Motion Notes
Overview
Mechanics – 2 parts

Kinematics

Dynamics
• Characteristics of motion
• Causes of motion
Kinematics

Imagine a three legged stool and each
leg is a fundamental parameter of
motion:
• Position (distance/location)
• Time
• Speed
Kinematics – Position

Position (linear measure)
• Where an object is located at specific point in
•
•
time
Units of meters
Can be described in terms of:
• Distance (scalar)
• measure from one position to another
• Displacement (vector)
• measure from one position to another in a direction
• distance from the start point to the finish point in a
straight line, in a certain direction
Kinematics - Speed

Definition
• Rate of change of position
• Average Speed (scalar) = distance traveled
time taken to travel the distance
• Velocity (vector) = displacement
time taken
•
When traveling in a straight line, speed and velocity have
the same magnitude.
• vav = d/t
• Units: meters/second (m/s)

Distance is to speed (both scalar quantities)
as displacement is to velocity (both vector
quantities).
Example – speed


Usain “Lightning” Bolt won the World
Track & Field 100m sprint in 9.58
seconds. What was his average speed
in m/s?
Solve: avg speed = distance/time
• Speed (v) = 100/9.58 = 10.438 m/s
Example - segments


A traveler uses a cab to travel east for
1000 m @ 25 m/s then north for 1700 m
@ 10 m/s. How long is the trip?
Solution: break the trip into segments
and use d = v*t or t = d/v to find the
segment times – then add.
• Seg 1: t = 1000/25 = 40 seconds
• Seg 2: t = 1700/10 = 170 seconds
• Total time = 40 + 170 = 210 seconds.
Kinematics - Acceleration


How do you pass a car on I-81?
Definition
• Rate of change of velocity
• a = v/t, or
• a = (vf – v0)/t, where
• vf = final (ending) velocity
• v0 = initial (starting) velocity
• t = time taken for the velocity to change
• units: meters/sec/sec, or meters/sec2 (m/s2)
• VECTOR!

Average speed (alternative formula)
• Vav = (v0 + vf)/2
Example - acceleration

A funny car accelerates from zero to 300
mph (135 m/s) in 5 seconds. What is its
acceleration?
Solve: acceleration = (vf – vo)/t

g-forces?

• a = (135 - 0)/5
• a = 27 m/s/s
• a = 27 m/s2
Kinematics – Graphs
(Distance vs Time)




slope = velocity or
speed of the object
Steep (left) slope =
higher speed
Zero slope (flat line)
= object stationary
Slope can be + or –
indicating motion
direction
Kinematics – Graphs
(Velocity vs time)




slope is acceleration
Steep (left) slope =
higher accel’n
Zero slope (flat line) =
zero accel’n or the
object is moving at
constant speed
Slope can be + or –
indicating speed
direction
Practice - Handouts

H/O Interpretation of Motion Graphs

H/O d-t & v-t graph worksheet

H/O Position Time
• D-T & V-T
• D-T & V-T
• D-T & V-T