S3 Mechanics

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S3 Mechanics

• Speed and Velocity

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Mechanics

1. Revision of Distance and Displacement
2. Definition of Speed
3. Definition of Velocity Speed and Velocity
4. Comparing Speed and Velocity

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1. Revision of Distance and Displacement
Time
, Distance
position change displacement
A car starts at A and travels 10 metres to B. B
is 6 metres due east of A. (P.60 IV,1) i) The
total distance travelled from A to B is 10
metres (10 metres is the length of the
path) ii) The displacement travelled from A to B
is 6 metres due east (6 metres is the
shortest distance or the length of a straight
line connecting A and B) iii) The position change
from A to B is 6 metres due east (Position
change displacement )
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New Concepts
and Velocity
Speed
We will explore some activities on the School
Sports Day and see how they are related to
physics.
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An event on Sports Day
Runner Peter Joel
Event 100 m 100 m
Time recorded 11.5 seconds 12.2 seconds
100m
Which runner should win the race?
Peter
Why?
Method 1 Consider the time taken for a certain
distance travelled
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Indoor Rowing Competition(????)
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Each pulling causes the machine to travel a
distance
A meter on the machine records the total distance
travelled
Teams are requested to row for 2 hours, 3 hours
or more.
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Two teams competed in a final for 2 hours
Team A recorded a distance travelled of 4500 m.
Team B recorded a distance travelled of 5600 m.
Can you tell me which team should win?
Why?
Method 2 Consider the distance travelled within
a certain time interval
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Runners of different times and distances
On a sports day
Runner Victor Magus
Event 800 m 1500 m
Time recorded 134 seconds 247 seconds
Can you decide which one runs faster with the
previous methods?
Who runs faster?
Why?
800 m /134 seconds 5.97 m per second (Distance
travelled in one second is 5.97 metres)
Victor
1500 m /247 seconds 6.07 m per second Distance
travelled in one second is 6.07 metres
Magus
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2. Definition of Speed

• The distance travelled by Victor is 5.97 metres
  in one second
• The distance travelled in one second by Victor is
  5.97 metres
• The speed of Victor is 5.97 metres per second
• Speed distance travelled in one second, OR
• Speed rate of change of distance travelled
• Speed is a scalar.

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2. Definition of Speed

• If the speed is not the same over the total time
  interval then

Example A car travelled 10 m in 2 s. What is
its average speed? Total distance travelled 10
m Total time interval 2 s
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Speed

• Practice
• Text Book
• Answer parts (i), (ii) and (iii) of the question

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Speed
The following ticker-tape is divided into 4
portions. Time interval of each portion is 0.1 s.
The distance travelled in each portion is
shown. Calculate i) the average speed of each
portion ii) The overall average speed
i) 0.5 m/s, 0.6 m/s
0.4 m/s 0.2 m/s ii)
(0.050.060.040.02)/(0.10.10.10.1) 0.425
m/s
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3. Definitions of Velocity
total distance travelled
_____________________
Average speed
total time interval
total displacement travelled
________________________
Average velocity
total time interval
Velocity is a vector as displacement is a vector.
Velocity is the rate of change of
displacement.
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4. Comparing Speed and Velocity
100 m
100 m
100 m
100 m
8.70 ms-1
8.20 ms-1
8.70 ms-1
8.20 ms-1
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Speed and Velocity

• Practice
• Answer part (iv) of the question

Average velocity (204030)m/45 s 2
ms-1
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Orienteering (????)

• Orienteering (????)
• Paths are not fixed and are chosen by the
  participants.
• (With wisdom and considering Physical
  Conditions)
• Worksheet (Please discuss in groups)

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Conclusion of Orienteering

• Is the winner the team with the highest speed
  along the path?
• Is Average Speed or Average Velocity more
  important in Orienteering?

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4. Comparing Speed and Velocity
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What have we learnt?

• We have learnt the followings in this lesson.
• Speed is a scalar and Velocity is a vector
• vs/t (examples, practice)
• v speed, s distance, t time interval,
• v velocity, s displacement, t time interval
• Apply the concept of rate of change in exercise
  or worksheets

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End Thank you very much

• April-May 2004