Kinematics sign convention – gravity during upward throw When a body is thrown upward, should gravitational acceleration be taken as zero, positive g, or negative g in the vertical equation of motion?

Introduction / Context:
Projectile and vertical throw problems hinge on consistent sign conventions. Gravity does not switch off during ascent; it constantly accelerates the body downward. The statement claims g is taken as zero on upward motion—this must be evaluated.

Given Data / Assumptions:

• Vertical 1-D motion under uniform gravity g.
• Choose upward as the positive axis (common convention).
• Neglect air resistance for clarity.

Concept / Approach:
With upward positive, gravitational acceleration is a constant negative value: a = −g. Kinematic equations such as v = u + a t and s = u t + (1/2) a t^2 hold at all times, including ascent, apex, and descent. Setting g = 0 would eliminate the deceleration responsible for the body slowing down and turning.

Step-by-Step Solution:

Verification / Alternative check:
Energy method: kinetic energy decreases on the way up because gravitational potential energy increases by m g h. If g were zero, there would be no energy exchange to halt the upward motion—contradicting observation.

Why Other Options Are Wrong:

• Correct near the top / in vacuum: gravity acts everywhere near Earth; vacuum removes drag only.
• g = 0 while going up: physically incorrect; gravity does not switch off.

Common Pitfalls:
Confusing sign convention with magnitude. You always include g; the sign depends on chosen positive direction.

Final Answer:
Incorrect – use g with downward sign (−g if upward is positive)