Question 20N.2.HL.TZ0.1a(ii)
| Date | November 2020 | Marks available | [Maximum mark: 2] | Reference code | 20N.2.HL.TZ0.1a(ii) |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Outline | Question number | a(ii) | Adapted from | N/A |
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 . The aircraft hovers motionless above the ground. A package is suspended from the aircraft on a string. The mass of the aircraft is and the combined mass of the package and string is . The mass of air pushed downwards by the blades in one second is .
Outline, by reference to Newton’s third law, how the upward lift force on the aircraft is achieved.
[2]
Blades exert a downward force on the air ✓
air exerts an equal and opposite force on the blades «by Newton’s third law»
OR
air exerts a reaction force on the blades «by Newton’s third law» ✓
Downward direction required for MP1.
The question required candidates to apply Newton's third law to a specific situation. Candidates who had learned the 'action and reaction' version of Newton's third law generally did less well than those who had learned a version describing 'object A exerting a force on object B' etc. Some answers lacked detail of what was exerting the force and in which direction.
