An example of balanced and unbalanced forces
Force applied to a load can produce "balanced" and "unbalanced" force scenarios.
Retort stands and bosses, 2
Pulley, single, on clamp
Sand (1 small bag) or block of wood
Forcemeter, reading up to 10 N
String or other cord (not elastic)
Beam, horizontal, 1 m
Health & Safety and Technical notes
If tall stands are used, G-clamps will be essential to prevent toppling by enthusiastic student demonstrators.
A pulley fixed to a beam on the ceiling of the laboratory is ideal if such a beam is available. A pulley fixed to a clamp system that cannot topple is adequate. You can prevent toppling by using G-clamps to secure the clamp bases to the benchtop.
a Raise a small bag of sand or block of wood steadily and quite slowly, using the length of string that goes up and over the pulley. Let students pull it in the same way, and feel for themselves whether the force is the same however far the cord is pulled.
b Pull the cord with the forcemeter. Show that the force is constant.
c Allow the force to vary. This produces unbalanced force, so that the load speeds up (it accelerates) or slows down (it decelerates, or has negative acceleration).
1 You applied a constant force to the moving load through the string. This can balance its weight and the forces of friction that act at the pulley. When the forces are balanced there is no acceleration (though it does have velocity).
2 You could hold the load in a fixed position, and compare the force with that required for steady motion. You should find that the force is smaller. This is only because no effort is now needed to overcome frictional forces. The force, as before, is constant. Again the combined forces acting on the load are balanced. The load has no acceleration (or velocity). The common feature of the balanced forces is the absence of any change in velocity (acceleration).
This experiment was safety-checked in October 2004