Understanding Negative Velocity and Displacement on a Velocity-Time Graph

In summary, the area under a velocity-time graph represents the displacement, and the integral assigns a negative sign to areas below the x-axis. However, this concept of negative displacement can be understood without relying on integration. It is simply a result of negative velocity below the time axis.
  • #1
grzz
1,021
21
Homework Statement
Is area under a velocity-time graph a distance or displacement?
Relevant Equations
Velocity = rate of change of displacement with time.
I think the area required is a displacement.
 
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  • #2
Depends if you mean area or signed area (integral)
 
  • #3
Thanks
 
  • #4
grzz said:
Homework Statement:: Is area under a velocity-time graph a distance or displacement?
Relevant Equations:: Velocity = rate of change of displacement with time.

I think the area required is a displacement.
The area under a curve is always, by definition, a positive number. If a curve crosses the ##x## axis, then the total area between the curve and ##x## axis is the sum of all the separate areas.

The integral, however, assigns a negative sign to areas below the ##x## axis. The integral may be positive or negative or zero.

In terms of physics, the integral of a velocity time graph gives the displacement. Whereas, the total area under a velocity time graph gives the total distance.
 
  • #5
Hence such a question makes sense only to students who are familiar with integration. Am I correct to say this?
 
  • #6
grzz said:
Hence such a question makes sense only to students who are familiar with integration. Am I correct to say this?
Not necessarily. You can take an area below the ##x## axis to be negative, without relying on integration.
 
  • #7
A student who knows about integration will not ask why the area can be negative while one who does not know about integration will ask why the area below the x axis is negative. What answer can I give him then?
 
  • #8
grzz said:
A student who knows about integration will not ask why the area can be negative while one who does not know about integration will ask why the area below the x axis is negative. What answer can I give him then?
If we are talking about a velocity-time graph, then velocity is negative below the time axis. And displacement is negative. That is nothing to do with integration.
 

FAQ: Understanding Negative Velocity and Displacement on a Velocity-Time Graph

What does negative velocity indicate on a velocity-time graph?

Negative velocity on a velocity-time graph indicates that the object is moving in the opposite direction to the positive reference direction. It does not mean the object is slowing down; rather, it is traveling in the reverse direction.

How can you determine displacement from a velocity-time graph with negative velocity?

Displacement is determined by calculating the area under the velocity-time graph. If the velocity is negative, the area under the curve will be below the time axis, contributing a negative value to the total displacement. Sum both positive and negative areas to find the net displacement.

What does a negative slope on a velocity-time graph represent?

A negative slope on a velocity-time graph represents a negative acceleration, meaning the object is decelerating if moving in the positive direction or accelerating if moving in the negative direction.

Can an object have a negative velocity and a positive displacement?

Yes, an object can have a negative velocity and a positive displacement. This situation occurs when the object initially moves in the positive direction, accumulating positive displacement, and then reverses direction, moving with a negative velocity.

How do you interpret a velocity-time graph where the velocity crosses from positive to negative?

When the velocity crosses from positive to negative on a velocity-time graph, it indicates that the object has changed direction. The point where the velocity crosses the time axis (velocity equals zero) represents the moment when the object stops and reverses its direction of motion.

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