# Can someone check my work?

1. Apr 29, 2005

### ninjagowoowoo

Q:
A NASA satellite has just observed an asteroid that is on a collision course with the Earth. The asteroid has an estimated mass, based on its size, of 4.85×10^9 kg. It is approaching the Earth on a head-on course with a velocity of 593 m/s relative to the Earth and is now 4.85×10^6 km away. With what speed will it hit the Earth's surface, neglecting friction with the atmosphere?

I found g using (mass of earth)(mass of sat)/(r^2)
of course since x = 4.85×10^6 km = 4.85×10^9 m
so g=
(5.98e24)/(4.85e9) <--since one of the r's cancels out with the mass of sat
then i used v^2 = vo^2 + 2ax
using g as a and everything else is pretty explanitory.

Where'd I go wrong?

2. Apr 29, 2005

### ninjagowoowoo

.. i forgot the G in the equation... but yeah it's still wrong..

Last edited: Apr 29, 2005
3. Apr 29, 2005

### siddharth

You can use the equation v^2=u^2+2as, if "a" is constant. When we usually use this forumla (ex: projectile motion),we assume the value of g to be fairly constant.

In this case, the gravitational force is not constant, and is varying with distance. So you cannot use the forumla v^2=u^2 + 2as.

Ill give you a hint. There is another way (a much easier one) you should approach the problem with. What do you usually use when you want to find final velocities?
(Example, an object is dropped at a height h.What is its velocity when it reaches the ground?)