1. Freefall

2. Lesson Structure Part 1 – Freefall without Air Resistance
Dynamics
v-t graph
Part 2 – Freefall with Air Resistance
Plot Baumgartner’s v-t graph
Dynamics: 3 stages of freefall (w/AR)
v-t graphs

3. Red Bull Stratos
On 14 Oct 2012, Felix Baumgartner set the world record for sky diving, diving down 39 km, and reaching a speed of 1360 km/h (368 m/s)
In this topic, we will study both the kinematics and dynamics of freefall

4. Plot v-t graph of Felix Baumgartner
Link:
Start from: 20 s mark of jump (3:56 of video), record velocity every 5 seconds until 50 s mark
This may be used as a starter activity, before or after doing v-t graph (with air resistance)
I recommend the teacher write down the v values by pausing the video every 5 seconds, then use powerpoint chart tools to plot the graph instantaneouly for the students to see

5. freefall without air resistance

6. Definitions (to memorize)
A Gravitational Field is a region in which mass experiences a force due to gravitational attraction
Gravitational Field Strength (g) is defined as the gravitational force acting per unit mass
Weight = mass x gravitational field strength

7. Freefall w/o Air Resistance
Two videos of freefall without air resistance:
1) Vacuum:
2) On the moon:

8. Freefall w/o Air Resistance
What did you observe?
When there is no air resistance
Objects accelerate downwards when dropped
This acceleration is the same for ALL OBJECTs, regardless of mass or size
This acceleration is called “acceleration due to freefall”

9. Freefall w/o Air Resistance
Consider the free body diagram of an object in freefall
Fresultant = mg
But, Fresultant = ma
ma = mg
a = g
Recall “g” is called ‘gravitational field strength’. It is also called ‘acceleration due to freefall’, and it has both units of Nkg-1 as well as ms-2. mg
Draw students’ attention that m is cancelled out, which is why a is the same regardless of mass of object.

10. Acceleration due to Freefall
Since acceleration due to freefall = g
And g depends on the strength of the gravitational field
Therefore, acceleration due to freefall is less on the moon than on Earth
This explains why in the moon video, the objects still accelerate downwards, but more slowly compared to on Earth

11. v-t graph of an object in freefall
Recall: in a v-t graph, gradient is acceleration
In a freefall (w/o air resistance) situation, acceleration is constant, hence gradient is constant (i.e. straight line graph)

12. Scenario 1: Object dropped from rest
If upwards is positive
If downwards is positive v v t t
Emphasize that the gradient of the graph will give you 10 (if freefall on Earth)

13. Scenario 1: Object dropped from rest
If upwards is positive
If downwards is positive s s t t

14. Scenario 2: Object thrown upwards
If upwards is positive
If downwards is positive v v t t
Explain how the graph comes about in 3 stages
Going upwards (and decelerating)
Coming to a stop
Change direction and going downwards (accelerating)
Emphasize that acceleration is always constant through, gradient is constant throughout, because resultant force (mg) is constant throughout, even when the object is at rest
This is the standard example of how to have a zero velocity but a non-zero acceleration.

15. Scenario 2: Object thrown upwards
If upwards is positive
If downwards is positive s s t t
Note that the graph shape should be quadratic

16. Tips for Solving Problems
Sign convention is important!
Acceleration always acts downwards
If upwards is taken as positive, acceleration =-10
If downwards is taken as positive, acceleration = 10

17. Practice Task 1
An object was dropped from rest. Determine how long it takes for its velocity to reach 30 ms-1.

18. Practice Task 2
A ball was thrown upwards before it fell down a cliff. If the initial velocity of the ball was 15 ms-1, determine the final velocity of the ball after 5 seconds.

19. Freefall with Air resistance

20. Plot v-t graph of Felix Baumgartner
Link:
Start from: 20 s mark of jump (3:56 of video), record velocity every 5 seconds until 50 s mark
This may be used as a starter activity, before or after doing v-t graph (with air resistance)
I recommend the teacher write down the v values by pausing the video every 5 seconds, then use powerpoint chart tools to plot the graph instantaneouly for the students to see

21. v-t graph of Felix Baumgartner

22. Freefall with Air Resistance
Stage 1: Object just released from rest, instantaneous velocity zero
No air resistance, because v = 0
Fresultant = mg
Fresultant = ma
a = g
Object has a downward acceleration of g (i.e. 10 ms-2) mg

23. Freefall with Air Resistance
Most of the time in real life there is air resistance
Recall for air resistance:
Always opposes direction of motion
The faster the object, the greater the air resistance
If velocity is zero, there is no air resistance
When an object is undergoing freefall under air resistance, we can split its motion into 3 different stages

24. Freefall with Air Resistance
Stage 2: Object starts to accelerate downwards starts to encounter air resistance
Fresultant = mg – Fair
Fresulatnt = ma
a = g – (Fair/m)
a < g
Still accelerating downwards, but with lower acceleration
Fair mg

25. Freefall with Air Resistance
Since object is still accelerating
Velocity (downwards) is still increasing
Since Fair increases when velocity increases
Fair keeps on increasing
Until Fair = mg (stage 3)
Fair mg

26. Freefall with Air Resistance
Stage 3: Fair = mg
Fresultant = 0
Since Fresultant = 0, object no longer accelerates
Downwards with uniform velocity
This velocity is called terminal velocity
Fair mg

27. v-t graph (with Air Resistance)
If upwards is positive
If downwards is positive v v vT t t vT

28. s-t graph (with Air Resistance)
If upwards is positive
If downwards is positive s s t t
The constant slope signifies the terminal velocity

29. Parachutes
A parachute works by increasing the air resistance of a freefalling object
Usually the parachute is opened when the object is already falling (may or may not be terminal velocity)

30. Parachutes
The moment the parachute opens, depending on how fast the object is currently falling, the Fair may be larger than the mg
Resultant force is upwards
Objects starts to decelerate (but still going downwards)
As velocity decreases, Fair decreases
Until resultant force is zero again
Terminal velocity (slower than before)
Fair mg

31. Quiz 4

32. Assignment Assignment 3b Topic 3 Paper 1 Qn 2, 5, 9, 17 Paper 2 Qn 2
Assignment 4 Topic 2
Paper 1 Qn 9, 14
Paper 2 Qn 6

33. Physics Website!!! COPY THIS DOWN YOU !&%#&^!@#*(&^!@#&%#)&*^ STUDENTS