Positive or negative velocity and acceleration


by Leonhart
Tags: accelartion, negative, positve, velocity
Leonhart
Leonhart is offline
#1
Sep13-09, 11:34 AM
P: 2
1. The problem statement, all variables and given/known data
A motion detector is placed on the top of a ramp sloping down. A cart is placed at the other end of the ramp (bottom of the ramp). The cart is pushed up the ramp towards the motion detector (top of ramp), and stopped at it's highest point.


2. Relevant equations
1. Is the object speeding up or slowing down?
2. Is the velocity of the object positive or negative?
3. Is the velocity getting more positive/less positive/more negative/less negative?
4. Does the object have a constant positive or negative acceleration?


3. The attempt at a solution

1. When we push the cart, it's speeding up, but it'll slow down over time because it's going up slope, so is it speeding up or slowing down?
2. I can't determine whether it's positive or negative because the cart is moving toward the motion detector, which would make it negative...right? But it's initial position is at the bottom of the ramp, which makes it negative.
#3 and #4 are the same as above. I can't tell which one is negative or positive.


Also, the graphs from the motion detector shows:
Position vs. time: sloping down
Velocity vs. time: sloping up

I've tried to read the textbook, but I'm still confused. I know that direction of an object determine whether its velocity is negative or positive.
Up, right: positive.
Down, left: negative.
In the textbook, there's a chart like this:
Initial velocity--------------Accelearation-----------------Motion
(+)-------------------------(+)-------------------------speeding up
(-)-------------------------(-)--------------------------speeding up
(+)-------------------------(-)--------------------------slowing down
(-)-------------------------(+)--------------------------slowing down
(-)or(+)--------------------(0)--------------------------constant velocity
(0)-------------------------(-)or(+)---------------------speeding up from rest
(0)-------------------------(0)--------------------------remaining at rest

EDIT: Thanks in advance.
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Redbelly98
Redbelly98 is offline
#2
Sep15-09, 06:36 PM
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Welcome to PF

Quote Quote by Leonhart View Post
Also, the graphs from the motion detector shows:
Position vs. time: sloping down
Velocity vs. time: sloping up
If you have these graphs, don't they indicate whether velocity etc. are positive or negative?
Leonhart
Leonhart is offline
#3
Sep16-09, 12:40 AM
P: 2
Quote Quote by Redbelly98 View Post
Welcome to PF



If you have these graphs, don't they indicate whether velocity etc. are positive or negative?
Thank you for your answer, but I've already turned in and graded my lab. I kinda understand it now. The thing that messed me up is the direction.

In the text book, it says "down" direction is negative, and "up" direction is positive. Since the detector is on top of the ramp, I would assume that the direction away from the detector, which is going DOWN the ramp is negative, but it turns out, AWAY from the detector is positive. >.<

Thanks for your answer. ^-^


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