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Physics

If an object moves starting from rest then v o = 0 , so the velocity of that object in
accelerated motion is defined as v = a∙t .
In accelerated motion the object’s acceleration is positive because the final velocity is
greater than the initial velocity: v > v o

Deceleration is a type of non-uniform motion (where the speed is not constant) when the
velocity of an object decreases at the same rate in equal time intervals.
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Consider this example: a driver moves with 60 km/h velocity, then stops at the traffic lights.
In this case the velocity of vehicle decreases, so the vehicle decelerates. In decelerated
motion, acceleration is negative because the final velocity is less than the initial velocity:
v < v o

Therefore, the equation of motion for the deceleration of an object is:
v = v o – a∙t

Worked example

What is the speed of a train that moves off from a station after 0.5 minutes, if its
acceleration is 0.9 m/s ?
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Given: SI: Solution:

t = 0.5 min 0.5 ∙ 60s = 30s v = v o + a∙t
m
a = 0.9 / s 2
m
v o = 0 / s
M m 2 ∙ 30s
v = 0 + 0.9 / s
c
Find: v
m
Answer: v = 27 / s
INV Explore the motion of a ball rolling down an inclined plane and across a flat surface.

1. How does the ball move at different stages?

2. Does the angle of inclination of the sloping plane affect the motion of the ball?

EXT Use rubber bands to listen to the sound of the ball as it rolls down. Can you use 5 rubber
bands and space them apart in such a way that you can hear a noise at equal time intervals?

OVER TO YOU!

1. Explain the phrase ‘the rate of change of velocity is constant’. Draw a labelled diagram.
2. Why is acceleration a vector quantity? Give examples to support your answer.
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