Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

PHYSICS · Stage 7 · Chapter 2: Forces

Physics 2.4 Air resistance — Drag Forces and Terminal Velocity

5 min read 6 parts Lesson 4 of 7

0%read
Lesson summary

2.4 Air resistance — Drag Forces and Terminal Velocity — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how air resistance is produced, where it is a problem and where it is useful, and explain terminal velocity and falling in a vacuum.Air resistance = the …

  • When noticed
  • Slows things down
  • Streamlining
  • Wind tunnels
  • Parachutes
  • Group cycling

Part 1 of 6What air resistance is

1Air resistance1 point
= the force on an object moving through air, also called drag.
  • Drag acts in the opposite direction to the motion.
2Cause
— an object moving through air collides with the particles in the air and effectively pushes the air out of the way, and these collisions provide the resistance.
3When noticed
— you notice it when walking into a wind, cycling fast or leaning out of a car window.
4Factors2 points
— air resistance depends on the speed of the object and the area of the object in contact with the air.
  • Faster objects push more air out of the way, so they experience more air resistance.
  • A large parachute has a high drag, and a cyclist crouching down reduces drag.

Part 2 of 6Reducing air resistance

1Slows things down
— air resistance slows things down, and it is less if the area in contact with the air is small.
2Streamlining1 point
— it reduces air resistance by changing the flow of air over a car or plane.
  • Cyclists pull in their arms, crouch forward and wear special helmets.
3Wind tunnels1 point
— scientists use them to find the best shape for vehicles, and smoke shows the path of air.
  • Streamlined cars use less fuel.
4Fuel use
— cars use less fuel at lower speeds, a roof box increases fuel use, and aircraft cruise at high altitude where the air is thinner.

Part 3 of 6Using air resistance

  • Parachutesthey increase the area in contact with the air, which increases air resistance; rocket cars use parachutes to brake.
  • Aircraftflaps are spread to slow down the aircraft when it lands, and designers think carefully about reducing air resistance in flight and using it on landing.
  • Group cyclingcyclists ride in a group because the leader takes most of the drag.

Part 4 of 6Falling and terminal velocity

1Start of a fall
— the only significant force is weight, so the object accelerates.
2Speeding up
— as speed rises, air resistance increases.
3Terminal velocity1 point
= the constant speed reached when the drag equals the weight, so the forces are balanced and the resultant force is zero.
  • Without a parachute the parachutist reaches a high, steady speed.
4Stages2 points
— weight is greater than drag at first, so acceleration is large.
  • As speed rises drag increases and acceleration gets smaller.
  • When drag equals weight, there is no resultant force and the speed is constant.
5Skydiver2 points
— terminal velocity is about 55 m/s (200 km/h) in free fall.
  • When the parachute opens, the area increases, drag becomes much greater than weight, and the skydiver slows down.
  • A new, lower terminal velocity of about 5 m/s is reached, which is safe for landing.
6Speed-time graph
— it rises steeply, becomes less steep and levels off at terminal velocity, then falls sharply after the parachute opens and levels off at a lower speed.

Part 5 of 6Do heavier things fall faster?

  • In aira hammer falls faster than a feather because of air resistance.
  • In a vacuumif there is no air there is no air resistance, so all objects fall towards the Earth at the same rate.
  • Moon testin 1971 an astronaut dropped a feather and a hammer together on the Moon, where there is no air, and they hit the ground at the same time.
  • Tennis ball and cricket balldropped together they hit the ground at about the same time when they have a similar size, because the air resistance on each is similar compared with their weights.

Part 6 of 6Investigating drag

  • Methodcompare paper cones or cupcake cases dropped from the same height, timing the fall over about 2 m, repeating three times and calculating speed.
  • Variableschanging the area or number of cases changes the drag and the terminal speed.
  • Controlscontrol the release height, the shape and the person timing.
  • Resulta larger case or bigger parachute gives more drag and a lower terminal speed.
  • Limitationsair currents indoors, a short fall distance, and a video analysis gives better data than a stopwatch.

Last updated: