1. 6.4 Daily life examples of force
Skiing squirrel
Tension in a string
Normal reaction
Friction
Check-point 3
Weight — the pull of gravity
Check-point 4 1 2 3 ? 4 ?
6.4 Daily life examples of force

2. Skiing squirrel There are different forces acting on this ‘skier’.
Can you identify them?
6.4 Daily life examples of force

3. 1 Tension in a string Tension – force in a string – either left/right
(depending on what it acts on)
tension
Tension can act in opposite directions.
6.4 Daily life examples of force

4. 2 Normal reaction
The force acting on a boy by the surface in contact with him is normal reaction.
Any force to
balance
the weight?
normal reaction
weight
An upward force provided by the surface
acting perpendicular (normal) to the boy
Net force acting on the boy = 0
6.4 Daily life examples of force

5. 2 Normal reaction No matter he stands at A (a horizontal surface) or
B (an inclined surface)
normal reaction
normal reaction
the normal reaction acting on boy is perpendicular (normal) to the road surface.
6.4 Daily life examples of force

6. a Where does friction come from?
When the block moves,
the tiny bumps catch each other
friction (opposing motion)
bumps of surfaces
Likewise… v v
friction
friction
Friction arises when object slides
(or tends to slide) over another object.
6.4 Daily life examples of force

7. b Reducing friction and using friction
Friction opposes or prevent the motion of an object.
People find ways to reduce it, for example:
6.4 Daily life examples of force

8. Experiment 6e Gently push them and observe their motion.
Frictionless motion
Gently push them and observe their motion.
6.4 Daily life examples of force

9. Experiment 6e
Frictionless motion
The ring puck, balloon punk and vehicle on linear air track all travel
at a nearly constant speed along a straight line.
Video
6.4 Daily life examples of force

10. b Reducing friction and using friction
However, friction is essential in our daily life.
For example:
6.4 Daily life examples of force

11. 3 Friction
6.4 Daily life examples of force

12. c Fluid friction Fluid: gas/liquid Moving in liquid friction
(e.g. air resistance)
Moving speed
, fluid friction
6.4 Daily life examples of force

13. e.g. fast-moving car experiences large air resistance
c Fluid friction
moving speed
, fluid friction
e.g. fast-moving car experiences large air resistance
streamlining the car body 
air passes car body smoothly 
friction
6.4 Daily life examples of force

14. c Fluid friction For the ball-bearing: At beginning, vball-bearing = 0
force(s) acting on it:
weight
• weight 
• air resistance 
net force:
downward
Accelerates:
downwards
6.4 Daily life examples of force

15. but at a decreasing value
c Fluid friction
For the ball-bearing:
v gradually 
force(s) acting on it:
• weight
(constant) 
• air resistance 
()
air resistance
net force:
downward v
Accelerates:
downwards
weight
but at a decreasing value
6.4 Daily life examples of force

16. When air resistance = weight,
c Fluid friction
For the ball-bearing:
v   air resistance 
When air resistance = weight,
 net force =
 acceleration =
air resistance
 Newton’s 1st law v
 move at constant speed
such v is called terminal speed.
weight
6.4 Daily life examples of force

17. Experiment 6f 1 Drop a ball-bearing and observe its motion.
Motion affected by fluid friction
1 Drop a ball-bearing and observe its motion.
2 Release a parachute and watch its motion.
6.4 Daily life examples of force

18. Experiment 6f
Motion affected by fluid friction
We get…
Both ball-bearing and the parachute fall with constant speeds after accelerating for a short time.
Video
Video
Simulation
6.4 Daily life examples of force

19. d How air resistance affects the motion of free falling objects
is similar to a skydiver taking a free fall
6.4 Daily life examples of force

20. d How air resistance affects the motion of free falling objects
v 
air resistance
 air resistance 
When air resistance = weight v
 net force = 0
weight
move at constant (terminal) speed
(as high as 60 m s–1)
6.4 Daily life examples of force

21. d How air resistance affects the motion of free falling objects
Video
6.4 Daily life examples of force

22. d How air resistance affects the motion of free falling objects
v / m s1
fair = mg,
a = 0,
vterminal
open parachute, fair  greatly 60
a 
fair
fair > mg,
ve a
v , fair  (<mg), +ve a
v , fair  mg
fair = mg,
v'terminal 2
t / s
dive, v = 0, air resistance (fair) = 0
6.4 Daily life examples of force

23. 3 Friction E
6.4 Daily life examples of force

24. Check-point 3 A trolley is... 2 A book rests on...
6.4 Daily life examples of force

25. A trolley is connected to a block by a string.
Check-point 3 – Q1
A trolley is connected to a block by a string.
(a) In the figure, draw the directions of tension acting on the trolley and acting on the block.
tension
tension
6.4 Daily life examples of force

26. The acceleration of the trolley = = ( ) 4/1 4 m s-2 downwards
Check-point 3 – Q1
(b) If the tension in the string is 4 N, what is the acceleration of the block?
The acceleration of the block is (equal to/ greater than/smaller than) that of the trolley.
By F = , ma
The acceleration of the trolley
= = ( )
4/1
4 m s-2
downwards
6.4 Daily life examples of force

27. A book rests on an inclined plane.
Check-point 3 – Q2
A book rests on an inclined plane.
Which of the following is the correct free-body diagram of the book?
6.4 Daily life examples of force

28. Which of the following is the correct free-body diagram of the book?
Check-point 3 – Q2
Which of the following is the correct free-body diagram of the book? A B C D
6.4 Daily life examples of force

29. 4 Weight  the pull of gravity
Weight is the gravitational force acting on an object by the Earth.
Measured in newton (N)
if an object falls freely, its weight W gives it an acceleration of g.
Applying F = ma ,
W = mg
6.4 Daily life examples of force

30. 4 Weight  the pull of gravity
W = mg
Taking g to be 10 m s2,
the weight of a 1-kg mass is
W = 1 kg  10 m s2
= 10 N
6.4 Daily life examples of force

31. 4 Weight  the pull of gravity
6.4 Daily life examples of force

32. 4 Weight  the pull of gravity
a Mass and weight
Mass
Weight
a measure of an object’s inertia
gravitational force
change from place to place, depends on g
same in everywhere
SI unit: kg
SI unit: N
symbol: m
symbol: W
6.4 Daily life examples of force

33. a Mass and weight
Gravitational force (g) is different on the surfaces of Earth, the Moon and other planets:
6.4 Daily life examples of force

34. a Mass and weight
Video
Simulation
6.4 Daily life examples of force

35. According to Newton’s 2nd law,
a Mass and weight
According to Newton’s 2nd law,
the gravitational force
⇒ acceleration
weight on the Moon = 1/6 weight on the earth
Simulation
6.4 Daily life examples of force

36. b Beam balance & spring balance
The weights of 2 different masses, m1 & m2, in the same place:
W1 = m1g & W2 = m2g
Weights of different objects are proportional to their masses in a particular place.
6.4 Daily life examples of force

37. b Beam balance & spring balance
 Beam balance can measure mass by comparing
weight of an unknown mass with
weight of known standard mass.
beam balance
unknown mass
standard masses
6.4 Daily life examples of force

38. b Beam balance & spring balance
A spring balance measures weight but NOT mass.
It gives different values of weight of the same object at different places.
spring balance
6.4 Daily life examples of force

39. c Apparent change in weight in a lift
When you travel in a fast lift,
you can feel a loss or gain of weight.
How does this happen?
6.4 Daily life examples of force

40. c Apparent change in weight in a lift
Video
Simulation
6.4 Daily life examples of force

41. c Apparent change in weight in a lift
There are 2 forces acting on the girl:
unchange
weight W &
normal force R by the lift floor
– can be measured by a weighing scale W R
– scale reading shows the weight felt by the girl, i.e. the girl's apparent weight
may change
6.4 Daily life examples of force

42. c Apparent change in weight in a lift
R changes when travelling in the lift.
10/F
G/F
decelerates downwards
R > W
rest at G/F
rest at 10/F
R = W
accelerates downwards
R = W
uniform velocity
R < W
R = W
6.4 Daily life examples of force

43. c Apparent change in weight in a lift
By Newton's 2nd law,
the acceleration of the lift and the girl can be found if R is known. W R
6.4 Daily life examples of force

44. 4 Weight  the pull of gravity
6.4 Daily life examples of force

45. Check-point 4 Mark and label all forces... 2 The force pulling the...
6.4 Daily life examples of force

46. A squirrel of 1.2 kg is skiing on water.
Check-point 4 – Q1
A squirrel of 1.2 kg is skiing on water.
Mark and label all forces acting on the squirrel.
normal reaction
tension
friction
weight
6.4 Daily life examples of force

47. Check-point 4 – Q2
If the force pulling the squirrel is 5 N, the friction of water is 3 N. What is the acceleration of the squirrel?
normal reaction
tension
friction
weight
By F = ma ,
5 – 3 = 2  a
a = 1.67 m s2 (towards right)
6.4 Daily life examples of force

48. The End
6.4 Daily life examples of force

49. Example 4 Ave. acceleration of cyclist = 2 m s–2
Acceleration of a cyclist
Ave. acceleration of cyclist = 2 m s–2
Mass of cyclist and bicycle = 84 kg
What is the average friction acting on him and the bicycle?
If his average driving force is 140 N,
6.4 Daily life examples of force

50. Example 4 Take the direction to the right as positive.
Acceleration of a cyclist
Take the direction to the right as positive.
Net force acting on the cyclist = 140 N − f
By F = ma,
140 − f = 84 × 2
f = −28 N (28 N towards the left)
6.4 Daily life examples of force

51. Return
6.4 Daily life examples of force

52. opposes the fall of raindrop
Example 5 E
Raindrop
Forces A and B act on a raindrop as it falls to the ground. B
air resistance
What are A and B ?
Why does it fall with a constant speed?
weight
A = B, but opposite  A
 net force on raindrop = 0
By Newton’s 1st law, it falls at constant speed.
6.4 Daily life examples of force

53. Return
6.4 Daily life examples of force

54. Example 6
Thrust of Long March 2F rocket
At lift-off, Long March 2F rocket (carried with Shenzhou 6 spacecraft) has an lift-off thrust of N.
Its total initial mass is kg.
What is the initial acceleration of the rocket and the spacecraft?
6.4 Daily life examples of force

55. Example 6 By Newton’s 2nd law of motion, F = ma U − mg = ma
Thrust of Long March 2F rocket
Thrust U
Rocket's (with spacecraft) mass = kg
By Newton’s 2nd law of motion,
F = ma
U − mg = ma
− = a
a = 1.72 m s2
(upwards) mg
6.4 Daily life examples of force

56. Return
6.4 Daily life examples of force

57. Example 7 Find the acceleration of the lift in (a) 0-5 s, (b) 20-25 s.
Apparent change in weight in a lift
scale reading/N
time/s 5 20 25
450
500
550 28
mass of girl = 50 kg
when lift is stationary on 10/F, reading of weighing scale = 500 N
lift takes 25 s to travel to G/F
Find the acceleration of the lift in (a) 0-5 s,
(b) s.
6.4 Daily life examples of force

58. Example 7 (a) Take downwards as +ve. During 0-5 s, R < W and
Apparent change in weight in a lift
(a) Take downwards as +ve.
During 0-5 s,
R < W and
W  R = ma
 500  450 = 50a
 a = 1 m s2
The lift accelerates downwards at 1 m s2.
6.4 Daily life examples of force

59. Example 7 (b) During 20-25 s, W < R and W  R = ma
Apparent change in weight in a lift
(b) During s,
W < R and
W  R = ma
 500  550 = 50a
 a = 1 m s2
The lift decelerates downwards at 1 m s2.
6.4 Daily life examples of force

60. Return
6.4 Daily life examples of force