# Homework Help: Asteroid collides with Earth, new accel. of gravity?

1. Oct 12, 2005

### confusedbyphysics

This was on my test and I took a guess because I was so confused. The question is that an asteroid hits earth and now the Earth has a mass of 1.5 times its original mass and a radius of .75 times its original radius. What is its new force of gravity?

its driving me crazy because it seemed like it should be one of the easiest problems on the test but i was stumped for some reason. how should I have done this? Thanks!!!!

2. Oct 12, 2005

### arildno

How can "g" be expressed by Newton's law of gravitation?

Can't see the point, though, because if an asteroid with mass possibly exceeding half the mass of the Earth, there will be no one about noticing the change in "g"..

3. Oct 12, 2005

### confusedbyphysics

LOL, good point.

Is it F = G X m1 X m2 / r^2?

thats all I could think of...where F is gravitational force and r is .75 X the radius of the earth and one of the masses is 1.5 X the mass of earth. and G is 6.67 X 10^-11. but I dont know what the other mass would be. Do I just assign a number?? Or is this wrong altogether

4. Oct 12, 2005

### arildno

Yes, that is the gravitational FORCE.
What must then the ACCELERATION be of an object with the mass $m_{1}$ when exposed to that force?

5. Oct 12, 2005

### confusedbyphysics

a=F/m. ah, so the ms just cancel out when you set F = ma... I got a = 19.6 (not the answer I circled on the test CRAP!)