Register to reply 
DistanceTime graphsby KellyOsbourne
Tags: displacement, distance, distancetime graph, negative gradient, negative speed? 
Share this thread: 
#1
Feb1509, 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 distancetime graphs online and I saw that towards the later stage of the journey in some of the graphs, the gradient of the distancetime 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 distancetime 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 distancetime 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! 


#2
Feb1509, 11:19 AM

Emeritus
Sci Advisor
PF Gold
P: 9,781

I distancetime 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 distancetime graph the gradient represents the speed, which is always nonnegative. However, on a positiontime 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 positiontime graphs. 


#3
Feb1509, 11:24 AM

P: 3

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 distancetime graph or a displacementtime graph, and how I can distinguish between the two. Again, thanks so much :) 


#4
Feb1509, 11:30 AM

Emeritus
Sci Advisor
PF Gold
P: 9,781

DistanceTime graphs
Is your school hosting this website? 


#5
Feb1509, 11:33 AM

P: 3

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 distancetime graphs  Introductory Physics Homework  2  
Distancetime Graph and Velocitytime Graphs  Introductory Physics Homework  5  
Acceleration/time graphs  Introductory Physics Homework  3 