Physics · Rung 5 of 23

Forces

Drawing the arrows acting on something is half of physics solved, and the classic mistake is always the same: believing that something at rest has no forces on it. A book on a table has its weight pulling down and the table pushing up exactly as hard, and that is why it does not move — at rest does not mean no forces, it means forces tied. The other classic mistake is drawing an arrow forward on something already moving: to keep going at a constant speed you do not need a push, you need nothing to slow you down.

Where this sits

Comes after
Quantities
Opens up
Pressure and floating, Levers and balance

Exercises from this rung

Generated by the app, from easiest to hardest

  1. a parachute coming down at a steady speed

    • the support, upwards
    • the drawing is complete
    • the water's push, upwards
    • weight, downwards

    Why It descends without accelerating because the air resists exactly what it weighs. Constant speed, forces tied.

    easier

  2. someone hanging still from a bar

    • the rope, pulling
    • weight, downwards
    • friction, backwards
    • your push, forwards

    Why The arms pull up as much as the body weighs.

    in between

  3. a box you push along the floor

    • the support, upwards
    • weight, downwards
    • friction, backwards
    • the water's push, upwards

    Why The floor resists backwards. Without it, the box would carry on by itself once you stopped pushing.

    harder