Distance-Time graphs


by KellyOsbourne
Tags: displacement, distance, distance-time graph, negative gradient, negative speed?
KellyOsbourne
KellyOsbourne is offline
#1
Feb15-09, 11:02 AM
P: 3
I was told by my teacher that the definition for distance is just "the amount of ground covered", and that displacement is "the distance in a particular direction". I was looking at some distance-time graphs online and I saw that towards the later stage of the journey in some of the graphs, the gradient of the distance-time graph was negative. Their explanation for the negative gradient was that the object was moving towards the starting position. I do not understand that. Even if the object is moving towards the starting position, it is still covering "ground" and it would go up, right? And the gradient of a distance-time graph is the speed, and I didn't think there was anything called negative speed. I know about negative velocity, but negative speed? Please clear this for me! I want to know how the gradient of a distance-time graph can be negative, when distance is just the "ground" covered.

This could be a very stupid question :$ But I am sometimes very blind to the obvious. Please help! :)

Thank you a lot in advance!
Phys.Org News Partner Physics news on Phys.org
Physicists design quantum switches which can be activated by single photons
'Dressed' laser aimed at clouds may be key to inducing rain, lightning
Higher-order nonlinear optical processes observed using the SACLA X-ray free-electron laser
Hootenanny
Hootenanny is offline
#2
Feb15-09, 11:19 AM
Emeritus
Sci Advisor
PF Gold
Hootenanny's Avatar
P: 9,789
Quote Quote by KellyOsbourne View Post
I was told by my teacher that the definition for distance is just "the amount of ground covered", and that displacement is "the distance in a particular direction". I was looking at some distance-time graphs online and I saw that towards the later stage of the journey in some of the graphs, the gradient of the distance-time graph was negative. Their explanation for the negative gradient was that the object was moving towards the starting position. I do not understand that. Even if the object is moving towards the starting position, it is still covering "ground" and it would go up, right? And the gradient of a distance-time graph is the speed, and I didn't think there was anything called negative speed. I know about negative velocity, but negative speed? Please clear this for me! I want to know how the gradient of a distance-time graph can be negative, when distance is just the "ground" covered.

This could be a very stupid question :$ But I am sometimes very blind to the obvious. Please help! :)

Thank you a lot in advance!
Welcome to Physics Forums.

I distance-time graph cannot have a negative gradient, since as you say distance (and hence speed) is a scalar quantity. However, a position/displacement time graph can have a negative gradient since position/displacement is a vector quantity.

On a distance-time graph the gradient represents the speed, which is always non-negative. However, on a position-time graph, the gradient represents the velocity, which can be negative.

Could you provide a link to the graphs with the negative gradient, I have a sneaking suspicion that they are position-time graphs.
KellyOsbourne
KellyOsbourne is offline
#3
Feb15-09, 11:24 AM
P: 3
Quote Quote by Hootenanny View Post
Welcome to Physics Forums.

I distance-time graph cannot have a negative gradient, since as you say distance (and hence speed) is a scalar quantity. However, a position/displacement time graph can have a negative gradient since position/displacement is a vector quantity.

On a distance-time graph the gradient represents the speed, which is always non-negative. However, on a position-time graph, the gradient represents the velocity, which can be negative.

Could you provide a link to the graphs with the negative gradient, I have a sneaking suspicion that they are position-time graphs.
Thank you so much! Yes, that's what I thought :)
Here is the link:
http://www.golfranger.co.uk/speed.html

Please tell me if I made a mistake in interpreting whether it was a distance-time graph or a displacement-time graph, and how I can distinguish between the two.
Again, thanks so much :)

Hootenanny
Hootenanny is offline
#4
Feb15-09, 11:30 AM
Emeritus
Sci Advisor
PF Gold
Hootenanny's Avatar
P: 9,789

Distance-Time graphs


Quote Quote by KellyOsbourne View Post
Thank you so much! Yes, that's what I thought :)
Here is the link:
http://www.golfranger.co.uk/speed.html

Please tell me if I made a mistake in interpreting whether it was a distance-time graph or a displacement-time graph, and how I can distinguish between the two.
Again, thanks so much :)
The graph shown on that page is clearly incorrect. The negative gradient of the red curve indicates a negative speed, which is impossible.

Is your school hosting this website?
KellyOsbourne
KellyOsbourne is offline
#5
Feb15-09, 11:33 AM
P: 3
Quote Quote by Hootenanny View Post
The graph shown on that page is clearly incorrect. The negative gradient of the red curve indicates a negative speed, which is impossible.

Is your school hosting this website?
Thank you, I thought I was going bonkers!
Umm, no, my school's not hosting it. I just ran across it when I was looking for information to prepare a poster for school.


Register to reply

Related Discussions
Very simple calculus problem...graphs and velocity/time graphs to acceleration. Calculus & Beyond Homework 1
motion detector graphs, distance vs time Introductory Physics Homework 1
Drawing velocity time graphs from distance-time graphs Introductory Physics Homework 2
Distance-time Graph and Velocity-time Graphs Introductory Physics Homework 5
Acceleration/time graphs!!!!! Introductory Physics Homework 3