Question

Assertion: In the motion of projectile the horizontal component of velocity remains constant
Reason: The force on the projectile is gravitational force which acts only in vertically downward direction
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Assertion is incorrect but Reason is correct

Answer 1

Hint
In projectile motion neglecting air resistance the only force is acting is the gravitational force and this acts in the downward direction.

Complete step by step answer
Force is necessary to change the state of a body, either to bring it in motion from rest, to bring it to rest while it was moving already or to change the velocity of the motion of the particle.
If there is no external force on the particle then its state will not change according to the Law of Inertia. So if it is at rest then it will remain in rest OR if it was moving with some velocity then it will continue to move forever with that constant velocity.
In case of projectile motion, neglecting air resistance the only force acting is the gravitational force and this force is acting in downward direction it means the vertical component of the particle velocity changes continuously. As shown in the figure

                             

As there is no force acting in the horizontal direction then horizontal components of velocity will not change. Hence, both assertion and reason are correct and reason is the correct explanation of the assertion.
Therefore, (A) option is correct.

Note
Gravitational force is force due to gravity on its own weight of the body and this acting in the downward direction always its value is equal to mg. where m is mass of the body and g is acceleration due to gravity.