1. Aristotle’s old belief
Inertia 1
Force 1
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Inertia
Aristotle’s old belief
Constant speed motion requires constant force
Galileo’s law of inertia
Photo
It is as natural for a moving object to keep moving with a constant speed along a straight line as for a stationary object to remain at rest.

2. Inertia and Newton’s first law of motion
Force 1
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Inertia and Newton’s first law of motion
Galileo’s thought experiment
Diagrams
Newton’s first law of motion:
Every object remains in a state of rest or uniform speed along a straight line (constant velocity) unless acted on by an unbalanced force.
Photo

3. Inertia and Newton’s first law of motion
Force 1
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Inertia and Newton’s first law of motion
Force : changes the state of rest or uniform motion of an object (vector! Why?)
Inertia : the resistance of an object to a change in its state of rest or uniform motion in a straight line.
Mass : is a measure of inertia (scalar! Why?)
Mass can be considered as a measure of inertia
Inertial balance
Diagrams

4. Origin of wrong concept of Aristotle’s old belief
Force 1
Force 1
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Friction
Origin of wrong concept of Aristotle’s old belief
Nature of friction
Diagrams
Direction : opposite to the motion (velocity)
Examples of smooth surface
Photo
Demonstration of Newton’s first law on smooth surfaces

5. Unbalanced force (resultant force, net force)
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Unbalanced force (resultant force, net force)
Falling of object in liquid (terminal velocity)
Photo
Its relation with Newton’s first law of motion
Diagram
Friction-compensated inclined plane
Photo
Experiment to study resultant force
Diagram
Results
Calculation

6. Newton’s second law of motion
Force 3
Force 1
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Newton’s second law of motion
Acceleration is not zero if the resultant force is not zero.
Deductions from the results
1. Acceleration  force (mass is constant)
2. Acceleration  1/mass (force is constant)
3. F  ma or F = constant  ma

7. Newton’s second law of motion
Force 4
Force 1
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Newton’s second law of motion
Definition:
The acceleration of an object is directly proportional to, and in the same direction as, the unbalanced force acting on it, and inversely proportional to the mass of the object
Unit of force : Newton (N)

8. Newton’s second law of motion
Force 5
Force 1
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Newton’s second law of motion
1 newton of force will give a mass 1 kg an acceleration 1
constant in F = constant  ma becomes 1
Mathematical form of Newton’s second law
Direction of resultant force = direction of acceleration

9. Acceleration in free falling = 10 m s-2
Weight & Mass 1
Force 1
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Weight and Mass
Definition:
The force of gravity acting on the object is called the weight of the object and is measured in newton
Acceleration in free falling = 10 m s-2
1 kg of mass has a weight 10 N (downwards)

10. Instruments to measure weight and mass
Weight & Mass 2
Force 1
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Weight and Mass
Instruments to measure weight and mass
Photo
Weightlessness
Discussion about the restrictions of the above machine
Calculation

11. Addition of Forces (vectors)
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Addition of Forces (vectors)
More than one force acting on an object
Add them together to get ONE resultant force
F = ma can only be applied for resultant force
Tip-to-tail method (Revision)
Example 1
Calculation
Example 2
Calculation

12. Addition of Forces (vectors)
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Addition of Forces (vectors)
Method of resolving components
Adding forces or vectors without drawing diagrams
Example
Calculation
Examples for components of forces
Calculation

13. END of Force 1

14. Back to
Inertia 1
Force 1
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Galileo Galilei ( )

15. Inertia 2 Force 1 Next Slide
Small bearing is released from rest on a smooth track at A. A E D C F
A ball reaches point C which is of the same height
Same situation for D and E
If the track is infinite long, the ball will never stop.

16. Inertia 2 Force 1 Next Slide Galileo’ pin-and-pendulum experiment
Consider the swing of a simple pendulum

17. Back to Inertia 2 Force 1 Click Back to
The bob rises to the same height as before
Even we have a pin, the bob rises to the same height

18. Back to
Inertia 2
Force 1
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Isaac Newton ( )

19. Back to Inertia 3 Force 1 Click Back to
We set the platform into vibration and record the period.
Fix load on the platform and repeat the vibration, we find that a longer period can be found.
The larger the load, the longer the period.

20. Back to Force 1 Force 1 Click Back to
Friction is caused by the interlocking of surface irregularities.

21. Force 1
Force 1
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A mass placed on a thin layer of polystyrene beads on a glass plate
A balloon is blown up and attached to a short pipe

22. Back to Force 1 Force 1 Air-layer Ball Motion on a air track
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Air-layer Ball
Motion on a air track

23. An object is falling inside liquid.
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Force 2
Force 1
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An object is falling inside liquid.

24. Back to Force 2 Force 1 Click Back to
The object is falling downwards with constant velocity. Do you know why?
Liquid resistance is equal to the weight
No unbalanced force
liquid resistance
weight

25. Back to Force 2 Force 1 Click Back to
An inclined plane is prepared so that when we give the trolley a hard push, it moves down with constant velocity. It is called to be friction-compensated.
Careful adjustment for the plane is needed to achieve this situation.

26. Force 2
Force 1
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Identical elastic strings are used to pull the trolley.
At first, we use one string and then two, and three.
We always maintain the same length for all the strings so that each string produces the same force.
The accelerations in each case are recorded.
Friction-compensated inclined plane
trolley
elastic string

27. Back to Force 2 Force 1 Click Back to
One elastic string is used to pull several trolleys.
At first, we use one trolley and then two, and three.
We always maintain the same length for the string so that the string produces the same force in each case.
The accelerations in each case are recorded.
friction-compensated inclined plane
elastic string

28. Force 2
Force 1
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Different tape charts for different no. of strings with one trolley are shown.
1 string
2 strings
3 strings
a = 2 m s-2
a = 4 m s-2
a = 6 m s-2
We find that the acceleration is directly proportional to the no. of strings used (Force) when the mass of trolley is kept constant.

29. Back to Force 2 Force 1 Click Back to
Different tape charts for different no. of trolleys with one string are shown.
1 trolley
2 trolleys
3 trolleys
a = 2
a = 1
a = 0.67
We find that the acceleration is inversely proportional to the no. of trolleys used (mass) when 1 string is used (constant force).

30. Back to Weight and Mass 2 Force 1 Click Back to
Beam balance (measure mass)
Spring balance (measure weight)

31. Back to Weight and Mass 2 Force 1 Click Back to
Can we use the beam balance or spring balance on Moon to get correct readings of mass and weight of an object with 1 kg mass?
The acceleration due to gravity on Moon is only about 1.8 m s-2.
1 kg slot-mass is still needed to balance the object.
The reading from the spring balance = 1  1.8 = 1.8 N!
Mass is the same anywhere while weight depends on position and is not a constant even for the same object.

32. Back to Addition of Force 1 Force 1 Click Back to
Two forces 3 N and 4 N are acting on an object (2 kg) as shown below. What are the resultant force and acceleration?
2 kg
3 N
4 N N
3 N (3 cm in length)
Use a scale of 1 cm to 1 N to draw the forces in the form of arrows. The direction of the force is indicated by the arrow.
4 N (4 cm in length)

33. Back to Addition of Force 1 Force 1 Click Back to
Attach the tip of an arrow to the end of another arrow. (Tip-to-tail method)
Draw an arrow from the starting point to the end point. It is the net force.
5 N (5 cm)
53.1°
4 N (4 cm in length)
3 N (3 cm in length)
Length of the arrow : 5 cm
Direction : N 53.1°E
Direction of net force : N53.1°E
Magnitude of net force : 5 N (Why?)
Direction of acceleration : N53.1°E (Why?)
Magnitude of acceleration :

34. Addition of Force 2 Force 1 Next Slide
By using the concept of tip-to-tail method, one force can also be separated into two different forces, for example,
Use a scale of 1 cm to 1 N
(10 sin 60°cm)
60°
10 N (10 cm)
10 cos 60° N
(10 cos 60° cm)
10 sin 60°N

35. Addition of Force 2 Force 1 Next Slide
We want to add the following two forces using the method of resolving components. Scale : 1 cm to 1 N
60°
30°
5 N
6 N

36. Addition of Force 2 Force 1 Next Slide 60° 30° 5 N 6 N y x
We place them together on xy-coordinate plane with both the nails at the origin.
5 sin 60°N
30° y x
5 cos 60°N
6 cos 30°N
6 sin 30° N
Each force can be represented by a force (component) along x-axis and a component along y-axis.

37. Addition of Force 2 Force 1 + - Next Slide y
Add the components along x-axis together. Then add the components along y-axis. +
6 sin 30° N
5 sin60° N -
6 cos 30° N
5 cos 60° N x
They are of the same direction and we can add them like scalars.

38. Addition of Force 2 Force 1 - + Next Slide
Combine the components of force along each axis to form the net force vector. y -
6 cos 30° N
5 cos 60° N +
6 sin 30° N
5 sin 60° N  x

39. Back to Addition of Force 2 Force 1 Click Back to
Magnitude of the net force:
Direction of force ():

40. Addition of Force 2 Force 1 Next Slide
HK Convention & Exhibition Centre
By resolving components, the roof will not fall even no support is directly below the roof

41. Addition of Force 2 Force 1 Next Slide Hong Kong Space Museum
By resolving components, the roof will not fall even no support is directly below the roof

42. Back to
Addition of Force 2
Force 1
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The top of a tunnel