A planet moves around the sun in an elliptical orbit with the sun at one of its foci. The physical quantity associated with the motion of the planet that remains constant with time is

• A

  velocity
• B

  centripetal force
• C

  linear momentum
• Correct answer
  D

  angular momentum

Solution

A planet revolves around the sun, is an elliptical orbit under the effect of gravitational pull on the planet. So, torque, $C=r \times F=r F \sin 180^{\circ} n=0$ $$ \text{As } \mathrm{C}=\frac{\mathrm{dL}}{d t} ; \mathrm{soL}=\mathrm{a} \text { constant } $$ $\Rightarrow$ Angular momentum is constant.

Hence, the correct answer is option: 'D'.