DP Physics (last assessment 2024)

Question 20N.2.HL.TZ0.1a(iv)

Date	November 2020	Marks available	[Maximum mark: 2]	Reference code	20N.2.HL.TZ0.1a(iv)
Level	HL	Paper	2	Time zone	TZ0
Command term	Calculate	Question number	a(iv)	Adapted from	N/A

a(iv).

[Maximum mark: 2]

20N.2.HL.TZ0.1a(iv)

A company delivers packages to customers using a small unmanned aircraft. Rotating horizontal blades exert a force on the surrounding air. The air above the aircraft is initially stationary.

The air is propelled vertically downwards with speed $v$ . The aircraft hovers motionless above the ground. A package is suspended from the aircraft on a string. The mass of the aircraft is $0.95 kg$ and the combined mass of the package and string is $0.45 kg$ . The mass of air pushed downwards by the blades in one second is $1.7 kg$ .

(a(iv))

Calculate the power transferred to the air by the aircraft.

[2]

Markscheme

ALTERNATIVE 1

power $« = rate of energy transfer to the air = \frac{1}{2} \frac{∆ m}{∆ t} v^{2} » = \frac{1}{2} \times 1.7 \times 8 . 1^{2}$ ✓

$= 56 « W »$ ✓

ALTERNATIVE 2

Power $« = Force \times v ave » = (0.95 + 0.45) \times 9.81 \times \frac{8.1}{2}$ ✓

$= 56 « W »$ ✓

Examiners report

HL only. It was common to see answers that neglected to average the velocity and consequently arrived at an answer twice the size of the correct one. This was awarded 1 of the 2 marks.

Syllabus sections

Core

Core » Topic 2: Mechanics » 2.3 – Work, energy, and power

Core » Topic 2: Mechanics

Question 20N.2.HL.TZ0.1a(iv)

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