Secondary 3 Physics
Chapter 5 Turning Effects of Forces
Worksheet 5B
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Date:___________
Class: _____________
Chapter 5 Turning Effects of Forces
Moments
1.
(a) State the Principle of Moments.
(b) In an experiment to verify the principle of moments, a student pivoted a metre rule on a
needle through a hole at the 50cm mark. Then weights W1 and W2 were hung on opposite
sides of the pivot using thin cotton loops the positions of the cotton loops were altered
until the ruler balanced. The loop holding W1 was a distance d1 from the pivot and that
holding W2 was a distance d2 from the pivot as shown in the diagram.
d1
d2
W1
W2
The student repeated the experiment three times with different pairs of weights .the
results obtained are shown in the table below.
(i) Complete the table.
W1 / N
d1 / m
W2 / N
d2 / m
0.5
32
0.4
40
0.4
45
0.6
30
0.4
40
0.8
20
0.5
36
0.6
30
W1 x d1 /Nm
W2 x d2 /Nm
(ii) Explain how the experiment verifies the principle of moments.
Chapter 5 Turning Effects of Forces Worksheet 5B
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(c) The student then decided to use the principle of moments to measure the weight of the
metre rule. He suspended the ruler from a loop of cotton placed at the 30 cm mark (P)
and balanced the ruler by suspending a 50 g mass from the 6 cm mark as shown in
Fig 1.
Fig 1
P
50 g
W
(i) Calculate the weight of the 50 g mass and the moment it exerts about P.
(take g = 10 N/kg)
(ii) If the weight of the ruler is W, what is the moment of this force about P?
(iii) Use the principle of moments to calculate the value of W.
(iv) Calculate the upwards force exerted on the ruler by the cotton at P and explain why
this force does not have to be included in the calculation of W.
Chapter 5 Turning Effects of Forces Worksheet 5B
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2.
Fig 2 shows an electrically operated railway crossing barrier. The barrier is pivoted at P
and in order to make it easier to raise the arm of the barrier, the weight of the arm is
balanced by the counterweight M. The weight of the arm is 250 N and its centre of mass is
2.40 m from P. The centre of mass of M is 0.40 m from P. Calculate W, the weight of M
which just balances the weight of the arm.
Fig 2
3.
Fig. 3 below shows a uniform metre rule, weight W, pivoted at the 75 cm mark and
balanced by a force of 2 N acting at the 95 cm mark.
0 cm
50 cm
75 cm
95 cm
Fig 3
(a) Calculate the moment of the 2 N force about the pivot.
W
2N
(b) Use the principle of moments to calculate the value of W.
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4.
A uniform metre has a mass of 90 g and its centre of mass is at the 50 cm mark. Fig 4
shows this ruler suspended by a thread at the 70 cm mark, balanced by a mass of weight
W hanging from the 80 cm mark.
thread
80 cm
Fig 4
0 cm
50 cm
70 cm
W
100 cm
(a) Calculate the weight of the ruler.
(b) Calculate the moment of the weight of the ruler about the 70 cm mark (assuming that g
= 10N/kg).
(c) What is the moment of the weight W about the 70 cm mark?
(d) Calculate the value of the weight W.
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Center of Gravity
5.
(a) A student said that his centre of gravity is 0.8 m above the ground. Explain what he
means by that.
(b) A 300 g walking stick is balanced by a 100 g mass placed 15 cm from a pivot as shown
in Fig 5b. How far from the pivot is the centre of gravity of the stick?
Fig 5b
(c) A knife edge is placed at the 62.0 cm mark of the uniform metre rule shown in Fig 5c.
In order to balance the rule horizontally, a weight of 0.24 N is hung from one end of the
rule. Show this weight on the diagram and also mark the weight W of the rule acting in
the correct position. Calculate the weight W of the rule.
Fig 5c
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6.
A model of a Bunsen burner in the form of a thin piece of card is shown in Fig 6a. The
model has small holes drilled through it at K, L and M.
(a) Explain briefly how you would determine the position of the centre of mass of the
model.
Fig 6a
(c) Fig 6b shows a cross-section through the centre of a triangular prism. G shows the
position of the centre of mass of the prism. The prism is laid on one of its rectangular
faces on a horizontal table. Explain whether the prism should be placed on PQ, QR or
RP, for maximum stability.
Fig 6b
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7.
A trap-door AB 2.00 m long and weight 80 N is hinged at end A. it is opened by pulling on
a vertical string at end B as shown in Fig 7.
(a) Given that the centre of gravity of the trap-door is 1.20 m from A, Calculate T, the
tension in the string required just to lift it. State any assumptions that you have.
Fig 7
(b) Why, if the vertical string is attached to any other point on the trap-door between A and
B, will the tension required to open the trap-door be greater than that calculated in (a)?
Chapter 5 Turning Effects of Forces Worksheet 5B
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8.
Fig 8 shows an empty wheelbarrow which weighs 80 N. The operator pulls upwards on
the handles with a force of 20 N to keep the handles horizontal. The point marked M is the
centre of mass of the wheelbarrow.
(a) On Fig. 8 itself, draw arrows to show the other two vertical forces that act on the
wheelbarrow.
Fig. 8
(b) Determine
(i) the moment of the 20 N force about the centre of the wheel A,
(ii) the distance between points A and M.
Centre of Gravity and Stability
9.
Susan, the Ikea designer has designed a chair that looks like Fig 9. However, when her
manager sat on it, he toppled over.
Top of seat
What do you think Susan should do?
Fig. 9
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10.
A student observed that an empty conical flask in Fig. 10a is relatively easier to balance
compared to an identical flask filled partially with sand (Fig. 10b). Explain this observation.
Fig. 10a
11.
Fig. 10b
Fig 11a shows one end of a uniform block of wood resting on a table. The block is then
pushed so that it begins to tilt about the edge at Q. Draw another diagram onto Fig 11b to
show how far the block can be tilted before it topples.
S
R
P
Q
Push
Fig. 11a
12.
Fig. 11b
Why is a wine glass (as shown in Fig. 12) that is filled with wine less stable than an empty
glass?
Fig. 12
Chapter 5 Turning Effects of Forces Worksheet 5B
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13.
In Fig 13, there are two identical rectangular wooden blocks A and B. Block B has a layer
of lead attached to its base.
Fig. 13
(a) The boxes were tilted about edges PQ as shown in the figure. Explain why A topples
over at a smaller angle of tilt than B.
(b) State 2 conditions which can help to prevent a vase toppling over when tilted.
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