A planet is orbiting a star. Suddenly the star's gravitational force vanishes completely. Which statement(s) about the subsequent motion are correct?

Question

G1 Physics · Accepted Answer

Step 1: Immediately after gravity vanishes, no force acts on the planet, so by Newton's first law it continues with constant velocity along the tangent to its former orbit; A is true. Step 2: With zero force, torque about the star is also zero, so angular momentum L about the star remains constant; D is true. Step 3: Areal velocity is dA/dt=L/(2m), so it remains constant even during the subsequent straight-line motion; C is true and B is false. Final: A, C and D.