Velocity vs time and acceleration vs time

[pb23me]

Homework Statement

describe the net force needed tto produce the following velocity vs time and acceleration vs time graphs.
i don't have a pic so I am goin to explain... velocity vs time starts below the x-axis and increases at a constant rate to some point above the x axis. at that point velocity is then a horizontal line. acceleration vs time starts above the x-axis and decreases at a constant rate to some point below the x axis. these are just two questions on a test that i got wrong.

Homework Equations

The Attempt at a Solution

 

[tiny-tim]

hi pb23me!

show us what you tried, and then we'll know how to help!

 

[pb23me]

well I am just confused about when velocity was in the neg direction and slowing dwn i put neg force. also on the acceleration vs time i have no clue what's going on with the force when there is a line starting from the pos side that decreases at a constant rate into the neg side.

 

[pb23me]

for the acceleration do you say it is a pos force causing it if it is on the pos side and neg force on the neg side? regardless of the acceleration line going towards or away from the axis of time??

 

[tiny-tim]

hi pb23me!

well I am just confused about when velocity was in the neg direction and slowing dwn …

don't say that!

if velocity is in the negative direction and "slowing down", then the velocity is increasing

the speed is getting less, but the velocity (measured in the positive "x" direction) is getting larger

don't think of velocity as if it was speed!

acceleration vs time graphs:

force = mass times acceleration, so the force always has the same sign as the acceleration

and of course, if the force is positive, it's to the right, and if it's negative, it's to the left

velocity vs time graphs:

the acceleration is always the slope (the tangent) of the velocity

does that help? ​

 

[pb23me]

thanx that helps a lot however... so if velocity is in the negative direction and slowing down you say that velocity is increasing. So if it is in the negative direction and speeding up it must be decreasing. If i am correct in that observation how is that so?

 

[The legend]

yes you are correct in your observation.. see velocity is a vector so it going in -tve x direction means it has some -tve magnitude... then if you still accelerate in negative x then your velocity still gets more and more -tve magnitude...

Example : in the beginning it had v1=-5 (say) and after some time it has v2=-10... which among them is greater?
Hope you get it