Question

a particle moving with an initial velocity of 5 m/s is subjected to uniform acceleratation of -2.5m/s^2 . find the displacement in the next 4 seconds.

Answer 1

First of all, since we want to find displacement. Find the time i whcih the particle stops.

u = 5 m/s 
v = 0
a = - 2.5 m/s^2
t = ??

v = u + at
0 = 5 - 2.5t
2.5t = 5 
t = 2 s

So, the particle stops after 2 s.

Distance travelled within these two seconds -
s = 5*2 + 1/2*(-2.5)*(2^2)
s = 10 - 5 = 5 m

Now, since the object is travelling for more 2 sec and we have 
u = 0
v = ??
t = 2 
a = 2.5 (in opposite direction)

s = ut + 1/2at^2
s = 0 + 5
s = 5 m in opposite direction.

So, displacement = 5 - 5 = 0

Answer 2

u=5
a=-2.5
t=4
s=ut+1/2at^2
s=5*4+1/2(-2.5)4*4
s=20-20=0
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