# Momentum and acceleration due to gravity

1. Dec 17, 2006

### pharaoh

what object is dangerous for the target, the one that has greatest momentum or the one that has least momentum and why? what is safest?
I answeres all the parts that contain momentum and velocity and force but i stucked in this one

also
what is the value of g (acceleration due to gravity) if the radius doubled?
is the right formular for this question is Fg= G m1m2/r^2, or do i have to substitute the Fg with m*a, then do the rest

2. Dec 17, 2006

### ShawnD

Things with momentum are dangerous because it means they're moving. Are you more scared of a knife on a table or a knife thrown at you?
That equation you posted is correct. Everything should factor out leaving 1/(r^2). How does the gravity change when the r in 1/(r^2) is changed from 1 to 2, or from 2 to 4?

3. Dec 17, 2006

### pharaoh

so you mean that Fg is the same as g right
and the object that has greatest momentum is the most dangerous

4. Dec 17, 2006

### pharaoh

if i doubled the radius then the g= 1/4

5. Dec 17, 2006

### cristo

Staff Emeritus
No, Fg is not the same as g. From the equation, F=(Gm1m2)/r^2 which gives m2g=(Gm1m2)/r^2 and so g=Gm1/r^2

Correct

Right idea: the new value of g obtained when doubling the radius will be 1/4 times the old value of g. So, write $g_{new}=\frac{1}{4}g$