# Thread: Newtons second law, help needed.

1. ## Newtons second law, help needed.

A 0,51kg heavy stone is in free fall, how heavy is the stone?

2. ## Re: Newtons second law, help needed.

Found out, i was looking at the wrong formula

3. ## Re: Newtons second law, help needed.

Would you mind sharing the answer? And what does "how heavy" exactly mean in the question?

4. ## Re: Newtons second law, help needed.

F=m*a second law

5. ## Re: Newtons second law, help needed.

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

6. ## 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.