Terminal velocity GCSE physics lesson: weight, air resistance and a skydiver's graph

Terminal velocity: practice

Show your working on paper. Use g = 9.8 N/kg. Find the weight, then the resultant force, before a = F ÷ m. Give forces in N and accelerations in m/s². The stretch is optional.

Diagnostic question: choose and explain

1. A skydiver is falling at her terminal velocity. She opens her parachute. What happens in the next second?
   1. She moves upwards: air resistance is now bigger than her weight
   2. She keeps falling, but slows down: air resistance is now bigger than her weight
   3. She keeps falling at the same speed: the forces are still balanced
   4. She keeps falling, speeding up more slowly: her weight is still bigger

Practice questions

1. A skydiver has a mass of 65 kg. Use g = 9.8 N/kg. What is her weight, in newtons?
2. The moment she steps out of the plane she is hardly moving down, so there is no air resistance on her yet. What is her acceleration, in m/s²?
3. An 80 kg skydiver is falling and the air resistance on him is 300 N. What is the resultant force on him, in newtons?
4. What is his acceleration at that moment, in m/s²? Give it to 2 decimal places.
5. A skydiver weighs 700 N. Put each moment of her jump in the group that says what her speed is doing.They sort 6 items into Speeding up and Steady velocity and Slowing down.
6. On a skydiver's velocity–time graph, what does the gradient show?
   1. How far she has fallen
   2. Her acceleration
   3. Her velocity
   4. The air resistance on her
7. When her parachute opens, a skydiver's velocity falls from 50 m/s to 8 m/s in 3 s. On average, by how much does her velocity fall each second, in m/s²?
8. Two skydivers of the same size and shape, one 60 kg and one 90 kg, jump together and fall flat. Which reaches the higher terminal velocity, and why?
   1. Neither: everything falls with an acceleration of 9.8 m/s²
   2. The 60 kg one: lighter things fall faster
   3. The 90 kg one: it needs more air resistance to balance its bigger weight, so it falls faster before they balance
   4. The 90 kg one: heavier things get less air resistance
9. Why does a bigger parachute give a lower terminal velocity?
   1. It makes the skydiver's weight smaller
   2. Its bigger area gives more air resistance at each speed, so air resistance equals her weight at a lower speed
   3. It keeps air resistance bigger than her weight all the way to the ground
10. An 80 kg skydiver opens his parachute. Just after, the air resistance on him is 2000 N. Use g = 9.8 N/kg. How quickly is he slowing down at that moment, in m/s²? Give it to 1 decimal place.
11. Stretch (Higher): for a 50 kg skydiver falling flat, the air resistance in newtons is about 0.4 × v², where v is her speed in m/s. Use g = 9.8 N/kg. What is her terminal velocity, in m/s?

Support: Draw the skydiver with weight and air resistance arrows, the bigger force the longer arrow. Bigger down: speeding up. Equal: steady. Bigger up: slowing down.

Terminal velocity: exit

Use g = 9.8 N/kg. Find her weight first.

1. A 60 kg skydiver is falling and the air resistance on her is 450 N. What is her acceleration, in m/s²?
2. Later, the air resistance on her has grown to 588 N. What happens to her speed now?
   1. She stops falling
   2. She slows down
   3. It stays the same: she falls at her terminal velocity
   4. She keeps speeding up at 9.8 m/s²

Exit check

An acceleration from the resultant force, and what happens once air resistance equals weight, show who has the idea.

A 60 kg skydiver falls and the air resistance on her is 450 N. What is her acceleration?

Terminal velocity

Use g = 9.8 N/kg. For each skydiver, draw a box with an arrow for her weight and one for air resistance, the bigger force the longer arrow. Give units.