Chapter 8: Motion &
Forces
Lesson 1: Describing
Motion
Reference Point
► The
starting point you choose to describe
the location, or position, of an object.
 Example: I am at the library.
► Position:
describes an objects distance and
direction from a reference point.
 Includes direction, distance, & reference point.
 Example: I’m at the library which is down the
hall
Displacement
► The
difference between the initial, or
starting, position and the final position
Displacement
Motion
► An
object’s change in position relative to a
reference point
Speed
► The
distance traveled divided by the time
interval during which the motion occurred
 Constant speed: when an object moves the
same distance over a given unit of time
(gradual speed increases)
 Changing speed: when an object moves a
greater distance each second.
► Average
Speed = total distance
total time
Speed
Constant Speed
A uniform distance is covered for each unit of time.
A constant speed graph shows a constant &
positive slope
Velocity
► The
speed and direction of a moving object.
 Constant velocity: an object moves with
constant speed and its direction doesn’t change.
 Changing velocity: Velocity changes when an
objects speed or direction of motion changes
Velocity
Velocity
► Velocity
appears to be very similar to
speed, however, when describing the
velocity of an object you need to provide a
magnitude and a direction
 Magnitude – the speed of the object
 Direction – the direction the object is moving
Difference between Speed & Velocity
► Speed:
That's how fast I am moving
► Velocity: That's my speed and direction
► Motion: Keep moving
► Direction: Which way, anyway
► Acceleration: Let's go faster
Acceleration
► The
measure of the rate of velocity over a
period of time
 An object accelerates if its speed, direction, or
both change
►Speeding
up: velocity increases in the same direction
as its motion (positive acceleration).
►Slowing down: velocity decreases in the opposite
direction as its motion (negative acceleration).
►Changing direction: velocity changes due to direction
Acceleration
Acceleration and Velocity
► As
velocity increases, so does acceleration
► As velocity decreases, so does acceleration
► When direction changes, so does
acceleration
► When there is a constant velocity, there
is no acceleration
Lesson 2: Graphing
Motion
Distance-Time Graph
►A
graph that shows how distance and time are
related
► Y-axis:
shows distance an object travels from
its reference point
► X-axis:
shows how an objects position changes
during each interval
► Can
be used to compare the motion of two
different objects
Distance-Time Graph
Constant: shows a
straight, diagonal line
on the graph
Rest is shown with a
horizontal line
Changing: Shows a
curving line. This
shows acceleration.
Describe the motion in each section of the graph.
Decelerating
Stopped
Steady speed
Accelerating
Speed-Time Graph
► Shows
the speed changes during an interval
► y-axis:
the speed of an object
► x-axis:
shows time
of time
If the graph is a horizontal line, there is no
change in velocity, therefore there is no
acceleration (the slope is 0).
If the acceleration is positive then the
slope is positive (the line moves upward to
the right).
If the acceleration is negative, then the
slope is negative (the line moves downward
to the right).
If you make a graph by
hand it should always
be on graph paper.
The graph should fill the
available space. Carefully
choosing the best scale is
necessary to achieve this.
The graph should always
have a title.
Always label the
x and y axes in 3
ways: title,
numerical
values, and
units.
Lesson 3: Forces
Force
►
A push or a pull on an object
►
Unit of force is the newton (N)
►
Arrows show size & direction of force
►
Non contact force: a force that one object applies to another
without touching it.
 Example: Electrical force (static) causes socks to stick
together when pulled out of the dryer.
►
Contact force: a push or pull one object applies to another
object that is touching it
 Example: a finger pressing a button
Non-Contact Force
►
Gravity: an attractive force that exists between all objects that
have mass (the amount of matter in an object)
►
Gravitational force depends on mass: mass increase =
gravitational force increase
►
Gravitational force depends on distance: the
greater the distance the smaller the force
►
Mass & Weight are different:
 Weight is the measure of the gravitational pull on an objects
mass
 Weight depends on the masses of the objects and the
distance between them
► the
moon has less mass therefore you weigh less on the moon!
Contact Force
► Friction:
a contact force that resists the sliding motion
of two surfaces that are touching
► Rough
surfaces produce more friction, smooth
surfaces produce less friction
► Air
resistance: frictional force between air and objects
moving through it
 Example: crumbling paper into a ball has less
surface area in contact with the air. Therefore,
resistance decreases making it fall faster.
Combining Force
Balanced Forces: forces are equal in
size and opposite in direction
Unbalanced Forces: forces act in
opposite directions, but they are not
balanced
Combining Force
Newton’s Laws of Motion
► Law
1 : if the net force acting on an object is
zero, the motion of the object does not change.
 AKA inertia: the tendency of an object to
resist a change in its motion
Newton’s Laws of Motion
► Law
2 : the acceleration of an object is equal to the
net force exerted on the object divided by the objects
mass
 The greater the mass, the greater the force
 Acceleration (a)= force (f)
mass (m)
Newton’s Laws of Motion
► Law
3 : for every action, there is an equal and
opposite reaction
 When an object exerts a force on a second object,
the second object exerts an equal force of equal
size in the opposite direction
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