Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Alinta on 28/09/2012 07:11:09

Title: Negative position and negative velocity graphs
Post by: Alinta on 28/09/2012 07:11:09

Hi, it's me with the beginners questions again,

I've been looking at graphs that show 'free fall' as a negative position or velocity, and therefore they look upside down on the graph (simple position vs. time and velocity vs. time graphs). Why is it that the downwards pull of gravity is always seen as a negative velocity? Or is this just the particular examples that I have found? Is this only in cases where the free falling thing has been thrown upwards to begin with, and therefore is returning to it's original position?

Because I am interested in how these things affect the human body, I am imagining a person standing, and then slipping over, or free falling to the ground from the distance of their own height - because this is downwards, is it still seen as a 'negative' despite their original position being upright? Or is it impossible to use this as an example?

Thanks,
Alinta

Title: Re: Negative position and negative velocity graphs
Post by: yor_on on 28/09/2012 09:10:13

hmm?

take a look here for the convention Convention for Direction in Gravity Equations. (http://www.school-for-champions.com/science/gravity_direction_convention.htm) does it really state that free fall is a negative? To me a free fall would be equivalent to being 'at rest' locally, neither negative nor positive. Although comparing you may get a different answer, defining your 'system'.

Title: Re: Negative position and negative velocity graphs
Post by: Alinta on 28/09/2012 10:02:36

Ah thanks yor_on, I think your link's explanation of "Velocities in the same direction as gravity are positive and velocities in the opposite direction are negative vectors." probably sums it up.

These were the graphs I was looking at http://www.physicsclassroom.com/Class/1DKin/U1L5c.cfm (http://www.physicsclassroom.com/Class/1DKin/U1L5c.cfm), which only seem to show negative directions, which may have confused me?

Title: Re: Negative position and negative velocity graphs
Post by: yor_on on 28/09/2012 10:27:05

Take a look at this one too. What is slope? (http://www.basic-mathematics.com/what-is-slope.html) And I definitely agree :) It's somewhat confusing.

Title: Re: Negative position and negative velocity graphs
Post by: evan_au on 28/09/2012 11:04:15

When talking about projectile motion (cannons, balls, rockets, etc), you are often interested in the height that the projectile reaches; "Up" = Positive.

Against this is gravity, always dragging you down. "Down" = Negative.

If you roll a ball off a cliff (instead of throwing it off the cliff), and take the top of the cliff as the zero point, then the height is only negative.
If the zero is taken at the bottom of the cliff, then the height starts off positive.
It all comes down to the convention you adopt...