Complete the table below by filling in the missing lengths - Leaving Cert Mathematics - Question 7 - 2021
Question 7
Complete the table below by filling in the missing lengths.
Swing 1 2 3 4 5
Length of Arc (cm) 45 81 729 20 200
$T_n = 45(0.9)^{n-1}$
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Worked Solution & Example Answer:Complete the table below by filling in the missing lengths - Leaving Cert Mathematics - Question 7 - 2021
Step 1
Complete the table below by filling in the missing lengths.
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Answer
The lengths of the arc can be calculated from the formula:
Ln=45(0.9)n−1
Swing
Length of Arc (cm)
1
45
2
45(0.9)^{1} = 40.5
3
45(0.9)^{2} = 36.45
4
45(0.9)^{3} = 32.805
5
45(0.9)^{4} = 29.5245
Step 2
Find the arc length of swing 25.
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Answer
Using the formula for the arc length:
T25=45(0.9)24
Calculating:
T25≈3.6extcm
Step 3
Find the total distance travelled by the tip of the pendulum when it has completed swing 40.
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Answer
The distance can be found by summing up the lengths for the first 40 swings:
S40=45+40.5+36.45+...+(45(0.9)39)
Using the series sum formula:
Sn=1−ra(1−rn)
where a=45 and r=0.9, we find:
S40≈443extcm
Step 4
Swing p is the first swing which has an arc length of less than 2 cm. Find the value of p.
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Answer
Set the equation 45(0.9)p−1<2. Solving for p gives:
p=⌈log0.9(452)+1⌉
Calculating gives approximately:
p≈31
Step 5
If the length of the pendulum is 1 m, show that the angle, θ, of swing 1 of the pendulum is 26°, correct to the nearest degree.
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Answer
Using the formula for the arc length:
L=rθ
Here, r=100 cm and L=45 cm:
45=100θ⇒θ=10045=0.45extradians
Converting to degrees:
θ=0.45×π180≈26°
Step 6
Hence, find the total accumulated angle that the pendulum swings through.
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Answer
The total angle for swings 1 to 40 is:
∑=26+26(0.9)+26(0.92)+...+26(0.939)
This series sums to:
Total=26×1−0.91−(0.9)40≈260°
Step 7
Hence, or otherwise, find the total distance travelled by the tip of the pendulum when it has moved through half of the total accumulated angle.
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Answer
Half of the total accumulated angle is:
2260=130°
Distance travelled is:
S=2π(1)×360130≈225extcm
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