I keep getting “Projectile trajectory” questions wrong in Motion in a Plane. What's the trap?
assuming the speed is constant because the path looks smooth.
The usual trap: assuming the speed is constant because the path looks smooth. How to avoid it: at the top only v_y is zero; v_x is still u cos(theta). Go back to the definition — Eliminating time from x = u cos(theta)t and y = u sin(theta)t - gt^2/2 gives a quadratic y(x), so the ideal projectile path is a parabola.
Key point
Projectile trajectory
Common mistake
assuming the speed is constant because the path looks smooth.
Memory tip
at the top only v_y is zero; v_x is still u cos(theta).
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