# I Does Newton's or Galileo's first law depend on gravity?

1. Sep 29, 2016

### parshyaa

Does newton's or galileo's first law depends on gravity ?
• Galileo said that ball rolling on a downward sloping plane speeds up while ball rolling on upward sloping plane slows down therefore he concluded that ball remains with constant velocity(unaccelarated) on a horizontal plane
• here gravity is the reason for increase or decrease in velocity on a downward or upward sloping plane because there is a component of 'g' ie gcos(theta) along or in opposite direction of the motion of ball , therefore ball will be in horizontal motion with constant velocity if there is no external force (except gravity) it means we will not include gravity force in the total force acting on a body .
• Therefore newton's first law will be , if the total force acting on a body (not including gravitational force in total force) equals zero then object will move with constant velocity or remains unaccelarated(a=0)
• Then why in the example of book on a table, where a = 0, we take W - N(contact force) =0 why not only N = 0(here W - N is downwards)

2. Sep 29, 2016

### phinds

No, Newton's first law does not say anything about gravity because it has, fundamentally, nothing to do with gravity which is simply one specific case of an external force acting on a body. You should state laws precisely if you want to understand them. YOU are the one who brought gravity into Newton's first law and then you asked about the law depending on gravity. Simply read the law as stated and you would have answered your own question.

3. Sep 29, 2016

### Staff: Mentor

Newtons 3 laws don't depend on Newtonian gravity. I don't know about Galileo.

Because $N\ne 0$. You cannot change that, it is a measurable physical fact.

4. Sep 29, 2016

### parshyaa

I know that newtons first law do not depends on gravity but I have observed that galileo concluded that if there is no force, object In motion will move with constant velocity. He concluded it from his experiment or observation of rolling balls from downward and upward sloping planes, he observed that velocity of a ball increases when it moves on downward sloping plane and it decreases on upward sloping plane
(Why speed increases or decreases on upward or downward sloping plane ???
Ans : because of componet of g )
From this he concluded that ball rolling on horizontal plane will neither speed up or slow down means it will remain constant or unaccelarated this conclusion will be valid only when we will remove every force (except gravity)
Then why cant I say that newtons first law does depends on mass , I know that this is wrong but I want to know why

5. Sep 29, 2016