CAIE IGCSE Physics 1.5.2.3 Apply the Principle of Moments to Questions

Practise calculating clockwise and anticlockwise moments, applying the principle of moments to find unknown forces or distances, and explaining counterweights.

Exam points

Calculate moments about pivots and apply the principle of moments to forces, masses and distances in equilibrium.

[Maximum number: 4]

Fig. 3.1 shows two children balanced on a seesaw. A seesaw is a length of wood which rotates about a central pivot.

Fig. 3.1

(a)

Child B moves 0.050 m further away from the pivot.

[ 4 ]

(i)

Explain why the seesaw rotates clockwise.

[ 1 ]

(ii)

Child A puts on a backpack and the seesaw now balances.

Calculate the mass of the backpack.
mass = kg [3]

[ 3 ]

(mass of backpack = ) $45 /(1.6 \times 9.8)) 2.9(\mathrm{~kg})$
(at balance) sum of clockwise moments = sum of anti-clockwise moments
$\mathbf{OR}(900 \times 0.85)=1.6 \times(450+W)$
(clockwise moment =) $765(\mathrm{~N} \mathrm{~m})$ OR (moment due to backpack =) $45(\mathrm{~N} \mathrm{~m})$ $\mathbf{OR}(W=)[\{900 \times 0.85\}-\{450 \times 1.6\}] / 1.6$

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CAIE IGCSE Physics 1.5.2.3 Apply the Principle of Moments to Questions question 1

Question (a)

Question (i)

Question (ii)