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ASSIGNMENT 1.6: Show that the equations (a) vf = vi + at (b) S = vit + ½ at2 are dimensionally correct.

Solution

As dimensions do not depend on the magnitude of the quantity, therefore both vi and vf have same dimensions = [ms-1] = [LT-1]                                                     ….                                           (1)

Dimensions of acceleration, a = ms-2 = [LT-2]        …                             (2)

Dimensions of distance, S = [L]                                                                  (3)

Dimensions of time, t = [T]                                                                          (4)

Now put the values of the dimensions of velocity, acceleration and time in equation (a).

[LT-1] =  [LT-1] + [LT-2][T] = [LT-1] + [LT-2T] = [LT-1] + [LT-1]

⇒[LT-1] = 2[LT-1].

Since 2 is dimensionless number, therefore, [LT-1] = [LT-1]. Therefore, the equation is dimensionally correct.

  • Put the respective dimensions from the above four equations, we have [L] = [LT-1][T] + ½ [LT-2][T2] ⇒ [L] = [LT-1T] + ½ [LT-2T2] = [LT-1+1] + ½ [LT-2+2] = [LT0] + ½ [LT0] = 3/2 [L]. And since 3/2 is dimensionless number, therefore, the equation becomes [L] = [L]. Thus the equation is dimensionally correct.
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