1. 3.5 Action and reaction: Newton’s third law
What will happen to the balloon?
Paired forces
Action-and-reaction pairs
Check-point 7 1 2
Book 2 Section Action and reaction: Newton's third law

2. What will happen to the balloon?
What will happen when the balloon is let go?
It will move away from the hand.
Why does it move this way?
Air is ejected.
What else work by the same principle?
Rocket, hovercraft etc.
Book 2 Section Action and reaction: Newton's third law

3. Book 2 Section 3.5 Action and reaction: Newton's third law
1 Paired forces
Forces always occur in pairs. Here are some more details of paired forces.
Expt 3e
Paired force
Book 2 Section Action and reaction: Newton's third law

4. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3e
Paired force
1 Push a ‘push-and-go’ toy car a few times. Place it on the card laid on a thin layer of polystyrene beads. Observe what happens.
Book 2 Section Action and reaction: Newton's third law

5. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3e
Paired force
2 Two students, A and B, pull a rope
(a) in turns
(b) at the same time
Observe what happens.
Book 2 Section Action and reaction: Newton's third law

6. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3e
Paired force
3 Switch on the fan cart and place it on a bench top. Observe the air flow direction and the moving direction of the cart.
Book 2 Section Action and reaction: Newton's third law

7. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3e
Paired force
4 Partially fill a water rocket with water and pump air into it. Pull the trigger to release the rocket.
Observe what happens.
Book 2 Section Action and reaction: Newton's third law

8. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3e
Paired force
Video
Video
Video
Video
3.13 Expt 3e - Paired force
Book 2 Section Action and reaction: Newton's third law

9. Book 2 Section 3.5 Action and reaction: Newton's third law
1 Paired forces
Forces occurs in pairs and are in opposite directions. How about their magnitudes?
Expt 3f
Newton’s third law
Book 2 Section Action and reaction: Newton's third law

10. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3f
Newton’s third law
1 Set up the apparatus. Place two force sensors on the trolleys.
Book 2 Section Action and reaction: Newton's third law

11. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3f
Newton’s third law
2 Do data-logging in the following cases:
(a) Push trolley A momentarily to hit trolley B.
(b) Push both trolleys A and B.
Obtain a force-time (F-t ) graph in each case.
Book 2 Section Action and reaction: Newton's third law

12. Book 2 Section 3.5 Action and reaction: Newton's third law
Experiment 3f
Newton’s third law
3 Repeat step 2 with a harder push and with the mass of trolley A doubled.
Video
3.14 Expt 3f - Newton’s third law
Book 2 Section Action and reaction: Newton's third law

13. Book 2 Section 3.5 Action and reaction: Newton's third law
1 Paired forces
Expt 3f shows that:
If A exerts a force on B, B exerts an equal but opposite force on A.
Newton’s third law of motion:
To every action, there is an equal and opposite reaction. The action force and the reaction force act on different interacting objects simultaneously.
Book 2 Section Action and reaction: Newton's third law

14. Book 2 Section 3.5 Action and reaction: Newton's third law
1 Paired forces
Note:
Newton’s third law applies to two objects interacting with one another, but not to one object by itself.
The naming of ‘action’and ‘reaction’ is just a convention.
Book 2 Section Action and reaction: Newton's third law

15. 2 Action-and-reaction pairs
act on different objects interacting with each other
have equal magnitude all the time
act in opposite directions
Book 2 Section Action and reaction: Newton's third law

16. 2 Action-and-reaction pairs
E.g. a person sitting on a chair
Forces acting on the person:
weight
normal reaction by chair
weight
normal reaction by chair
not an action-and-reaction pair
Book 2 Section Action and reaction: Newton's third law

17. 2 Action-and-reaction pairs
Forces forming action-and-reaction pairs:
normal reaction (force on person by chair)
weight (force on person by Earth)
force on Earth by person
force on chair by person
Example 17
Pushing a car
Book 2 Section Action and reaction: Newton's third law

18. Book 2 Section 3.5 Action and reaction: Newton's third law
Example 17
Pushing a car
A person is pushing a car but it does not move.
‘The car does not move because the pushing force on the car by the person is balanced by the force applied on the person by the car.’
Comment on this statement.
Book 2 Section Action and reaction: Newton's third law

19. Book 2 Section 3.5 Action and reaction: Newton's third law
Example 17
Pushing a car
Incorrect.
Pushing force on the car by the person and the force applied on the person by the car:
form an action-and-reaction pair
act on different objects
never balance each other
Book 2 Section Action and reaction: Newton's third law

20. Book 2 Section 3.5 Action and reaction: Newton's third law
Example 17
Pushing a car
It is the friction on the car by the ground that balances the pushing force on the car by the person.
Net force = 0
Remains at rest
Book 2 Section Action and reaction: Newton's third law

21. 2 Action-and-reaction pairs
Video
3.15 The fan cart
The motion of a fan cart is ‘stolen’ by a sail
Example 18
Book 2 Section Action and reaction: Newton's third law

22. Book 2 Section 3.5 Action and reaction: Newton's third law
Example 18
The motion of a fan cart is ‘stolen’ by a sail
Air is blown in a forward direction.
(a) When the cart is placed on the bench, it moves backwards. Why?
By Newton’s third law:
The cart exerts a force F on the air
The air exerts an equal and opposite force F’ on the cart
pushing the cart backwards
Book 2 Section Action and reaction: Newton's third law

23. Book 2 Section 3.5 Action and reaction: Newton's third law
Example 18
The motion of a fan cart is ‘stolen’ by a sail
(b) A ‘sail’ is mounted on the cart in front of the fan. Draw all the horizontal forces acting on the fan, air and the sail.
force on air by the fan F
force on the fan by air F’
force on air by the card F’’’
force on the card by air F’’
Book 2 Section Action and reaction: Newton's third law

24. Book 2 Section 3.5 Action and reaction: Newton's third law
Example 18
The motion of a fan cart is ‘stolen’ by a sail
(c) Why does the cart not move after a sail is added?
By Newton’s third law:
The magnitude of the forces are the same
Net force on the cart (= F’ + F” ) = 0
It does not move.
Book 2 Section Action and reaction: Newton's third law

25. Book 2 Section 3.5 Action and reaction: Newton's third law
Check-point 7 – Q1
Draw and label the action-and-reaction pair that causes the motion in each case.
(a) The car moves on the card.
force on car by card
force on card by car
Book 2 Section Action and reaction: Newton's third law

26. Book 2 Section 3.5 Action and reaction: Newton's third law
Check-point 7 – Q1
(b) A and B move when they pull the string.
force on B by string
force on string by A
force on string by B
force on A by string A B
Book 2 Section Action and reaction: Newton's third law

27. Book 2 Section 3.5 Action and reaction: Newton's third law
Check-point 7 – Q1
(c) The fan cart moves when the fan is on.
force on fan by air
force on air by fan
Book 2 Section Action and reaction: Newton's third law

28. Book 2 Section 3.5 Action and reaction: Newton's third law
Check-point 7 – Q1
(d) The launch of the water rocket.
force on rocket by water
force on water by rocket
Book 2 Section Action and reaction: Newton's third law

29. Book 2 Section 3.5 Action and reaction: Newton's third law
The End
Book 2 Section Action and reaction: Newton's third law