Q5.6 Wait for weight (T)
An hourglass is weighed on a sensitive balance. First, when sand is dropping
steadily, from the upper to the lower part of the hourglass, and later, after
the upper part is empty. Are the two weights the same or not?
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Hint
What happens to the momentum in the steady state? Is it changing?
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Solution
In the steady state, with sand falling in
a smooth stream, there is no rate of change
in the total momentum of the hourglass and
sand. Thus the total external force is zero,
and the upward force from the balance is
equal to the downward gravitational force
mg. The balance thus reads the same as
it does after all the sand has come to to
rest at the bottom.