Question

An object of mass 5kg is sliding with a constant velocity of $4m{s^{ - 1}}$ on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
(A) $32N$
(B) $0N$
(C) $2N$
(D) $8N$

Answer 1

To solve this question we have to find the net external force acting on the body. Then we need to use Newton’s second law of motion to find the force required.
Formula used: The formula used to solve this question is given by
$\Rightarrow F = ma$, where $F$ is the external force acting on a body of mass $m$ and having the acceleration $a$.

Complete step by step solution:
According to the question, the object is placed on a frictionless horizontal table. This means that the table cannot offer friction to the object. Also, there is no external force acting on the body. So, the net external force on the body is equal to zero. From the Newton’s second law of motion we have
$\Rightarrow F = ma$
According to the question, $m = 5kg$ and $F = 0$. So we have
$\Rightarrow 0 = 5a$
Or $5a = 0$
From this we get
$\Rightarrow a = 0$
So, the acceleration of the body is equal to zero.
From the first equation of motion, we have
$\Rightarrow v = u + at$
Substituting $a = 0$, we get
$\Rightarrow v = u + 0t$
$\Rightarrow v = u$
According to the question $u = 4m{s^{ - 1}}$. Therefore
$\Rightarrow v = 4m{s^{ - 1}}$
So, the velocity at any time $t$ is equal to the initial velocity of $4m{s^{ - 1}}$.
Since the velocity of the object is already constant, we do not need to apply any force to keep it constant. Thus, the force required to keep the object moving with the same velocity is $4m{s^{ - 1}}$.

Hence, the correct answer is option (B).

Note:
In this question, we have been told that the object is moving with a constant velocity. So, we do not have to work out for checking the external forces on the blocks to determine its acceleration. A constant velocity implies zero acceleration. So we can directly choose option B as the correct answer.