There is a force acting on a rock moving in a straight line in outer space at constant speed? Give reasons. What happens if this rock comes near a planet?
Answers
There is no force acting on the object since it is moving in the outer space where there is no gravitational force. But according to the question, there may be some or the other gravitational force which is acting on the object and making it move in a straight line at constant speed. When it comes near a planet it collides with the planet due to the gravitational force of the planet,causing explosion.
The obvious force acting on the rock is gravity, which follows an inverse-square law between all objects everywhere (you can dismiss anything small or far away as negligible). There is also photon pressure - light impacting on the rock, possibly pressure from the solar wind (atomic particles), and possibly magnetic drag if the rock is magnetic.
In the solar system, gravity from the sun is the major force, or near a planet, then from the planet. Jupiter’s gravity affects everything, more-or-less, though the effect is small at large distances or small timescales.
If the rock comes near a planet, it will follow an orbital trajectory around the planet’s centre of mass. That has the shape of a hyperbola for an object with more than escape velocity (e.g. it came from somewhere else, rather than being launched from the planet by a rocket). If the hyperbola intercepts the planet’s surface, the rock will crash and make a crater. If it does not, it will accelerate towards the planet and zoom past at high speed, then slow down again the other side, gradually reaching its original speed but in another direction.