Question

A body of mass 2 kg moving on a horizontal surface with an initial velocity of 4 m/s comes to rest in 2 seconds. If one wants to keep the body moving on the same surface with the velocity of 4 m/s, the force required is
(A) Zero
(B) 4 N
(C) 8 N
(D) 16 N

Answer 1

Hint: If acceleration occurs in the opposite direction, that situation is called deceleration, which is known as negative acceleration or retardation. It acts in the opposite direction of velocity and it is responsible for reducing the speed.
Deceleration = (Final Velocity – Initial Velocity)/Time
$\left[ {Deceleration = \dfrac{{v - u}}{t}} \right]\dfrac{m}{{{s^2}}}$

Complete step by step explanation:
Initial velocity of body, u = 4 m/s
Final velocity of body, v = 0
Taken time, t = 2 seconds
Mass of the body, m = 2 Kg
Acceleration, a =?
According the 1st equation of linear motion
$v = u + at$
Put the following values in this equation ……..
$0 = 4 + a(2)$
$2a = - 4$
$\left[ {a = - 2} \right]\dfrac{m}{{{s^2}}}$
This negative acceleration is known as deceleration.
Let’s discuss the required force,
Force = mass x acceleration
$F = ma$
$F = 2 \times 2$
$\left[ {F = 4} \right]$ Newton
Hence, they can move the body using 4N force.

Hence, the correct option is B.

Note: First law of motion explains that Everybody continues to be in a state of rest or uniform motion in a straight line, except insofar as it may be compelled by force to change that state. Newton’s first law of motion defines the inertia of state with respect to any point.
Inertia of rest - Changes in body position due to its inertia of rest. Example as: When a branch of a fruit tree is shaken, the fruits fall down because the branch comes in motion and the fruit tends to remain at rest. Hence, they get detached.
Inertia of motion -Changes in body position due to its inertia of motion. Example as: When a horse at full gallop stops abruptly, the rider on it falls forward because of inertia of motion of the upper part of the rider’s body.
Inertia of Direction - Changes in body position due to its inertia of direction. Example as: Use of an umbrella to protect us from rain is based on the property of inertia of direction because the rain drops cannot change their direction of motion.