### Q.5:- A constant retarding force of 50 N is applied to a body of mass 20 kg moving initially with a speed of 15 ms^{–1}. How long does the body take to stop?

**Answer:-**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*

∴ a = -50/20 = -2.5 ms^{-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*

∴ t = -u / a = -15 / -2.5 = 6 s