Clockwise moments Forces / N Distance from axis / cm Clockwise moments / N cm Anticlockwise moments Forces / N Distance from axis / cm Anticlockwise moments / N cm
Sum of anticlockwise moments / N cm
Sum of clockwise moments / N cm
Conclusion Forces up equal forces down. (Law 1: Vector sum of forces equals zero.) Clockwise moments equal anticlockwise moments about the chosen point. (Law 2: Vector sum of moments equals zero.)
Precautions Ensure that the Newton balances are hanging vertically to avoid friction and to ensure that readings are parallel to the weights.
Use a more accurate instrument to measure upward forces. Ensure that the metre stick is horizontal so that forces are perpendicular to the metre stick and therefore distances can be read directly from the metre stick.
Use a narrow fulcrum when finding the centre of gravity of the metre stick so as to get an accurate reading on the metre stick.
Note
If the metre stick is not horizontal, the laws can still be verified by this experiment. The perpendicular distances from the line of action of the force to the axis will all be cos times the reading on the metre stick, where is the angle of the metre stick to the horizontal. By dividing the equation (clockwise moments anticlockwise moments) by cos we get an equation that is independent of .Therefore it does not matter if the metre stick is horizontal or not.
cm
Density
DENSITY Density ( ) is mass per unit volume.
r DENSITY
r r m
V density
m mass V volume
68 INVESTIGATING PHYSICS
It is a scalar quantity with an SI unit of kilogram per metre cubed (kg m3). The symbol for density is the Greek letter , pronounced rho.
Note: The SI unit of volume is metre cubed (m3). r
