The half-life of a radioactive nucleus is 5 years. The fraction of the original sample that would decay in 15 years is:
A block of certain mass is placed on a rough floor. The coefficients of static and kinetic friction between the block and the floor are 0.4 and 0.25 respectively. A constant horizontal force \( F = 20 \, \text{N} \) acts on it so that the velocity of the block varies with time according to the following graph. The mass of the block is nearly (Take \( g = 10 \, \text{m/s}^2 \)): 
The amount of time taken for half of a particular sample to react is known as Half-life.
We can describe exponential decay by any of the three formulas

