Chapter 2—Motion and Force - Palmer ISD

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Chapter 2

Motion and Force

Section 1

Describing
Motion

Motion and Speed


Motion

occurs when an object
changes its position


*Motion doesn’t always have to be
seen to have taken place


Reference

Point
-
used to determine
how far something moves


Ex: mailbox while driving

Relative Motion


Relative Motion


Not all motion is obvious


Everything is in motion relative
to something else


Ex: the Earth is always in
motion around the sun, etc.

Distance and Displacement


Distance

describes how far something
has moved


SI base unit to measure distance is
METER


Displacement

the distance and
direction of an object’s change in
position
from the starting point


See figure 2 on pg. 39

Speed


Speed
-
the distance an object travels
per unit of time


Ex: miles per hour/ feet per minute,
etc.


Rate

any change over time, so
--


**
Speed is a RATE


Equation:

Speed = Distance / Time



(
typically measured in m/s)

Practice Calculating Speed
-
pg. 42
-
# 1 and 2


*
Most speeds aren’t constant, they
either slow down or speed up


Average

speed
-
describes speed of
motion when speed is changing

(driving on a 3 hr. trip)


Instantaneous

Speed
-
speed at a given
point in time


Ex: speed on a speedometer




Velocity


Velocity
--
includes the speed of an
object AND the direction of its
motion


*
Velocity can change EVEN IF
speed remains constant


Change in

Velocity

can be EITHER
a change in speed OR in direction
of movement

Remember:

*To Calculate Velocity, it is the
same as calculating speed, but you
must also report a DIRECTION
-
north, south, east, west, etc.


*One example of change in
velocity

when a race car at a
constant speed is rounding a curve,
velocity is changing, due to
direction of travel changing

Plate Tectonics


The motion of the Earth’s crust creates
the continental drifting that occurs
every year


Plate Tectonics Theory
-
Pangea


Plates move so slowly that their speeds
are given in units of centimeters per
year.


Australian plate
-
1 of fastest

moves north at
an average speed of 17 cm./yr and San
Andreas Fault (in CA) about 1 cm/yr

Section 2

Acceleration


Acceleration
-
the rate of change of
velocity (velocity = speed w/direction
)


When the velocity of an object
changes, the object is accelerating


So: Acceleration occurs when an
object changes its SPEED, its
DIRECTION or BOTH

Acceleration:


If speed is increasing = positive
acceleration (acceleration)


If speed is decreasing = negative
acceleration (deceleration)


Ex: horse on a carousel
-
acceleration
due to direction only

Equation for finding Acceleration:


Acceleration = change in velocity / time

OR

Acceleration = final velocity


initial




velocity / time

(usually reported in m/s/s or m/s^2)


(if motion is in a straight line, (no change in direction,
use change in speed to calculate acceleration)

Amusement Park Acceleration:

Wooden and steel roller coaster provide
different thrills due to different materials

Wooden
-
swaying effect

Steel
-
more loops, steep inclines, drops, etc
.

Section 3: Motion and Forces


Force:
a push or pull that one
body exerts on another


*A force can cause the motion of
an object to change


ex: tennis racket on a ball


ex: billiards

Balanced forces
: forces that
are equal, but in opposite
directions *they do NOT
change the direction of the
force

NET FORCE
-
when

two balanced
forces are combined the net force
will be zero (if equal but in
opposite directions)

Unbalanced forces
: forces that
are unequal and in opposite
directions

*the greater force will cause
an object to move in that
direction

NET FORCE
: with unbalanced
forces, will be the DIFFERENCE
BETWEEN the two forces

Inertia
: The tendency of an
object to resist ay change in
its motion

*If an object is moving, it will
keep moving at the same speed
and direction unless an
unbalanced force acts on it

*Or: velocity of the object
remains constant unless a force
changes it

Newton’s First Law of Motion:
(Also called Law of Inertia)

*An object in motion will remain
in motion unless acted upon by an
outside force

*An object at rest will remain at
rest unless acted upon by an
outside force

Ex: what happens in a car crash
-
pg. 55