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

thilina-g | College Teacher | (Level 1) Educator

Posted on

The force exerts a deceleration on the body and it can be found by,

`F = ma`

`50 = 20 xx a`

`a = 2.5 ms^(-2)`

The deceleration is `2.5 ms^(-2).`

To find the time taken to stop the mass, we can use, v = u+at

`0 = 15 -2.5t`

`t = 15/2.5`

`t = 6 s.`

Therefore, it would take 6 seconds to stop the mass.

sanjeev5757 | High School Teacher | (Level 1) Honors

Posted on

Given that: m=20kg., mass of body

u=15m/s, initial velocity

F=50N,force of retardation

the deceleration act on the body can be found by Newton's second law as:

F=ma

=>50=20.a

=>a=5/2 m/s2 (deceleration)

from third equation of motion,under retardation :v2=u2-2as

=>0=225-2.5/2.s

=>s=225/5

=>s=45m.

body will move 45m before being to stop.

from first equation of motion: v=u-at(a is negative,for retardation)

0=15-5/2t

t=15*2/5

t=6sec.

body will take 6sec.to come to stop.

Hope,you will be satisfied.