# Let's think crack u'r brain with this qn.

1. Feb 19, 2005

### gunblaze

ok...

note: Earth's pull of gravity starts 2.7 billion light years away.<Got this info from somewhere else>(may not be true, u are free to use u'r own)

A comet which is floating freely in space, suddenly, got attracted by earth's gravity and started accelerating at 10m/s-2 towards earth...What would be the speed juz before it touches earth's soil?

2. Feb 19, 2005

### Janus

Staff Emeritus
First off, the Earth's gravity pull does not "start" at any given distance from the Earth, it reaches to inifinty.

Secondly it does not maintain full surface strength at all distances, it falls off in strength by the square of the distance you are from the center of the Earth. IOW, if you moved twice as far from the center of the Earth as you are now sitting on the surface, the acceleration due to gravity would drop to 1/4 that of the surface, or to 2.5/m/sec^2.

At 2.7 billion lightyears away, said comet would start to accelerate at about 6*10^-42 meters/sec^2, our in other words, you could watch it for a very long time with out even noticing that it was moving.

As it slowly got closer to the Earth, it acceleration would increase. By the tim it struck the Earth it would be moving at about 11 km/sec. (Assuming it started at reast with respect to the Earth.)