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Date May 2021 Marks available 2 Reference code 21M.1.SL.TZ1.11
Level Standard Level Paper Paper 1 Time zone Time zone 1
Command term Sketch and Show Question number 11 Adapted from N/A

Question

If a shark is spotted near to Brighton beach, a lifeguard will activate a siren to warn swimmers.

The sound intensity, I, of the siren varies inversely with the square of the distance, d, from the siren, where d>0.

It is known that at a distance of 1.5 metres from the siren, the sound intensity is 4 watts per square metre (W m-2).

Show that I=9d2.

[2]
a.

Sketch the curve of I on the axes below showing clearly the point (1.5, 4).

[2]
b.

Whilst swimming, Scarlett can hear the siren only if the sound intensity at her location is greater than 1.5×10-6 W m-2.

Find the values of d where Scarlett cannot hear the siren.

[2]
c.

Markscheme

I=kd2              (M1)

4=k1.52              M1

I=9d2                AG


Note:
The AG line must be seen for the second M1 to be awarded.
          Award no marks for substituting 1.5 and 4 into I=9d2 (i.e., working backwards).


[2 marks]

a.

        A1A1

 

Note: Award A1 for correct general shape (concave up) with no I-intercept, passing through the marked point 1.5, 4; the point must be labelled with either the coordinates or the values 1.5 and 4 on the x and y axes. Award A1 for the curve showing asymptotic behavior (i.e. I tends to 0, as d tends to infinity), extending to at least d=6; the curve must not cross nor veer away from the horizontal asymptote.


[2 marks]

b.

1.5×10-69d2               (M1)

Note: Award (M1) for a correct inequality.


d2450m  2449.48            A1


Note: Award A0 for d=2450.


[2 marks]

c.

Examiners report

[N/A]
a.
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b.
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c.

Syllabus sections

Topic 2—Functions » SL 2.5—Modelling functions
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Topic 2—Functions

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