PASCO ME-8949 EQUAL ARM BALANCE - Manual (Page 2)

2
Equal Arm Balance 012-05921A
®
Suppose the balance is mounted vertically. A standard
mass can be hung vertically from a groove or loop on one
side of the balance. An object of unknown mass can be
hung from a groove or loop at the same distance on the
other side of the Balance. If the balance doesn’t rotate
then the masses are equal. This is because the net torque
caused by the gravitational forces on one side of the bal-
ance is equal in magnitude and opposite in direction to the
net torque caused by the gravitational forces on the other
side of the balance. Thus, the Equal Arm Balance can be
used to determine a gravitational mass by comparison
with a standard mass. It can also be used to explore the
concept of torque as a vector quantity.
Setup
Vertical Setup
➀
Set up a base and support rod and attach a right angle
clamp to the support rod.
➁
Attach the Balance axle to the right angle clamp so
that the plane of the Balance is vertical.
➂
For use as an Equal Arm Balance hang masses from
loops at equal distances from the axis of rotation. Al-
ternatively, hang masses with light string or wire loops
attached to them on each side of the center of the Bal-
ance using the upper grooves.
➤NOTE For testing the torques that result from
gravitational forces, hang a standard mass from one
of the grooves or loops on one arm of the Balance.
Place another mass at a groove or loop on the oppo-
site side of the Balance that is a different distance
from the center. How much more or less mass is
needed to achieve a balance? What is the torque on
each side of the balance.
Horizontal Setup
➀ Set up a base or table clamp and support rod and at-
tach a multi-clamp to the top of the support rod.
➁ Attach the Balance axle to the multi-clamp so that the
plane of the Balance is horizontal.
➤NOTE For testing the torques that result from
applied forces, pull with a steady force on one of
the loops on one arm of the Balance. For simplic-
ity, pull in a direction that is in the same plane as
the balance but is perpendicular to the bottom edge
of the Balance. Exert another force at loop on the
opposite side of the balance. This force should still
be in the same horizontal plane as the Balance, but
it can be at a different distance from the center or in
a different direction. How much more or less force
is needed to balance that standard torque? Can you
calculate the magnitude and direction of the torque
on each side of the balance?
➤ CAUTION: The total mass hanging from the
Balance should never exceed one kilogram and the net
pulling force on the Balance should never exceed 10 N.
Suggested Experiments
This experiment has been adapted from Unit 5: One
Dimensional Forces, Mass, and Motion
What is mass?
Philosophers of science are known to have great debates
about the definition of mass. If we assume that mass re-
fers somehow to “amount of stuff”, then we can develop
an operational definition of mass for matter that is made
up of particles that appear to be identical. We can assume
that mass adds up and that two identical particles when
combined have twice the mass of one particle; three par-
ticles have three times the mass; and so on. But suppose
we have two objects that have different shapes and are
made of different stuff, such as a small lead pellet and a
silver coin. How can we tell if these two entities have the
same mass?
Ideas about Mass and Its Measurement
➀
Attempt to define mass in your own words without
using the word “stuff”.
➁
How many different ways can you think of to deter-
mine whether a lead pellet and a silver coin have the
same mass?
F
r
Axis of
Rotation
Axis of Rotation
Fig 3: Using Right Hand Rule to Find
Direction of Torque
τ