Transcript Chapter 2—Motion and Force

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 DIRECTIONnorth, 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