1. Mechanics and kinematics
Unit Two
Mechanics and kinematics
Describing motion

2. How far did the object travel?”
In science,
Motion is defined as
The change in position for a particular time interval. You can then start describing motion with the question, “
How far did the object travel?”

3. Distance and displacement
HOW FAR?
Distance and displacement

4. How far did the object travel?
There are actually two ways to answer this question.
First is by getting the total length of the path travelled by the object.

5. Example: Ali ran 10m to the east, then 5m to the south, and another 10m to the west. So it has travelled a total of 25 meters. (DRAW)
The other way is by measuring the distance between the initial position and final position of the object.
Ali has travelled 5 meters to the south.

6. In science, the first measurement gives the distance travelled by the object while the second measurement gives its displacement.

7. Adobe file: Here are more illustrations showing the difference between distance travelled (represented by broken lines) by an object and its displacement (represented by continuous lines).

8. P.O. C Distance Displacement
Can you give one difference between distance and displacement base on the given examples?
P.O. C
Distance
Displacement
Definition
It is the actual path that the body covered
It is the distance covered in a certain direction
It is the shortest distance between final and initial points
Type of quantity
Scalar
Vector

11. 1-Calculate the distance and displacement when an object moves from (A) to (B).
2-Calculate the distance and displacement when an object moves from (A) to (A) again .

12. 1-Calculate the distance and displacement when an object moves from (A) to (C).
2-Calculate the distance and displacement when an object moves from (A) to (A) Again.

13. In which case can displacement be equal to zero?
Is it possible to get zero displacement?

14. Speed velocity and acceleration

15. Speed is the term used to describe the distance traveled in a unit of
time
The distance can be in a straight line or around a corner

16. Speed 1-

17. Speed speedometer 2-Instantaneous Speed :
the speed at any given instant in time=actual speed
speedometer

18. average velocity.
Example1
The physics teacher walks 4 meters East, 2 meters South, 4 meters West, and finally 2 meters North. The entire motion lasted for 24 seconds.
Determine the average speed.

19. Answer
The physics teacher walked a distance of 12 meters in 24 seconds; thus, her average speed was 0.50 m/s.

20. Velocity
Velocity is the term used to describe the displacement traveled in a unit of time
It describes how fast and in what direction a body is moving

21. Now the formula :
V = displacement/time

22. For motion in straight line you can use + or – to indicate the direction
V=+ 10 m/s
V= -10 m/s

23. P.O. C Speed Velocity Vector Scalar Type of quantity
DEFINATION
It is the distance traveled in a unit of time
It is the displacement traveled in a unit of time
Type of quantity
Scalar
𝒗= 𝒔 𝒕
Vector
𝐕𝐞𝐥𝐨𝐜𝐢𝐭𝐲= Displacement Time

24. Acceleration is the rate of change of velocity
Acceleration has direction and magnitude.
Thus, acceleration is a vector quantity.

26. Now the formula of accelerstion is :
𝒂= ∆𝒗 ∆𝒕
𝒂= 𝒗−𝒖 𝒕 𝒇 − 𝒕 𝒊
  𝒂: Acceleration (m/ s 2 )
(𝒗: ( 𝐟𝐢𝐧𝐚𝐥 𝐯𝐞𝐥𝐨𝐜𝐢𝐭𝐲 (m/s).
𝒖: initial velocity (m/s )
𝒕 𝒇 : time of the end of motion( Sec )
: 𝒕 𝒊 : time of the beginning of motion
 ∆v = change in velocity
( subtract, final – initial)
a = v-u

27. GRAPHS

28. Motion graphs

30. Higher the speed more the gradient
Low speed means less
slope or gradient
High speed means more
slope or gradient

31. 2. Speed or (velocity) Vs time graphs
1. On a speed or (velocity) -time graph the gradient of the line is the acceleration
2. On a speed or (velocity ) - time graph the area under the the line is equal to the distance or displacement traveled

35. Example1 In the graph below describe the motion of the object?
average velocity.
Example1
In the graph below describe the motion of the object?

36. In region A the object is moving with constant acceleration.
In region B the object is moving with constant velocity.
In region C the object is again  moving  with
 constant  acceleration, but when compared with region A: 1. the acceleration is slower because the slope is less steep 2. the acceleration is negative because the slope is downwards.
The total distance travelled by the object can be
calculated by measuring the area under the graph.

37. This area can be divided into two triangles and one rectangle.
The area of triangle A = ½ base x height
            = 0·5 x 10 x 20 = 100.
The area of triangle C= ½  x (70 - 30) x 20 = 400.
The area of rectangle B = (30 - 10) x 20 = 400.
The distance travelled is the total area = A + B + C
              = 100 + 400 + 400 = 900 m.

38. Equation Of Motion

39. What are equation of motion?
Whenever object moves and you want to describe its motion…
There are four equations of motion to find The relationships between displacement, time, velocity, and constant acceleration
They apply if the motion is in a straight line And acceleration is uniform

40. Five
Quantities Or
Variables

41. Equation of motion 𝒗=𝒖+𝒂∆𝒕 ∆𝒔=𝒖∆𝒕+ 1 2 𝒂 ∆𝒕 2 ∆𝒔= 1 2 (𝒖+𝒗)∆𝒕
Does not include s
∆𝒔=𝒖∆𝒕+ 1 2 𝒂 ∆𝒕 2
Does not include v
∆𝒔= 1 2 (𝒖+𝒗)∆𝒕
Does not include a
𝒗 2 = 𝒖 2 +2𝒂∆𝒔
Does not include t

42. Derive the first equation of motion
𝑎= 𝑣−𝑢 ∆𝑡
𝑣−𝑢=𝑎∆𝑡
𝒗=𝒖+𝒂∆𝒕

43. Example 1 A car accelerate form rest at 3ms-2
A car accelerate form rest at 3ms
Along a straight road. How far does the car travelled after 4 s??
S= 24 m

44. Example 2
A car is travelling 20 m/s along a straight road, The driver puts the brakes on for 5 s. If this causes a deceleration of 3ms What is the car’s final velocity? -2 -1

48. A racing car reaches a speed of 42 m/s
A racing car reaches a speed of 42 m/s. It then begins a uniform negative acceleration, using its parachute and braking system, and comes to rest 5.0 s later. Find the distance that the car travels during braking.
حساب المسافة اللازمة لتوقف سيارة ، بمعرفة سرعتها الابتدائية وزمن التوقف.

49. What is free fall .
Free falling is a motion under force of gravity as the only force acting on the moving object.
The acceleration on an object in free fall is called the acceleration due to gravity, or free-fall acceleration
Practically, free falling can only
take place in vacuum

50. Air Resistance
In air…
A stone falls faster than a feather
Air resistance affects stone less
In a vacuum
A stone and a feather will fall at the same speed.
When air resistance is negligible, mass has no effect on the downward motion on an object.

51. . Free Fall
The acceleration of gravity (g) for objects in free fall at the earth's surface is 9.8 m/s2.
Galileo found that all things fall at the same rate.

52. . Free Fall The rate of falling increases by 9.8 m/s every second.
Height = ½ gt2
For example:
½ (9.8 )12 = 4.9 m ½(9.8)22 = 19.6 m
½ (9.8)32 = 44.1 m
½ (9.8)42 = 78.4 m

53. . Free Fall
Acceleration due to gravity is constant during upward and downward motion.
Free-fall acceleration on Earth’s surface is
(9.8 𝑚/ 𝑠 2 ≈ 10 𝑚/ 𝑠 2 ) at all points
in the object’s motion.
When the body fall down the velocity increases by rate = 9.8 m/s for each second
g = +9.8𝑚/ 𝑠 2
When the body thrown up the velocity decreases by rate = 9.8 m/s for each second
g = -9.8𝑚/ 𝑠 2

54. . Equation of Motion for Free Fall
𝒗 =𝒖+𝒈∆𝒕
∆𝒅=𝒖∆𝒕+ 𝟏 𝟐 𝒈 ∆𝒕 𝟐
𝒗 𝟐 = 𝒖 𝟐 +𝟐𝒈∆𝒅 

55. Free Fall - example
1) A coin is dropped in a vacuum tube, Find the coin's velocity after.30 seconds.
Vf = ?
t = .30 seconds
a = g = m/s2 (falling)
Vi = 0 m/s
vf = vi +at
Vf = vi +gt
Vf = m/s2 x .30 s
Vf = -3 m/s

56. Free Fall - example
2) If a rock is dropped from a building, and it takes 18 seconds to reach the ground, how tall is the building? 56

57. Sample Problem
1. A worker drops a wrench from the top of a tower 80.0 m tall. What is the velocity when the wrench strikes the ground?
2. How far above the floor would you need to drop a pencil to have it land in 1 s?
H.B solving problem numbers 1 and 2 page 27 ٚ

58. Force There are forces all around us.
We make use of forces to do a lot of things in our everyday life.
Force is a push or a pull acting upon an object.
Whenever we push or pull an object, we exert a force.
We cannot see a force, but we can see and feel its effects.

59. The following diagrams shows activities that involve ………….forces.
These diagrams shows activities that involve Pushing forces

60. The following diagrams show activities that involve ………….. forces.
The following diagrams show activities that involve pulling forces.

61. Force Non Touch Force Touch Force Air Resistance Force
acts on an object only
by touching it.
Normal force
Air Resistance Force
acts at a distance (long-range force).
Gravitational Force Magnetic force

62. The effects of forces
The effects of a force include
(a) force changes the shape of an object
(b) force changes the position of an object
(c) force changes the speed of a moving object
(d) force changes the direction of a moving object

63. Force can change the shape of an object
Pressing, bending and stretching can change the shape of an object.
(a) Bending
(b) Stretching
(c) Pressing

64. Force changes the speed of a moving object such as:
Move it
Stop it
Increase or decrease its speed

65. force changes the position of an object
Hit a ball
move
move

66. Force changes the direction of a moving object.
A force can change the direction of an object's movement.

67. The unit of force
We measure force in units called Newton (N).
(1N resultant force acting on 1kg produces an acceleration of 1m/s2).
We can measure a force using a spring balance or Newton meter.

68. Equal forced
Two forces are equal when the two forces (magnitude) and direction are the same.

69. Balanced forces
When the forces acting on an object are balanced, they cancel each other out. The net force (RESULTANT) is zero.
Effect :
the object at is at rest [ velocity = 0]

71. Question 1
Forces of 12 N and 5 N both act at the same point, but their directions can be varied.
a) What is their greatest possible resultant?
b) What is their least possible resultant?
c) If the two forces are at right angles, find the resultant.

72. Question 2
Which one of the following effect of force on an object is not true ?
Change its shape
Change its position or direction
Change its colour
Change its speed