Air Resistance

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Air Resistance

Learn about air resistance and terminal velocity. A frictional force caused by air particles while an object travels through the air.

Air Resistance Learning Targets

  • I know how air resistance changes during a fall
  • I know how to draw a free body diagram of a falling object
  • I can determine net force and acceleration of an object
  • I can describe terminal velocity

Free Body Diagrams

  • A free body diagram shows all the vector forces and their direction acting on an object
  • The dot in the middle represents the object
Free Body Diagram

Air Resistance

  • Air resistance is a force that pushes backwards as an object moves through the air
  • The faster an object falls the greater the opposing air resistance

Air Resistance and a Vacuum

  • A vacuum is an area without air molecules
  • Space is also a vacuum.  Vacuums can be created on earth by sucking out the air particles.
  • Surface area affects the amount of air resistance under normal situations
  • A greater surface area will collide with more air particles and increase air resistance
No Air Resistance In A Vacuum

Q1: If Caleb has a mass of 76 kg, what is his weight on Earth with an acceleration due to gravity of 10 m/s²?

1

Normal force and weight on a horizontal surface

  • Have the same magnitude
  • Use the weight equation (Fw = mg) for magnitude of FN
  • FN = mg but up

Q2: If Caleb has a mass of 76 kg, what is the normal force on him?

2

Q3: What is the net force on Caleb as he sits on a horizontal surfaced seat?

Fnet = 0 N

Net force

Q4: When 76 kg Caleb first jumps out, there is no air resistance.  What is his acceleration at this point?

Fnet = 760 N Down

There is no normal force in the air

No Air Resistance

Q5: What is Caleb’s new net force when air resistance is 200 N?

3

Net Force With Air Resistance 2

Q6: What is Caleb’s new acceleration with a net force when air resistance is 200 N up?

4

Q7: What is Caleb’s new net force when air resistance is 500 N?

5

Skydiving-Stickman-Third-Out-of-Plane

Q8: What is Caleb’s new acceleration now with air resistance of 500 N?

6

Q9: What is Caleb’s new net force when air resistance is 760 N?

7

Skydiving-Stickman-Forth-Terminal-Out-of-Plane

Q10: What is Caleb’s new acceleration now that there is 760 N of air resistance?

0 N net force and 0 m/s² acceleration  

As an object falls:

  • It’s weight pulls down causing acceleration
  • Air resistance increases in the opposite direction
  • The result is that net force and acceleration decrease
  • The object increases its speed but by less
  • Until terminal velocity

Terminal velocity is the point when a persons weight equals the air resistance force

At terminal velocity:

  • There is no net force
    • Fair= -Fw
  • Therefore no acceleration
  • A person falls with constant velocity
Air Resistance
  • Terminal velocity depends on the shape of an object
  • Terminal velocity is around 53 m/s or 122 mph for a human skydiver

Q11: What shape does a velocity time graph have at terminal velocity?

Flat line with no slope

Q12: Describe air resistance compared to weight, net force, and acceleration at terminal velocity

  • Air resistance is equal to the objects weight
  • No Net Force
  • No Acceleration

VT Graph of Skydiver

Q13: What is 76 kg Caleb’s weight at terminal velocity?

760 N down

Q14: What is the air resistance on 76 kg Caleb at terminal velocity?

760 N up

Q15: What is the net force on Caleb at terminal velocity?

0 N

Air resistance up and weight down cancel out

Q16: What is the acceleration of Caleb at terminal velocity?

0 m/s²

No net force so no acceleration

Q17: What is 76 kg Caleb’s new net force when air resistance increases to 1000 N after pulling out the parachute?

17

more air resistance than weight with parachute

Q18: What is Caleb’s new acceleration now that there is a net force of 240 N up?

Q19: Is Caleb going up when the acceleration is up when the parachute opens?

No

Acceleration describes how velocity is changing per time.  He is still going down the whole time.

Q20: Is Caleb accelerating or decelerating when the parachute opens?

Caleb is decelerating

velocity is down but acceleration, the change in velocity per time, is up 

Air Resistance Questions:

1. Draw a force diagram of an object with a weight of 300 N when 100 N of air resistance is encountered?

Inside the rounded square below is a force diagram with the overall net on the outside

Air Resistance Net Force

2. What is the acceleration of an object with a weight of 300 N when 100 N of air resistance is encountered?

Weight is down and air resistance is up or in the reverse direction of motion. 

Note: down is positive in my solution

 

First find the net force:

Fnet = sum of forces = 300 + (-100) = 200 N down

Next determine the mass from the weight(FW):

Fw = mg

m = Fw / g

m = 300 / 10

m = 30 kg

Lastly determine the acceleration produced by net force:

F­net = ma

a = Fnet / m

a = 200/ 30

a = 6.67 m/s2 down

3. Draw a force diagram of a 40 kg object that encounters 60 N of air resistance?

First Find The weight (FW) to place in the force diagram.

FW = mg

FW = (40)(10) = 400 N down

Inside the rounded square below is a force diagram with the overall net on the outside

Weight is down and air resistance is up or in the reverse direction of motion

Force Diagram With Air Resistance

4. What is the acceleration of a 40 kg object that encounters 60 N of air resistance?

FW = mg

FW = (40)(10) = 400 N down

 

Fnet = sum of forces = 400 + (-60) = 340 N down

 

F­net = ma

a = Fnet / m

a = 340 / 40

a = 8.5 m/s2 down

Air Resistance and Terminal Velocity Quiz

1 / 8

What is the acceleration of a 1400 N skydiver at terminal velocity?

2 / 8

What is the weight of a 1400 N skydiver at terminal velocity?

3 / 8

What is the air resistance force on a 1400 N skydiver at terminal velocity?

4 / 8

What is net force of a 1400 N skydiver at terminal velocity?

5 / 8

What is the acceleration of a 1400 N  skydiver when she encounters 600 N of air resistance?  (use g = 9.8 m/s/s)

6 / 8

What is the mass of a 1400 N  skydiver when she encounters 600 N of air resistance?  (use g = 9.8 m/s/s)

7 / 8

What is the net force when a 1400 N skydiver encounters 600 N of air resistance?

8 / 8

What is the acceleration of a 1400 N skydiver that has no air resistance?

Your score is

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