# How do we measure velocity at the first place? (1 Viewer)

### Users Who Are Viewing This Thread (Users: 0, Guests: 1)

J

#### jby

##### Guest
In time dilation and length contraction, there is a velocity term.
How do we measure velocity at the first place?

#### chroot

Staff Emeritus
Gold Member
Relative to some other object, which you assume to be stationary.

- Warren

H

#### hemmul

##### Guest
a bit closer to truth

Actually, any reference frame is defined in a way, that in each point of space you have a spy - a being with a clock. Say an object is traveling in the space. Consider the following:
first event the object passes the coordinate (x1,y1,z1) of the space. the spy sitting at that point remembers the time of this event: t1
second event the object passes the coordinate (x2,y2,z2) of the space. the corresponding spy remembers the time of this event: t2
Now, when t2->t1, the instant velocity is defined as:
vx=(x2-x1)/(t2-t1);
vy=(y2-y1)/(t2-t1);
vz=(z2-z1)/(t2-t1);

Practically, there are no spies rather there are clock associated with a reference frame, relatively to which you want to determine the velocity

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving