2.
Why is the weight of the ruler not included in the calculation of moments in Question 1?
The weight of the ruler acts through its centre of gravity (at the 50.0 cm mark), which is at the pivot.
Since the distance between the pivot and its weight is zero, the moment of its weight about the pivot
(= weight × 0) is zero. Hence, the weight of the ruler can be ignored.
Conclusion:
For the metre rule to be balanced (i.e. in rotational equilibrium), the anti-clockwise moment (W1 × d1)
about the pivot must be
equal
to the clockwise moment (W2 × d2).
Reflection:
How does the plasticine in the experiment help to balance the metre rule in step 4?
When fixed to the metre rule, the plasticine contributes to the weight of the metre rule such that it shifts the
centre of gravity of the metre rule to the pivot. Since the weight acts at the pivot, it produces no turning effect
about the pivot.
Challenge Yourself:
Describe an experiment that makes use of the Principle of Moments to determine the mass of a metre
rule (between 50 g to 80 g). The only apparatus provided are a glass prism and a 50 g mass.
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Chapter 5 Turning Effect of Forces
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