A planet is moving in an elliptic orbit. If `T,V,E` and `L` stand, respectively, for its kinetic energy, gravitational potential energy, total energy and angular momentum about the centre of force, then

Options

A. E is always negative
B. L is conserved but direction of vector L changes
C. U is always positive
D. T is conserved

Answer

A

Explanation

(i) For the planet, the angular momentum (L) is conserved. But direction of L remains constant. Hence (b) is wrong.
(ii) K.E. changes as the velocity changes with distance. Hence (d) is wrong.
(iii) Gravitational P.E. `= -(GMm)/(R )` is negative. (c ) is wrong.
(iv) But the total energy (E ) is always negative .
`therefore` (a) is correct.