Question

Which of the following statements are not valid for a projectile? Take the upward direction as positive.

(a) The projectile has the same x velocity at any point on its path.

(b) The acceleration of the projectile is positive and decreasing when the projectile is moving upward, zero at the top, and increasingly negative as the projectile descends.

(c) The acceleration of the projectile is a constant negative value.

(d) The y component of the velocity of the projectile is zero at the highest point of the projectile’s path.

(e) The velocity at the highest point is zero.

Short answer

The statements that are not valid for a projectile are as follows:

(b) The acceleration of the projectile is positive and decreasing when the projectile is moving upwards, zero at the top, and increasingly negative as the projectile descends.

(e) The velocity at the highest point is zero.

Step-by-step solution

Step 1. Object velocity in projectile motion

The projectile includes horizontal velocity, which is parallel to the tangential plane of the surface.

The horizontal velocity can accelerate towards the center of the earth, and this acceleration is in the direction that is perpendicular to the horizontal velocity.

Step 2. Explanations for the given options

In the projectile motion, the horizontal component of the velocity remains the same during the entire flight.

Hence, option (a) is a valid statement.

The particle's acceleration during the projectile motion remains constant for the entire flight. It is never zero.

Therefore, option (b) is not a valid statement, and option (c) is valid.

At the highest point of the projectile motion, the vertical component of the velocity is zero, and the resultant of the magnitude of the velocities is lowest at this point.

Hence, option (d) is a valid statement, and option (e) is not a valid statement.

Therefore, options (b) and (e) are invalid statements.