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)=(a ebt cos ta ebt sin t)
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.
Show that the magnitude of the velocity of the ice-skater at time t is given by
a ebt√(1+b2).
Find the value of a and the value of b.
Find the magnitude of the velocity of the ice-skater when t=2.
At a point P, the ice-skater is skating parallel to the y-axis for the first time.
Find OP.
Markscheme
use of product rule (M1)
(˙x˙y)=(abebt cos t-aebt sin tabebt sin t+aebt cos t) A1A1
[3 marks]
|v|2=˙x2+˙y2=[abebt cos t-aebt sin t]2+[abebt sin t+aebt cos t]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.
=[a2 sin2 t-2a2b sin t cos t+a2b2 cos2 t+a2 cos2 t+2a2b sin t cos t+a2b2sin2 t]e2bt A1
use of sin2 t+cos2 t=1 within a factorized expression that leads to the final answer M1
=a2(b2+1)e2bt A1
magnitude of velocity is a ebt√(1+b2) AG
[4 marks]
when t=0,
A1
(M1)
A1
Note: Use of result from part (b) is an alternative approach.
[3 marks]
(M1)
A1
[2 marks]
(M1)
(A1)
correct substitution of their to find or (M1)
and (A1)
use of Pythagoras / distance formula (M1)
A1
[6 marks]