3-1 Speed Versus Velocity

Chapter 3 Motion (Ewen et al. 2005)

Objectives:•Distinguish between speed and velocity•Use vectors to illustrate and solve velocity problems.

Motion and Speed

Motion can be defined as an object’s change in position.How quickly and object changes its position is its speed—distance traveled per unit of time.

time

distancespeed

Units: mi/h, km/h,

m/s, or ft/s

Velocity

Velocity of an object is the rate of motion in a particular direction.The time rate of change of an object’s displacement. A vector quantity that give the direction of travel and the distance traveled per unit of time

Figure 4.2 The velocity, distance, and time for a car traveling at a constant velocity of 10 m/s to the right is shown in 1-s intervals.

t

svvavg

s = displacement

vavg = average

velocity or speed

t = timeUnits?

Constant Positive Velocity

Constant Negative Velocity

Example 1

Find the average speed of an automobile that travels 160 km in 2.0 h.

Example 2

An airplane flies 3500 mi in 5.00 h. Find its average speed.

Example 3

Find the velocity of a plane that travels 600 km due north in 3 h 15 min.

Example 4

A plane is flying due north (90o) at 264 km/h. Suddenly there is a wind from the east (180o) at 55.0 km/h. What is the plane’s new velocity with respect to the ground in standard position?

Note: Wind directions are given from the direction the wind is coming from, not the direction it is actually blowing. For example, a north wind is actually coming from the north but is actually moving southward.

Example 5

A plane is flying northwest (at 135o) at 135 km/h. Suddenly there is a wind from 30.0o south of west (at 30.0o) at 65.0 km/h. What is the plane’s new velocity with respect to the ground in standard position?

Figure 3.8 Although the boat is not pointed toward the dock, the combination of the boat’s velocity (green vector) plus the current’s velocity (blue vector) results in a perfect docking (red vector).

Figure 3.7 An example of how the velocity of a boat and the velocity of the current are combined so the resultant velocity is directed toward the desired location.