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Physics Form 4 Chapter 2 Force and Motion I

Displacement and velocity
3. (a) Displacement by an object is the distance travelled by the object in a
specific direction. N
(b) It is the distance travelled by the object from its initial position to its W E Delima
final position in a straight line.
(c) For the car travelling from Delima to Mengkibol, the displacement is S 45°
determined by measuring the length of the straight line connecting 800 m
Delima to Mengkibol in the direction of Mengkibol from Delima.
(d) Hence from Figure 2.4, its displacement is given by the following, Mengkibol 800 m
Magnitude of the displacement Figure 2.4
2
=  800 + 800 2
= 1 131 m
Its direction is south-west of Delima (or S45°W of Delima)
(e) Hence, displacement is a vector quantity. Chapter
4. (a) Velocity is defined as the rate of change of displacement.
(b) In the case of the car travelling from Delima to Mengkibol, the average velocity, v, is given by:
Displacement,s 2
v = Time taken, t

1 131 m
= 120 s = 9.4 m s –1
(c) Velocity is a vector quantity.

EXAMPLE 2.1 EXAMPLE 2.2

A cow walked along a curved path from P to Q, An athlete runs from the P Q
which is 70 m away from P. Q lies to the south-west starting point O to P which is 100 M
of P. The distance travelled by the cow is 240 m 100 m north of O. After that he
and the time taken is 160 s. runs to point Q which is 100 m 100 M N
U east of P. The total time taken
is 25 seconds. O

What is the (a) distance, (b) Figure 2.6
P
average speed, (c) displacement, and (d) average
velocity traveled by the athlete?
Q
Figure 2.5 Solution
Calculate the (a) average speed, and (a) Total distance traveled
(b) average velocity = 100 + 100 = 200 m
of the cow moving from P to Q.
(b) Average speed
Solution Total distance 200
–1
Total distance travelled = 240 m = Total time = 25 = 8 m s
Displacement = 70 m (c) Displacement O
Time taken = 160 s = 100 + 100 2
2
Total distance travelled
(a) Average speed = = 100 2 m
Time taken
240 m (d) Average velocity
= = 1.5 m s –1
160 s
Displacement 70 m = Displacement = 100 2 = 4 2 m s –1
(b) Average velocity = = Total time 25
Time taken 160 s
o
–1
= 0.44 m s in the south- in the direction N45 E from O.
west direction of P
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