# Newtons second law, help needed.

• Sep 19th 2012, 11:16 AM
controlsystem
Newtons second law, help needed.
A 0,51kg heavy stone is in free fall, how heavy is the stone?
• Sep 19th 2012, 11:21 AM
controlsystem
Re: Newtons second law, help needed.
Found out, i was looking at the wrong formula
• Sep 19th 2012, 11:31 AM
emakarov
Re: Newtons second law, help needed.
Would you mind sharing the answer? And what does "how heavy" exactly mean in the question?
• Sep 19th 2012, 06:07 PM
Oldspice1212
Re: Newtons second law, help needed.
F=m*a second law
• Sep 19th 2012, 09:40 PM
topsquark
Re: Newtons second law, help needed.
Quote:

Originally Posted by Oldspice1212
F=m*a second law

Actually Newton's 2nd is
$\Sigma F = ma$ (That's a capital sigma and means "sum of")

Which means you need to factor in the force of gravity as well. How would you do that?

-Dan
• Sep 20th 2012, 06:38 AM
HallsofIvy
Re: Newtons second law, help needed.
You still haven't answered the basic question, "what do you mean by "heavy", here?"

One way of defining "heavy" is the object's weight: "the force of gravity on an object" which is always "mg", the mass times the acceleration due to gravity. Since you give the mass in kg, use g= 9.81 m/s^2.

But more common is the force with which an object is pressing down on a floor, scale, your hand, etc. Here, since the object is not pressing down on anything there is no force- its "heaviness" is 0.