Physics 30
Speed, Velocity, and Acceleration
Speed
The speed of an object is the rate of change of the object’s position. Speed is an indication of
how fast an object is moving. The average speed of an object is the distance traveled divided by the time
required to cover the distance.
Average speed= distance/ elapsed time (units m/s)
Speed is therefore a scalar quantity because it only tells you how fast an object is moving, it does not tell
you anything about the direction the object is going.
Speed is the magnitude of velocity. Speed and velocity are generally not equivalent because total
distance and total displacement are generally not the same. When would they have the same
magnitudes?
Velocity
The velocity of an object is defined to be the ratio of its change in position to the time taken to
change the position.
Physics 30
The “sign” of the velocity indicates the direction of movement. A negative sign indicates movement in
the negative direction.
Acceleration
The acceleration of an object is defined to be the ratio of its change in velocity to the time taken
to change the velocity.
The “sign” of the acceleration indicates whether the velocity is increasing or decreasing. A positive sign
indicates that the velocity is increasing. It will also be an increase in speed if the object is traveling in the
positive direction. It is a decrease in speed otherwise.
A negative sign indicates that the velocity is decreasing. It will also be a decrease in speed if the object is
traveling in the positive direction. It is an increase in speed otherwise.
It is important to note that information about an object’s acceleration tells us how the object’s velocity
is changing. In order to know what this change in velocity is doing to the object’s speed, we must know
the direction the object is traveling.
As a rule, if the object’s velocity and acceleration are in the same direction (have the same sign), we can
say that the object’s speed is increasing. If the velocity and acceleration are in opposite directions (have
opposite signs), we know that the object’s speed is decreasing.
What can we say about changes in speed and/or velocity if the acceleration is either increasing or
decreasing?
Physics 30
Practice
Velocity
1. Pg. 46 # 1-4
2. Position vs time graphs pg. 48, 49 Read and complete the practice
3. Read section 2.4 then complete the practice problems on pg. 52
Acceleration
1. Read section 2.6, complete the practice a)- d) on pg. 56
Uniform Acceleration:
1. Section 2.13 Practice problems pg. 73-74 #1 -6
2. Pg. 80 # 1-19