IGCSE Physics 0625

1.5.2 Turning effect of forces

Objective

Describe an experiment to demonstrate that there is no resultant moment on an object in equilibrium.

Theory

An object is in equilibrium when the net force acting on it is zero and the net turning effect (moment) about any point is also zero. A moment is calculated as the force multiplied by the perpendicular distance from the line of action of the force to the pivot point. For equilibrium, the clockwise and anticlockwise moments must balance.

Experiment: Demonstrating No Resultant Moment in Equilibrium

Apparatus

1. A rigid lever (e.g., a ruler or a strong bar)
2. A low friction pivot point (e.g., a knife edge supported by a stand or a fulcrum block)
3. A mass hanger or a platform to hang masses from
4. Several masses (e.g., washers or small weights)
5. A string to suspend the masses
6. A ruler or measuring tape to measure distances
7. A protractor to measure angles (optional, for more precise angle measurements)

Procedure

1. Set up the lever with the pivot point in the middle.
2. Suspend the mass hanger from one end of the lever using the string.
3. Add masses to the other end of the lever until the lever is in a balanced (equilibrium) position – i.e., it remains horizontal and does not rotate.
4. Measure the distance from the pivot point to the mass hanger (this is the distance from the force to the pivot).
5. Measure the distance from the pivot point to the point where the masses are attached to the lever (this is the perpendicular distance from the force to the pivot).
6. Calculate the moment due to the mass on the hanger using the formula: Moment = Force x Distance. The force is the weight of the mass (mass x g).
7. Calculate the moment due to the masses on the other side of the lever using the same formula.
8. Observe that the two moments are equal and opposite, resulting in no resultant moment.
9. Repeat the experiment with different masses and different positions of the masses to confirm the principle.

Data Table

Mass (kg)	Distance to Pivot (m)	Force (N)	Moment (N m)
0.1	0.2	0.1 x 9.8	0.1 x 0.2 x 9.8
0.2	0.2	0.2 x 9.8	0.2 x 0.2 x 9.8
0.3	0.2	0.3 x 9.8	0.3 x 0.2 x 9.8

Analysis

The experiment demonstrates that when an object is in equilibrium, the net turning effect (moment) about any point is zero. This is because the clockwise moment is equal and opposite to the anticlockwise moment. The principle of moments states that the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point.

Error Analysis

Potential sources of error include: friction at the pivot point, inaccuracies in measuring distances, and slight inconsistencies in the placement of masses. Minimizing friction by ensuring the pivot is as low friction as possible and using careful measurements can reduce these errors.