育教係華頁四第張六第 A+AT+CE2 WAH KIU YAT PO
8
770英文中學會考試題預習專欄
堅道英文書院主編
物理科
PHYSICS (3)
MOTION IN STRAIGHT LINE-
MOTION Motion is the act of changing positLON.
SPEED AND VELOCITY: Speed and velocity-are- measures.
of the rate of motion of a moving body .-
Speed is a scalar quantity and indicates only the magnitude of the rate of motion.
Speed-
distance moved
time taken
(length/time)
Velocity is defined as the time rate of displace- ment, or defined as the rate of change of distance with
ime in a specified direction.
The acceleration is. not uniform. It is decreasing.
Velocity-
displacement time
(length/time)
Uniform velocity If a body traverses equal distances in equal time intervals, it is having a uni forn velocity.
ACCELERATION Acceleration ië defined as the time rate of change of velocity. The magnitude and
direction of velocity may change, or both quantities. may change together.
If only the speed changes, i.e, it the direction,
is unchanged t
三期星6日九十月—十年九六九一曆公年八十五國民華中
A car is moving
with uniform acceleration.
2gs
When a body is thrown up, it is desclerating.
1g becomes negativa in this case,
Example 1
A body falls freely and reaches ground after Bec. What is the velocity just before it strikes the ground? How far has it fallen (neglect air friction) in ft.?
Steps: (1) Recalling the useful equation involving
all given and known datai-
u + at
$ut + tat
Substitute numerical values
3) Solve algebraically
(4) Statement of answers with units attached
Solution
Since w
where u
+at
The initial velocity which is zero..
a - acceleration due to gravity, 32 ft/sec/sac
t
time in sec.
Vel. at the end of 3 sec is
32 £3
96 ft/seo
The distance fravelled is
where u is the initial valsoity
* is the time taken
If the direction change only
form
curvilinear motion, it is
Which is a circular motion and is
exclusive in our syllabus,
Displacement-time graph
The slope of the graph represents the velocity
of a particle moving in straight line. The directio
in which it moves is constant. The sense and magnitude of the displacement and velocity are shown on the graph.
A stone is tarown vertically upwards. After being released it travels with a uniform deceleration (from A to 0). At 0, its climax, it is under frea falling action. It drops with uniform agcòleration and returns to ground at B..
The total displacement is zerone area above the time axis is positive and that below it is magative)
TRANSLATORY MOTION
A. Uniform velocity.
The simplest conceivable motion is tha uniform velocity with acceleration equals
Ans
Example 2
10x 9
x32
144 ft.
The distance travelled is 144 ft. and the velocity then is 96 ft/seg
A train starts from rest from a station X and travels with uniform acceleration of 50 cm per sec for 20 sec. It then travels at uniform velocity for another 30 sec, and brakes are then applied so that a uniform retardation is obtained and the train comes to rest in a further 10 sec. Draw velocity-ti time graph of the motion. Using the graph, calculater. (1) the maximum velocity reached; (ii) the retardation; (iii) the total distance trav
car is at reat
(3)
A car travels in uniform velocity
The slope of the tangents along the graph decreases, indicating that the car is slowing down.
Svt
Uniformly accelerated linear motion,
This type of motion is characterized by the fact that the acceleration is constant both in magnitude and direction.
Thie motion is also necessarily a straight line (motion.
utat
which means the final velocity after an interval
of time (t) is equal to the initial velocity. plug the gain in velocity (at),
Since average velocity
and
VEL. { [m/sec)____
Type of motion Type of jacceleration
Distance covered
The
acceleration
TIME (SEC),
positive
120x1000
= 10,000 cm
Uniform vel.
281.
1000 = 30
30.000cm
200 osvered is
déceleration
negative
x 1000 x 10 im.5.000CM
+ 30,000 + 5,000)*
45,000 cm,
450 metras
The maximum velocity
retardation in reached is 10n per sec
= 100 cm/sec/sea
car travels with increasing speed
Velocity-time graph
A car starts at rest and travels in uniform velocity. It is alowed down and comes to rest. at A
(1) The slope of the graph represents the
acceleration.
The area between the grapn and the time axia represents the distance covered,
car is moving with constant velocity
zero acceleration,
·S
ut
at
another useful formula to rememoer is.
x t
2aS
285.
FREE FALL
U)/2F
A body or mass m accelerating under one and.. only one force, its weight the pull of the Earth. on it, is said to be in a state of free fall.fe The acceleration is designated as the accelera of gravity and is usually represented by a symbol "g" The value of 'g' varies according to latitudes and altitudes, and is approximately 32 ft/sec/sed, 980 cm/seo/sec,- or 9.8 m/sec/sec.
Under free-falling, the three equations or motion
becomes
Exercise
A motorist travels a distance of 10 miles in 20: minutes. During the trip he accelerates his car uniformly from 30 mph to 60 mph in 10 sec. (a) What is the average speed of the car for the entire trip in mph? (b) Is the speed cons
tant for the 10 miles? (0) What is the acceleration of the car in the 10 sec. inter (d) How far does the car go in the 10 sec. interval?:
A pebble is drepped inte a well. If the splash: is heard three seconds after the pebble ia released, how far is the water level below the the point of release?
A block sliding down a frictionless plane travels 10 on from rest in the firet second, 30 addi- tional ca in the second second, 50 additional om in the third second, 70 additional ca in the fourth second, and 90 additional cm in the fifth second. What is the magnitude of its acclerafion?.
A meter bicycle accelerates uniformly from rest at 5.5 ft per 880 per seo, then travels with uniform velocity, and finally retards uniformly te rest at 22 ft per ses per sec. If the ene mile journey takes 2 min. 5 sec, une a velocity- time graph te determine the maximum speed.
Solutions of Broroias 2 and 3 will be given on next issue.
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