Is Negative Speed Possible?

  • Thread starter Tabe
  • Start date
  • Tags
    Negative
In summary, velocity and acceleration are vector quantities that include direction, while speed and displacement are scalar quantities that do not have direction. It is not possible to have a negative speed, but it is possible to have a negative velocity. When describing an object's velocity in terms of a straight line, a scalar value with a sign can be used, or the full vector notation can be used. The magnitude of the velocity vector is always positive, while the direction is indicated by the sign on the unit vector.
  • #1
Tabe
42
0
:confused: I know that you can have a negative acceleration, which would be considered deceleration, but is it possible to have a negative speed? It may sound like a stupid question to some people, but I am completely baffled.
 
Physics news on Phys.org
  • #2
Don't worry :) I have that same misunderstanding when I started.

Velocity and Acceleration is a vector. That means it is a quantity WITH a direction. For example, I can assign the up direction as + and the down direction as -. Then when I drop an apple it will the acceleration = -g (negative), because it is going down. It is NOT possible to have a negative speed, but it is possible to have a negative VELOCITY.

Speed and Displacement is a vector - it is the quantity without direction.

Still don't understand? Try http://www.physicsclassroom.com/Class/1DKin/U1L1b.html
 
  • #3
No we cannot have a negative speed...speed is not a vector quantity. It has only magnitude, not direction. Its value indicates the rate at which an object's distance traveled changes with time, but not in what direction.

Velocity, on the other hand, is a vector quantity. To describe an object's velocity, it is not sufficient to indicate only how fast it is going, but also in what direction.

For the special case of motion in a straight line, we can define a coordinate system such that the line along which the object is traveling is one of the coordinate axes that we have defined (say, the x-axis for example). Then, full blown vectors are not strictly required to describe the velocity...an algebraic scalar (a number with a sign) would be sufficient. If an object is traveling at 10 m/s on the x-axis, then what we call the scalar component of the velocity in the x-direction [itex] v_x [/itex] is:

[tex] v_x [/tex] = 10 m/s if it is traveling in the positive x-direction

[tex] v_x [/tex] = -10 m/s if it is traveling in the negative x-direction

Since the object is traveling in a straight line, it obviously does not have components in any other direction (other than x!). So we can forget about the x and describe the object's velocity as v, a scalar that has an absolute value equal to the magnitude of the velocity vector, but unlike the magnitude, also has a sign that indicates the directional "sense" (+ or -) of the object's motion along the x-axis. We can write the velocity as

[tex] v [/tex] = 10 m/s if it is traveling in the positive x-direction

[tex] v [/tex] = -10 m/s if it is traveling in the negative x-direction

If you like, you can use the full blown vector notation instead:

[tex] \vec{v} = \text{(10 m/s)}\hat{i} [/tex]

(travelling in postive x-direction)

OR

[tex] \vec{v} = -\text{(10 m/s)}\hat{i} [/tex]

(travelling in negative x-direction).

One point of confusion to watch for (that only arises when the motion is confined along one axis and we choose to drop the x subscript). When using the scalar component notation, v indicates both magnitude (10m/s) and direction, which means it can be either postive or negative. In contrast, when using the vector notation, the same symbol "v" is actually the magnitude of the velocity vector:
[tex] v = |\vec{v}| [/tex]
so it is always positive (magnitudes are always postive). The direction is instead given by the sign on the unit vector:
[tex] \pm \hat{i} [/tex].
 
  • #4
Ok, thanks, that explains a lot. It also answers some of the other questions that I had about velocity.
 

1. Can negative speed actually exist in the physical world?

Currently, there is no evidence or scientific theory that supports the existence of negative speed in the physical world. All known laws of physics, including the theory of relativity, do not allow for negative speed. Therefore, the concept of negative speed remains purely hypothetical at this point.

2. How is negative speed different from negative velocity?

Negative speed and negative velocity are often used interchangeably, but they are not the same thing. Speed is a scalar quantity that measures how fast an object is moving, while velocity is a vector quantity that measures both speed and direction. Negative velocity means that an object is moving in the opposite direction of its positive velocity, whereas negative speed simply means that the object is moving at a negative rate.

3. Can negative speed violate the law of conservation of energy?

No, negative speed cannot violate the law of conservation of energy. This law states that energy can neither be created nor destroyed, only transferred or converted from one form to another. Negative speed does not involve any energy transfer or conversion and therefore, does not violate this fundamental law of physics.

4. Are there any real-life examples of negative speed?

As mentioned earlier, there is no evidence of negative speed in the physical world. However, it is a common concept in science fiction and has been used in movies, books, and other forms of media. In these fictional scenarios, negative speed is often portrayed as a way to travel faster than the speed of light and therefore, bend the laws of physics.

5. What are the implications of negative speed if it were to exist?

If negative speed were to exist, it would have significant implications for our understanding of physics and the laws that govern our universe. It could potentially open up new possibilities for faster-than-light travel and challenge our current understanding of space and time. However, until there is scientific evidence to support its existence, negative speed remains purely a theoretical concept.

Similar threads

Replies
4
Views
231
  • Introductory Physics Homework Help
Replies
13
Views
2K
  • Introductory Physics Homework Help
Replies
17
Views
661
  • Introductory Physics Homework Help
Replies
16
Views
3K
  • Introductory Physics Homework Help
Replies
16
Views
854
  • Introductory Physics Homework Help
2
Replies
44
Views
2K
  • Introductory Physics Homework Help
Replies
3
Views
3K
  • Introductory Physics Homework Help
Replies
1
Views
262
  • Introductory Physics Homework Help
Replies
6
Views
589
  • Introductory Physics Homework Help
Replies
17
Views
490
Back
Top