Transcript Chapter 5 Matter in Motion

Chapter 12
Matter in Motion
Section 1
Measuring Motion
ObservingA neutron
Motion
star is the core of a dead
• Motion is described
as a change in
position over time
when compared to a
reference point.
• A reference point is a
location that appears
to stay in place.
star. This one has been observed
to be traveling 400,000 mph after
its parent star exploded.
Speeds of different objects
•
Snail 0.03 mph or 1.3 cm/sec
•
Human = 30 mph or 13.3 m/s
•
Speed of Sound = 760 mph or 337 m/s
•
Car = 776 mph
•
Jet = 2,200 mph or 978 m/s
•
Rocket = 35,000 mph or 15,555 m/s
•
Speed of Light = 186,280 miles per sec or 300,000,000 m/s
or 345 m/s
or 670,608,000 mph
Speed depends on distance and
time.
• The following rate can
be used to calculate
speed
dis tan ce
speed 
time
• Include the proper
units
– m/s, km/h, mph
Distance
Speed
Time
• Examples
– What is your average
speed if you walk
4000m in half an hour?
d
r
t
d
speed 
t
4000m
4000
4000
m m
speed
speed 

half
an
30
min
1800
s hour
2.2m / s
• You traveled for 6 ½
hours at 110 km/h. How
far did you travel?
• How long will it take
to travel 810 meters
at a rate of 15 m/s?
d
d
r
t
d r xt
110km
d
x 6.5hours
hour
d = 715 km
r
t
d
t
speed
810 m
t
 54s
15 m / s
Speed vs Velocity
• Velocity is speed in a
particular direction
• Velocity changes as
speed or direction
changes
– Example
•
•
•
•
m/s
Km/hr
m/s forward
Km/hr south
speed
velocity
– Examples
• A car speeding up or
slowing down
• A rollercoaster turning
• A wheel spinning
– All examples of a
change in velocity
Combining Velocities
•
Velocities in the
same direction can
be added:
•
•
Example
•
1. Car at 10 m/s
2. Ball at 5 m/s
3. Combined velocity =
15 m/s
Velocities in
opposite directions
work against each
other
Example
1. Car at 10 m/s
2. Ball at 10 m/s
3. Combined velocity =
0 m/s
`
Since we were very young we have learned
to add velocities together to make objects
move faster and/or farther.
Acceleration
• Acceleration is the
rate at which velocity
changes (DV)
• Examples
• Use the following
formula to calculate
acceleration
Means “change in
velocity”
– Speed up
– Slow down
– Change direction
DV
A
t
final V  start V
A
t
– Units m/s/s (also
written m/s2 )

Spinning is
considered to be
accelerating
continuously since
the direction is
constantly
changing.
Acceleration Examples
• An airplane goes from 150 m/s
to 50 m/s in 20 seconds. What
is the acceleration?
final VDV start V
A A
tt
50  150  100
A

20
20
A = -5 m/s2
Negative acceleration
means the object is
slowing down.
• A car goes from a stop to
21 m/s in 6 seconds. What is
the acceleration?
final VDV
 start V
A A
tt
21  0 21
A

6
6
A = 3.5 m/s2
Positive acceleration
means the object is
speeding up.
Y-axis
Graphing Speed
rise
run
=
distance
time
DISTANCE
SLOPE =
X-axis
TIME
Graphing Speed
DISTANCE
SLOPE =
TIME
rise
run
=
distance
time
Section 2
Force
• A force is a “push” or
“pull” on an object.
– Forces have both size
and direction.
– Measured in newtons
(N)
• Examples
– Gravity (downward pull
due to the size of the
objects involved)
– Kicking a soccer ball
(causes the object to
change direction)
Forces in Combination
(Net Force)
• Forces in the same
direction add together
– Two train engines pulling a
large train.
– 4 jet engines pushing an
airplane
– How many students does it
take to push a school bus?
• Forces in opposite
directions subtract from
each other.
– Balanced forces create no
motion.
• One student trying to push
a bus.
• Tug-o-war between
equally strong people
– Unbalanced forces create
motion.
• Lifting a ball defeats the
force of gravity
Forces in the same direction are
added together to determine net
force.
• example: moving a car
20 N left
+ 5 N left
25 N Left
20 N
5N
Forces in different directions are
subtracted from one another.
• Example: a tug of war
(You subtract the smaller force from the larger force)
30 N Right
- 20 N Left
10 N Right
20 N
30 N
Elastic Forces
• Tension
– A magnitude of pulling
force on an object
– The stretching of an
object
• Compression
– A magnitude pushing
force on an object
– The squeezing of an
object
Section 3
Friction
• Friction is a force that
opposes motion
between two surfaces
that are touching.
– The amount of friction
depends on
• the force pushing the
objects together
• the roughness of the
surfaces
• Friction occurs
because all objects
have mass and even
smooth objects are
not perfectly smooth.
– “Hills and valleys” are
always present.
Types of Friction:
Kinetic Friction (motion)
1. Sliding Friction
- one surface
grinding over
another
- example
box across the
floor
2. Rolling Friction
- one surface rolling
across another
- example
wheels, axles,
ball-bearings
3. Fluid Friction
- one object moving
through a fluid (air,
water, oil, etc)
- example
swimming, flying,
parachute
Friction also exists when objects are not
moving.
• “Static” means not
moving.
SLIDING
STATIC
4. Static Friction
- maximum friction
experienced before
the object begins to
slide, roll or move
through a fluid
Low
High
FRICTION-O-METER
In space, no air means no fluid friction.
Lift
Thrust
Drag
Flight requires
some fluid friction
to hold the plane
up
(air friction)
Gravity
Slow fish
Fast fish
Ways to Change the amount of
friction
• Reducing friction
– Lubricant
• Oil
– Smoother surface
• Waxed surface, Teflon
• Increasing friction
– “Sticky” substances
• Glue, tape
– Increase the force pushing
the objects together
• Extra weight, “elbow
grease”, brakes on a car
– Rougher surface
– Change sliding to rolling
• Handcart, wheel-barrow
– Reduce the force pushing
the objects together (less
weight)
• Unload, lift
• Sand on the road, cleats