Gravitation - Result Question 57
60. A planet is moving in an elliptical orbit around the sun. If T, V, E and L stand respectively for its kinetic energy, gravitational potential energy, total energy and magnitude of angular momentum about the centre of force, which of the following is correct?
[1990]
(a) $T$ is conserved
(b) $V$ is always positive
(c) E is always negative
(d) L is conserved but direction of vector $L$ changes continuously
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Solution:
- (c) In a circular or elliptical orbital motion, torque is always acting parallel to displacement or velocity. So, angular momentum is conserved. In attractive field, potential energy is negative. Kinetic energy changes as velocity increase when distance is less. So, option (c) is correct.
