A constant retarding force of \[50N\] applied to a body of mass \[20kg\] moving initially with a speed of $15ms^{-1}$. How long does the body take to stop?

Hint: Using Newton’s laws, $F=ma\;;a=\dfrac{v}{t}$. Substitute the given values. Solve for t. A retarding force causes the acceleration of an object to be negative. In $F = ma$ , where F is the resultant force, the force acts against the direction of the object's current velocity is the retarding force.
Formula used: $F=ma\ $ and $F=ma\;,a=\dfrac{v}{t}$.

Complete step-by-step answer:
A retarding force causes the acceleration of an object to be negative. In $F = ma$ , where F is the resultant force, the force acts against the direction of the object's current velocity is the retarding force.

We know from Newton’s laws, $F=ma\ $ and$F=ma\;,a=\dfrac{v}{t}$.
Given that, $F=50N$ , $m=20kg$

Then $a=\dfrac{F}{m}=\dfrac{50}{20}=2.5$
We also know that $a=\dfrac{v}{t}$.
Given that, $v=15ms^{-1}$

Using the given $t=\dfrac{v}{a}= \dfrac{15}{2.5}=6 sec.$.
Thus it takes \[6\sec \] for a constant retarding force of \[50N\] to stop a body of mass \[20kg\]moving initially with a speed of $15ms^{-1}$.

Note:
A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. The forces are mainly of two types: contact force and non-contact force. Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is abbreviated by an "N”.Forces that resist relative motion (like air resistance or friction) are called 'retarding forces'. Sometimes they are also just called 'resisting forces', though. Whether the forces actually 'resist motion' depends on who's looking at a particular situation.

One Newton is the amount of force required to give a 1-kg mass an acceleration of $1 m/s^{2}$. Thus, the following unit equivalency can be stated: $1N=\dfrac{1kg\;m}{s^{2}}$