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**Speed & Velocity**

**Velocity is the change in displacement divided by the change in time required
for that to happen. Velocity has units of displacement 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".**
**This limit is the derivative; that is,**

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

**x = x**_{i} + v t

(c) 2005, Doug Davis; all rights reserved