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Date May 2021 Marks available 6 Reference code 21M.2.AHL.TZ1.6
Level Additional Higher Level Paper Paper 2 Time zone Time zone 1
Command term Find Question number 6 Adapted from N/A

Question

An ice-skater is skating such that her position vector when viewed from above at time t seconds can be modelled by

(xy)=(aebtcostaebt sint)

with respect to a rectangular coordinate system from a point O, where the non-zero constants a and b can be determined. All distances are in metres.

At time t=0, the displacement of the ice-skater is given by (50) and the velocity of the ice‑skater is given by (-3.55).

Find the velocity vector at time t.

[3]
a.

Show that the magnitude of the velocity of the ice-skater at time t is given by

aebt(1+b2).

[4]
b.

Find the value of a and the value of b.

[3]
c.

Find the magnitude of the velocity of the ice-skater when t=2.

[2]
d.

At a point P, the ice-skater is skating parallel to the y-axis for the first time.

Find OP.

[6]
e.

Markscheme

use of product rule                      (M1)

(˙x˙y)=(abebtcost-aebt sintabebt sint+aebtcost)                A1A1

 

[3 marks]

a.

|v|2=˙x2+˙y2=[abebtcost-aebt sint]2+[abebt sint+aebtcost]2                M1


Note:
It is more likely that an expression for |v| is seen.
         ˙x2+˙y2 is not sufficient to award the M1, their part (a) must be substituted.


=[a2sin2t-2a2bsintcost+a2b2cos2t+a2cos2t+2a2bsintcost+a2b2sin2t]e2bt          A1

use of sin2t+cos2t=1 within a factorized expression that leads to the final answer               M1

=a2(b2+1)e2bt          A1

magnitude of velocity is aebt(1+b2)          AG


[4 marks]

b.

when t=0, 

a=5          A1

abebtcost-aebtsint=-3.5          (M1)

b=-0.7          A1


Note:
Use of aebt1+b2 result from part (b) is an alternative approach.


[3 marks]

c.

5e-0.7×21+-0.72          (M1)

1.51  (1.50504)          A1


[2 marks]

d.

x˙=0         (M1)

aebtbcost-sint=0

tant=b

t=2.53  2.53086         (A1)

correct substitution of their t to find x or y         (M1)
x=-0.697  -0.696591  and  y=0.488  0.487614         (A1)

use of Pythagoras / distance formula         (M1)
OP=0.850 m  0.850297         A1


[6 marks]

e.

Examiners report

[N/A]
a.
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b.
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c.
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d.
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e.

Syllabus sections

Topic 3—Geometry and trigonometry » AHL 3.12—Vector applications to kinematics
Show 28 related questions
Topic 5—Calculus » AHL 5.13—Kinematic problems
Topic 3—Geometry and trigonometry
Topic 5—Calculus

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