Investigation of the laws of equilibrium for a set of co-planar forces

In this experiment a metre stick is suspended from two newton balances and four different "weights" are hung from the stick. The weights are increased and moved around until the stick is level. The two conditions for equilibrium should then hold: (i) the total force up = total force down and (ii) for any point on the metre stick the sum of the clockwise moments = the sum of the anticlockwise moments.

Note: The "weights" and balances can be moved by dragging the red section of string attached to each one. The weight of the metre stick, at the 50 cm mark, is 1 N and each hanging weight is 0.2 N. A small spirit level (assumed weightless) sitting on the stick indicates when it is level (remember the bubble always goes to the high side)

 

Procedure:

1. The system is in equilibrium when it starts. Move the balances or some of the weights or add weights to put it out of equilibrium.
2. Carefully move the weights until you get the system back in equilibrium (the bubble in the middle)
3. Into you copy draw the forces acting on the stick (don't forget the weight of the stick) and the position of each of the forces.
4. Pick any point on the stick and calculate the sum of the clockwise moments and the sum of the anticlockwise moments about that point.
5. Press "Reset" to repeat the experiment

Analysis:

To calculate moments about the zero mark the following is used:

sum of clockwise moments = w1×x1 + w2×x2 + 1×50 + w3×x3 + w4×x4, where the x values represent the distances from zero of each of the weights.

sum of anticlockwise moments = b1 reading×x5 + b2 reading×x6 , where x5 and x6 are the distances from 0

Precautions:

• Check that the centre of gravity is at the 50 cm mark on the metre stick
• Ensure that the Newton balances are vertical and the metre stick is horizontal when readings are taken