MOTION
An Introduction
Thoughts about Motion:
A Short History
Aristotle
• Greek philosopher
• (384-322 BCE)
Assumptions:
1.Natural laws could be
understood by logical
reasoning
2.Heavy objects fall faster than
light objects
3.Moving objects must have
forces exerted on them to
keep them moving
Galileo Galilei
• Italian (1564-1642)
Assumptions:
1. Natural laws could be
understood by
experimentation
2. Objects of different
weights fall at the same rate
(except air resistance)
Leaning Tower of Pisa
Assumptions
(continued)
3. Moving things, once
moving, continue in motion
without the application of
forces
(ignoring friction)
Sir Issac Newton
• English (1642 -1727)
Sir Issac Newton
Assumption:
Newton’s First Law :
Inertia
Every object remains at
rest or in motion (unless
acted upon by an outside
force)
Motion : Speed
1. = how fast an object is
moving
2. speed = distance / time
Units = mi/hr, km/s, m/s, ft/sec,
cm/s, in/s
Motion : Speed
• Average speed =
total distance covered
time interval
Speed examples
1. It took me 12.8 hours to drive to
Vegas.
(914 miles)
What was my average speed?
Speed Example 1
Av speed = total distance covered
time interval
Speed Example 1
= 914 mi
12.8 hr
=
71.4 mi / hr
Speed example 2
2. If I drive at an average speed
of 79 mph ( mi / hr) ,
how many miles can I cover in
4.5 hours?
Speed example 2
total distance = (av. speed) (time)
= (79 mi ) (4.5 hr) =
(hr)
355.5 miles
Speed example 3
3. How long will it take to drive to
Chicago (1000 miles) if your
average speed
is 63 mi/hr?
Speed example 3
Time =
distance =
av. speed
Speed example 3
(1000 mi) =
(63 mi/hr)
Speed example 3
15.9 hrs.
SPEED IS RELATIVE
•
•
•
•
Everything is moving
Earth is rotating (spinning)
Earth is orbiting around the sun
Galaxy is expanding
SPEED IS RELATIVE
• Motion is measured relative to
something
1. e.g. Train relative to track
2. Space shuttle relative to
Earth
3. Other examples?
SPEED IS RELATIVE
• How fast is the Earth is moving?
SPEED IS RELATIVE
30 km/sec
Relative to the sun
• So… you are moving 30 km/sec
• The desk is moving 30 km/sec
VELOCITY
• = speed plus
a DIRECTION
of motion
v = distance
time
VELOCITY Problem
• Two cars are driving in opposite
directions.
• Car 1 is going 60 mi/hr.
• Car 2 is also going 60 mi/hr.
• Do both cars have the same
speed?
VELOCITY Problem 1
• Yes
• Do both cars have the same
velocity? Why or why not?
VELOCITY Problem
• No.
• Because they are not traveling
in the same direction.
• They have the same speeds, but
opposite velocities.
Acceleration
Acceleration
• = (change in velocity)
(change in time)
• = ∆v/ ∆ t
• = v2 – v 1
t2 – t 1
Acceleration
• NOTE : Deceleration =
negative acceleration
• So, stepping on the brake =
− acceleration
Acceleration Problem 1
• A 1965 T-bird with a 390 cubic
inch engine can go from rest to
60 mi/hr in 8 seconds.
• What is its acceleration in
m/sec2?
Acceleration Problem 1
• What formula to use?
• How about the acceleration formula?
Acceleration Problem 1
• Acceleration = ∆v/ ∆ t
• = v2 – v 1
t2 – t 1
• Right, but we need m/sec, not
mi / hr, what to do?
Acceleration Problem 1
SO, we need to convert….
(6x101 mi)
(1km)
(103 m) (1 hr)
(hr) (6.2x 10-1 mi) (1 km) (3.6x103 s)
= 6 x 104 m
2.23x 102s
=
2.6 x 102
m/s
Acceleration Problem 1
• But, we need to know m/ s2
• (2.6 x 102 m/s) = 32.5 m/s2
(8 s)