This applet shows the forces on this (blue horizontal) beam, which is attached to the wall by a hinge about which the beam can rotate freely.

Suspended from the beam is a sphere. The mass of the sphere is indicated and can be increased by pressing the "b" (bigger) key or decreased by pressing the "s" (smaller) key. You can also move the mass to the left or right either by dragging it (small blue circle) with the mouse, or by pressing the left and right arrow keys. (Holding down the shift key while pressing the arrow keys will move the mass faster!)

The beam is held in horizontal position by the blue wire. You can change the angle of the wire by dragging the point (small blue circle) where it is fixed to the wall. You can also use the up and down arrow keys to accomplish this.

The colored arrows show the magnitudes and directions of the forces:


- string tension


- gravity, Mg, for the sphere


- reaction force of the wall on the beam


- gravity, mg for, the beam

This applet first assumes a massless beam, and the red arrow will then of course not be visible. If you click on the appropriate radiobutton, then the beam will become massive, and the red arrow will show.

Initially, the yellow and gray arrows representing the string tension and reaction force are not visible either. There are two ways of causing them to be displayed:

  1. (Easy) You hit the "r" (reaction force) and/or the "t" (string tension) key on your keyboard.
  2. (More challenging) You click on the point where you expect the endpoint of the force vectors to be located. If your answer is correct (with +- 20 pixels each in x- and y-direction), then the respective force arrow will show.