**Speed & Velocity**

**Velocity has units of distance divided by time. In the lab, we
will usually measure velocity in units of m/s. Velocity is also
measured in km/h (and even in mi/hr).**

**
**

**If a motorcycle travels 20 m in 2 s, then its average velocity
is**

**
**

**
**
**If an antique car travels 45 km in 3 h, then its average
velocity is**

**
**

**
Things do not
always move with a constant velocity. The velocity may change. The instantaneous
velocity is the velocity "right now", the velocity at some particular moment.
We usually shorten that and say simply "the velocity".**
**In common, everyday English, "velocity" and "speed" mean the
same thing. In Physics, there is a distinction that is sometimes
useful. **

**Velocity is positive if an object is moving to the right and it
is negative if an object is moving to the left.**

**Speed tells how fast an object is moving without saying
anything about its direction. Speed is always positive. Speed is the
"absolute value" of the velocity. Speed in the velocity information
without the sign or direction information.**

**
**

**
We can take the definition of average velocity, turn it around, and
write**
**x = x**_{i} + v_{avg} t
**where x**_{i} = initial position; that is, x_{i}
= position at time t = 0

**If we only want to know how far something has gone, that's like
starting at x**_{i} = 0,

**x = v**_{avg} t
**If the velocity is constant, then v = v**_{avg}
and

**x = v t **
or** **

**x = x**_{i} + vt

(C) 2003, Doug Davis; all rights reserved