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Date May 2019 Marks available 6 Reference code 19M.1.AHL.TZ1.H_5
Level Additional Higher Level Paper Paper 1 (without calculator) Time zone Time zone 1
Command term Find Question number H_5 Adapted from N/A

Question

A camera at point C is 3 m from the edge of a straight section of road as shown in the following diagram. The camera detects a car travelling along the road at t = 0. It then rotates, always pointing at the car, until the car passes O, the point on the edge of the road closest to the camera.

A car travels along the road at a speed of 24 ms−1. Let the position of the car be X and let OĈX = θ.

Find d θ d t , the rate of rotation of the camera, in radians per second, at the instant the car passes the point O .

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

let OX =  x

METHOD 1

d x d t = 24    (or −24)       (A1)

d θ d t = d x d t × d θ d x        (M1)

3 tan θ = x        A1

EITHER

3 se c 2 θ = d x d θ        A1

d θ d t = 24 3 se c 2 θ

attempt to substitute for θ = 0 into their differential equation       M1

OR

θ = arctan ( x 3 )

d θ d x = 1 3 × 1 1 + x 2 9        A1

d θ d t = 24 × 1 3 ( 1 + x 2 9 )

attempt to substitute for x = 0 into their differential equation       M1

THEN

d θ d t = 24 3 = 8   (rad s−1)       A1

Note: Accept −8 rad s−1.

 

METHOD 2

d x d t = 24    (or −24)       (A1)

3 tan θ = x        A1

attempt to differentiate implicitly with respect to t        M1

3 se c 2 θ × d θ d t = d x d t       A1

d θ d t = 24 3 se c 2 θ

attempt to substitute for θ = 0 into their differential equation       M1

d θ d t = 24 3 = 8 (rad s−1)       A1

Note: Accept −8 rad s−1.

Note: Can be done by consideration of CX, use of Pythagoras.

 

METHOD 3

let the position of the car be at time t be d 24 t from O       (A1)

tan θ = d 24 t 3 ( = d 3 8 t )        M1

Note: For  tan θ = 24 t 3 award A0M1 and follow through.

EITHER

attempt to differentiate implicitly with respect to t        M1

se c 2 θ d θ d t = 8        A1

attempt to substitute for θ = 0 into their differential equation       M1

OR

θ = arctan ( d 3 8 t )        M1

d θ d t = 8 1 + ( d 3 8 t ) 2        A1

at O,  t = d 24        A1

THEN

d θ d t = 8        A1

 

[6 marks]

Examiners report

[N/A]

Syllabus sections

Topic 5 —Calculus » AHL 5.14—Implicit functions, related rates, optimisation
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