Question

A planet is moving in an elliptical orbit. If $T,U,EandL$ are its kinetic energy, potential energy, total energy and magnitude of angular momentum respectively, then which of the following statement is true.

• A. $T$ is conserved
• B. $U$ is always positive
• C. $E$ is always negative
• D. $L$ is conserved but the direction of vector $\stackrel{\rightharpoonup }{L}$ will continuously change

Answer 1

The correct option is C $E$ is always negative

For the planet to move in any orbit, its total mechanical energy must be bound. Thus $E$ must be negative always.

$T$ will not be conserved because $v$ varies as $r$ changes.

potential energy can never be positive and $L$ is always conserved with direction of $\overrightarrow{L}$ always in one direction.