QUESTION #123

# If Newton's law of motion, which states that any object in motion will remain in motion, is true, then why is it that as a comet gets close to the sun and it melts, and a tail forms, and why wouldn't it be just a big ball of melted stuff?

Asked by: Nakul Jeirath

### Answer

Newton's First Law of Motion actually states that a body in motion will remain in motion UNLESS ACTED UPON BY AN OUTSIDE FORCE. In the case of a comet, there are a number of outside forces acting upon it.

The primary force on a comet, of course, is gravitational as it orbits the Sun. That force, however, acts more or less uniformly on all parts of the comet (ignoring tidal forces). Other forces, however, include the Solar Wind (a stream of subatomic particles emitted by the Sun) and sunlight itself. While these forces are small and have little effect on the large mass of the comet, they are large enough to influence the smaller mass of its tenuous tail. The small particles of dust and gas that form as the comet's material is heated near the sun are 'blown' outward, away from the sun and the comet's main body, because their small mass is more easily accelerated by the small forces mentioned above.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor