Ncert Exercises & Solutions

Question 5.5:

A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 m/s. How long does the body take to stop?

Retarding force, F = –50 N

Mass of the body, m = 20 kg

Initial velocity of the body, u = 15 m/s

Final velocity of the body, v = 0

Using Newton’s second law of motion, the acceleration (a) produced in the body can be calculated as:

F = ma
–50 = 20 × a

$\Rightarrow a = \frac{- 50}{20} = - 2.5 m / s^{2}$

Using the first equation of motion, the time (t) taken by the body to come to rest can be calculated as v = u + at

$\Rightarrow t = \frac{- u}{a} = \frac{- 15}{- 2.5} = 6 s$

Question 5.6: