1.8.1 Particle Moving Vertically Downward
When a particle is free falling under the attraction of the earth, the particle will be
subjected to an acceleration directed towards the centre of the earth.
All equation involving ‘a’ will be replaced by a= + g (= 9.81 m/s2). If it is a free fall,
the initial velocity is zero, i.e., v0 = 0. If there is some initial velocity, then the velocity = v0.
1.8.2 Particle Moving Vertically Upwards
In this case, we have to take, a= – g (-9.81 m/ s2) in ail the governing equations involving
Example 1.1. A body is moving with a velocity of 4 m/s. After 4 seconds the velocity of the body becomes 10 m/s. Determine the uniform acceleration of the body
Solution. v =v0 +at
10 =4+a x4
A = 10-4 /4 = 6/4 = 1.5 m /s2.
Example 1.2. A motorcycle is moving with a velocity of 30 m/s. The motorcycle is brought to rest by applying brake in 5 seconds. Find (i) the retardation ; (ii) distance travelled by the motorcycle after applying brake
Solution. v0 =30 m/s
v = 0
t = 5 s
v = v0 –at
0 = 30 –a x5
a = 30 /5 = 6 m/s2 retardation.
We know S = S0 +v0 t – 1/2 at2
S = 0 +30 x 5 -1/2 x 6 x(5)2
= 150 – 75 = 75m.