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Question 20N.2.HL.TZ0.b

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

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.

The package and string are now released and fall to the ground. The lift force on the aircraft remains unchanged. Calculate the initial acceleration of the aircraft.

[2]

Markscheme

vertical force = lift force – weight OR =0.45×9.81 OR =4.4 «N» ✓

acceleration =0.45×9.810.95=4.6 «ms-2» ✓

Examiners report

Well done by a good number of candidates. Many earned a mark by simply using the correct mass to find an acceleration even though the force was incorrect.

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