Transcript Velocity - SFSU Physics & Astronomy

Measuring motion
Two fundamental
components:
 Change in position
 Change in time
Three important
combinations of
length and time:
1. Speed
2. Velocity
3. Acceleration
Description of Motion
Speed  distance
time
average speed = total distance covered
time interval
instantaneous speed - the speed that
something has at any one instance
Units of speed
miles/hour…………….mph
kilometer/hour……….km/h
meters/second……….m/s
Converting Units
Converting feet to meters:
1 m = 3.281 ft
(this is a conversion factor)
Or: 1 = 1 m / 3.281 ft
316 ft × (1 m / 3.281 ft) = 96.3 m
Note that the units cancel properly – this is the
key to using the conversion factor correctly!
Average Speed
The average speed is defined as the distance
traveled divided by the time the trip took:
Average speed = distance / elapsed time
Is the average speed of the red car 40.0 mi/h,
more than 40.0 mi/h, or less than 40.0 mi/h?
Velocity
Velocity = {speed with a direction}
Examples:
70 mph is a speed.
70 mph North is a velocity.
Velocity
 Describes speed (How fast is it going?)
and direction (Where is it going?)
 Graphical representation of vectors:
length = magnitude; arrowheads =
direction
Acceleration
 Rate at which motion changes over time
 Speed can change
 Direction can change
 Both speed and direction can change
v f - vi
a=
t
Forces - historical background
Aristotle
Galileo and Newton
 Heavier objects fall
faster
 Objects moving
horizontally require
continuously applied
force
 Relied on thinking
alone
 All objects fall at the
same rate
 No force required for
uniform horizontal
motion
 Reasoning based
upon measurements
KINEMATICS
DYNAMICS
Description
Explanation
 Position
 Velocity
 Acceleration
 Forces
 Newton’s laws
Applications
 Momentum
 Circular motion
 Newton’s law of
gravitation
 Horizontal motion on
land
 Falling objects
 Compound (2-D)
motion
Applications
Aristotle on Motion
(350 BC)
 Aristotle attempted to understand motion by
classifying motion as either
• (a) natural motion
• forces acting at a distance
• (b) or violent motion
• contact forces
 “Large object tend to 'strive harder'.”
 He stated that “The Earth remains at
rest.”
Geocentric Model - Earth Centered Universe
Copernicus (1500's)
 "The Earth and
planets orbit the
Sun.”
 He reasoned this
from his
astronomical
observations.
Galileo (1600's)
 Scientist who supported
Copernicus
 Dropped objects with
different weights from the
Leaning Tower of Pisa
 Found that all objects fall at
the same rate if you can
account for air resistance


http://www.youtube.com/watch?v=YD6JYdKxRjo Pisa
http://www.youtube.com/watch?v=x7dUgiKzLSc Pisa

http://www.youtube.com/watch?v=WOvwwO-l4ps Moon
Free Fall
Free fall is a state of falling free
from air resistance and other forces
except gravity.
Galileo’s Incline Planes
Isaac Newton (1642-1727)
His three laws of
motion first
appeared in his
book called
Principia.
Newton’s First Law
a.k.a “Law of Inertia”
A body remains at
rest or moves in a
straight line at a
constant speed
unless acted upon by
an unbalanced force.
NET FORCE
 A force or a combination of forces
produces changes in motion
(accelerations).
10 N
10 N
10 N
20 N
m
=
m
10 N
=
m
10 N
=
m
20 N
0N
10 N
m
m
Normal up
SUPPORT FORCE
A table can supply an
upward support force
also known as a
normal force.
Weight down
Scales pushing up
When we say
“normal to”
we are saying
“at right angles to”.
Weight down
THE EQUILIBRIUM RULE
Examples of
Mechanical
Equilibrium:
Computer setting on a table
Scales pushing up
Normal up
Weight down
Weighing yourself on a set of scales
Hanging from a tree
Car parked on an incline
Friction
Tree
pulling up
Weight down
Normal
Weight down
Weight down
The Equilibrium Rule
F

0

EQUILIBRIUM OF MOVING THINGS
 Equilibrium is a state of no change.
 If an object moves in a straight line with no
change in speed, it is in equilibrium.
Examples:
Driving at constant velocity
Normal up
Air resistance
Force from road
Air
Resistance
Weight down
Terminal velocity in parachuting
Weight down
If an object weighs 10 lb, what must the air
resistance force be if the object is falling
and has reached terminal velocity?
(a) 10 lb
(b) 32 lb
(c) there is no way of telling without
knowing what the value of the
terminal velocity is
Newton's concept of motion said that the
natural state of an object was
(a) constant velocity
(b) constant acceleration
(c) constant net force