Maths-
General
Easy

Question

The displacement of a particle in time ‘t’ is given by S = t3 – t2 – 8t – 18. The acceleration of the particle when its velocity vanishes is

  1. 10    
  2. 10 units /sec2    
  3. 5 units/sec2    
  4. 20 units/sec2    

hintHint:

We are given the equation of displacement of a particle. We have to find the value of its acceleration at the point where velocity vanishes i.e. becomes zero. Acceleration is second order derivative of the equation and velocity is first order derivative of displacement.

The correct answer is: 10 units /sec2


    The given expression is s = t3 - t2 - 8t - 18.
    To get the velocity we will differentiate the above equation w.r.t "t" . And to find the point at which it's vanishing, we will equate the derivate with zero.
    s space equals space t cubed space minus space t squared space minus 8 t space minus 18
fraction numerator d s over denominator d t end fraction equals 3 t squared space minus 2 t space minus space 8
T h i s space i s space t h e space e q u a t i o n space o f space v e l o c i t y.
space
    W e space w i l l space e q u a t e space t h e space e q u a t i o n space w i t h space z e r o
0 space equals space 3 t squared space minus 2 t space minus 8
0 space equals space 3 t squared space minus space 6 t space plus 4 t space minus space 8 space space space space space space... left parenthesis f a c t o r i z a t i o n right parenthesis
0 space equals space 3 t left parenthesis t space minus space 2 right parenthesis space plus space 4 left parenthesis t space space minus space 2 right parenthesis
R e a r r a n g i n g space t h e space a b o v e space e q u a t i o n
left parenthesis 3 t space plus space 4 right parenthesis left parenthesis t space minus 2 right parenthesis space equals space 0
S o comma
space 3 t plus space 4 space equals space 0 space o r space t space minus space 2 space equals space 0
t space not equal to negative 4 over 3 space a s space t i m e space c a n n o t space b e space n e g a t i v e
t space equals space 2
    The point at which velocity is vanishing is t = 2
    Now, we will find the equation for acceleration by taking the second order derivative of displacement.
    fraction numerator d squared s over denominator d t squared end fraction equals space 6 t space minus space 2
L e t space u s space d e n o t e space a c c e l e r a t i o n space b y space " a "
a space equals space 6 t space minus space 2
    We will substitute the value of t = 2 to find the acceleration at required point.
    a = 6(2) - 2
    = 12 - 2
    = 10 units/sec2
    So, the acceleration at the point where velocity vanishes is 10units/sec2

    For such questions, we should know relation between displacement, velocity and time. We should also know how to find first and second order derivate.

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