.
Force changes are marked mixed in the log. Review is read-only. Up to 2,000 samples are kept; export before restarting. Progress resets on reload. During play, a translucent net display keeps the scene visible. Tap the cargo or select Player forces to add individual horizontal forces. Inspect pauses every actor and shows the full force diagram with fixed player lanes, outlined labels and a separate dashed net result. Vertical arrows start at the modeled center of mass; player arrows are offset for readability. Lengths are not to scale. Zero player forces have no arrow and remain in the textual readout. The canvas is visual, with cart measurements and all controls provided as text. Physical-device multi-touch and assistive-technology playtesting remain unverified.
MATCHED FORCE INVESTIGATION
Same force. Heavier cargo.
Predict first: What happens to acceleration when the mass doubles, but the force and time stay the same? Say or sketch your prediction before running the pair.
Two crew roles · +5 N each · +10 N total · 2 simulation seconds · from rest at 4 m
Each test resets to two crew, rightward delivery and same-side controls, replacing the current trial, history and checkpoint. Export anything to keep first. One person can run both tests. Results stay until reload.
Standard · 10 kg
Not tested yet.
Heavy · 20 kg
Not tested yet.
Run both trials to compare your prediction with measured acceleration.
The crew, signed force, duration, start position and initial velocity are controlled. Only mass changes. Gravity and support remain in the model; the floor is frictionless. A bigger normal force does not add friction. The shipment drawing does not set its mass.
PLAN → EXECUTE → REFLECT
Which way must the force point? Where will you coast and begin braking? Explore the galleries before moving the cargo.
Grip the cart. Push moves it away from you; Pull brings it toward you. Off removes your force, not its velocity. Save a checkpoint before trying a new idea.
Compare your stop with your prediction. Inspect forces or review the velocity graph, then Retry with a different plan.
OPEN from 0–12 s, CLOSED from 12–20 s, then repeat. The clock runs with the cargo simulation. Cross the beam’s center while open, then dock gently. It is a timing signal, not a solid wall: a missed window pauses the trial and offers a checkpoint retry. The same rule works for 1–4 players.
Need a starting plan?
One screen, one crew. The camera follows every player and the cargo, then zooms out to a readable limit. If you spread farther, an explicit regroup pause lets you restore the checkpoint. No camera forces, grip stretching, hidden drag or automatic braking.
Paused · Forces on the cargo
The world and clock are paused. Arrows are not to scale. Player arrows use offset lanes; vertical arrows start at the modeled center. The net is a sum, not an extra force. Closing leaves the cart paused. Press Run when ready.