Question

In a projectile motion the velocity:
A) is always perpendicular to the acceleration.
B) is never perpendicular to the acceleration.
C) is perpendicular to the acceleration for one instant only.
D) is perpendicular to the acceleration for two instants.

Answer 1

In order to find the correct answer of the given question, we need to know about the projectile motion in detail. Also, we need to know the relation between velocity and acceleration and how they both vary to each other.

Complete solution:
Projectile motion is the motion of a body when the body is projected or thrown into the air, when only the acceleration due to gravity acts on the body. Since only acceleration due to gravity is acting on the body which is in vertical direction, the velocity of the body in the horizontal direction is constant. At each point in the projectile motion, the velocity will have two components, i.e. horizontal and vertical components.
The velocity will be in horizontal direction and the corresponding acceleration will be along the vertical downward direction. In this case only at the highest point the velocity will be perpendicular to the acceleration.
Therefore, we can say that the velocity is perpendicular to the acceleration for one instant only.

Hence, option (C), i.e. is perpendicular to the acceleration for one instant only.

Note: A projectile is an object which is projected or dropped from a height and continues to be in its motion due to its own inertia and is only influenced by the downward force of gravity which is actually the acceleration due to gravity. If other forces were acting on the body then the object would not be said to be a projectile.