Transcript speed, velocity, chapt 2

MOTION
Do you have to see something move to
know that motion has taken place??
All you need to know is
that its position has
changed!
Motion occurs when an
object changes its
position RELATIVE TO
A REFERENCE POINT.
The dog’s position
has changed relative
to the tree
(reference point) so
you know motion must
have occurred.
After you choose your reference point,
you can create what is called a FRAME
OF REFERENCE
This is basically just a GRAPH!!
Y axis
X axis
DESCRIBING MOTION
1. DISTANCE
• Describes how far something has
moved.
QUICK REVIEW: Distance is
measured in what unit when you
use SI??
DESCRIBING MOTION
DISTANCE V/S DISPLACEMENT
Distance is HOW FAR SOMETHING
HAS TRAVELED from the reference
point (starting position)
Whereas
2. Displacement is DISTANCE and
the DIRECTION from the reference
point (starting position) something is.
EXAMPLE
Total Distance
Traveled =80 m
Displacement =
20 m North
DESCRIBING MOTION
3. Speed = How far something
travels (distance) in a given amount
of time
Speed (in meters/second) = distance (in meters)
time (in seconds)
(You could also state speed in km/h or
km/s or cm/yr)
EX. OF A SPEED PROBLEM
A car traveling at a constant speed
covers a distance of 750 m in 25 s. What
is the car’s speed?
Important info: Distance (d) = 750 m
Speed = d/t
Time (t) = 25 s
Speed = 750 m
25 s
= 30 m/s
Different Types of Speed
1.Constant Speed = when the speed hardly
changes. Like when you use cruise control
on your car.
2.Changing Speed = when the speed changes
over a given distance. Like when you ride
a bike or drive through downtown.
3.Average Speed = this describes speed of
motion when you have changing speed. It
is the total distance traveled divided by
the total time traveled.
Different Types of Speed
4. Instantaneous Speed = The speed at
a given point in time. Like going from
0 km/h to 60 km/h. Your speedometer
shows you your instantaneous speed.
-Instantaneous Speed changes when
you increase or decrease your speed
-Instantaneous Speed stays the same
when your speed is constant
How would these different types of speed look
if your graphed them?
time
Distance
Distance
CONSTANT SPEED
CHANGING SPEED
time
GRAPHING MOTION
Use a DISTANCE/TIME GRAPH
-Time = X axis
-Distance = Y axis
A graph of constant speed will represent
the motion with a straight line
To show two constant speeds
which are different, you use
SLOPE = steepness of the line
= speed
Steeper the slope = faster
the speed
GRAPHING MOTION
A graph of changing speed will have a zigzag or curved line
Putting it all together
FASTEST CONSTANT SPEED
DISTANCE
SLOWER CONSTANT
SPEED
What is the slope of this part
of the line?
CHANGING SPEED
TIME
Putting it all together
What is the average speed (slope)
during the 10 min to 20 min period?
1000
600
200
0
Change in distance
= 1200-600 = 600
Amount of time =
20-10 = 10 min
Avg. Speed = change in distance (600 m)
10
20
30= 60 m/min
Amount of time (10 min)
Putting it all together
What is the instantaneous
speed at t= 10 min? (it is the
slope of the line at that point)
1000
600
600m – 0m = 600 m
10min – 0min = 10min
600m/10min
= 60m/min
200
0
10
20
30
VELOCITY
Includes the SPEED of an object and the
DIRECTION it is moving
-Velocity of an object can change even
though the object’s speed doesn’t –
how??
-The velocity changes if the direction
changes even though the speed is
constant
How are Velocity and Speed
different??
Velocity includes
direction/speed doesn’t!
Example Problem with Velocity
Describe the velocity of an object that travels
north 6.9 m in 3 s, then turns and travels
south 2.8 m in 4 s.
Work this just like
you did for finding
average speed, but
now you include
direction!
6.9 m / 3 s = 2.3m/s
NORTH
Then
2.8 m / 4 s =
0.7m/s SOUTH
ACCELERATION
= Rate of change of velocity
-When velocity changes = object
is accelerating
-So: Acceleration occurs when an
object changes speed, changes
direction, or changes both speed and
direction
Acceleration due to a Change in
Speed
-Increasing Speed = Positive
Acceleration
-Decreasing Speed = Negative
Acceleration (Deceleration)
Graphing Positive and Negative
Acceleration
Time
Speed
Speed
Positive Slope =
Positive Acceleration
= like going downhill
(speeding up)
Negative Slope =
Negative
Acceleration = like
climbing a hill
(slowing down)
Time
Graphing Zero Acceleration
Zero Slope = Zero
Acceleration
Speed
Time
This means No
change in
Velocity = No
change in Speed
or direction
Putting them all together
Speed
0A
-A
+A
Time
Calculating Acceleration
Acceleration (A) m/s2 = Change in Velocity (m/s)
Final Velocity = Vf
Time (s)
Initial Velocity = Vi
A (m/s2) = Vf-Vi (m/s)
Time (s)
Calculating Acceleration
If there is no direction change and only
speed changes, then a change in velocity =
change in speed so you use:
A (m/s2) =Final speed – Initial Speed
Time
Example Problems
1. A plane on a runway reaches 80 m/s in
20 s. What is the planes acceleration?
Important Info: Final Speed = 80m/s
Time = 20 s
A = Vf-Vi
t
= 80m/s – 0m/s = 80 = 4m/s2
20s
20
Example Problem
A skateboarder is going 3m/s and stops in
2 seconds.
Important Info: Final Speed = 0m/s
Initial Speed = 3m/s
Time = 2 s
A (m/s2) = Vf-Vi = 0m/s – 3m/s = -1.5 m/s2
t
2s
Working Backwards
The acceleration of a car that comes to a stop
in 5 seconds is -4m/s2. What was the car’s
speed when it started accelerating
(decelerating)?
A=
-4m/s2
5s X
-4m/s2 = 0 – Vi
5s
t = 5s
A = Vf-Vi
t
-20 m/s = -Vi
20 m/s = Vi
X 5s
Get your book: Work these
problems!
pg. 51 # 6-7
Force
= A push or pull
-Can cause the motion of an object to
change (so it can change velocity)
EX: kicking a soccer ball, playing pool,
hitting a tennis ball
Does the Atmosphere have force? Do you
feel it?
Does gravity have force? Do you feel it?
Net Force
-When two or more forces act on an
object at the same time, the forces
combine and we call this NET FORCE
Balanced Forces
-Forces on an object that are equal in
size and opposite in direction
-The Forces cancel each other out
-Net Force = 0
Unbalanced Forces
-When two or more unequal forces act on an
object
Situation #1
-Forces are unequal and in opposite directions
-Object is moved in the direction of the larger
force
-Net force = difference between the two
forces
Unbalanced Forces
Situation #2
-Forces are applied in the same direction
=Forces are combined (added together)
-Net force = add two forces together
Inertia
= The tendency of an object to resist a change
in its motion
-The velocity (speed and direction) of an
object remains constant unless a force
changes it
This said, if an object is
at rest, it should stay at
rest unless a force causes
it to move. Time to test
this out.
Inertia and Mass
-The greater an object’s mass = The
greater its inertia
(the more it resists moving)
DUH! It takes more force to stop a
bowling ball by swatting it with a
paddle than it does to stop a tennis
ball!
Newton’s First Law of Motion
-An object moving at a constant
velocity(constant speed and direction) keeps
moving at that velocity unless an unbalanced
net force acts on it.
-AKA: The law of Inertia
Two students push on a box in the same
direction, and one pushed in the opposite
direction. What is the net force on the box if
each pushes with a force of 50 N.
50N
Subtract the opposite force
from your combined forces
50N
50N
Add your combined
forces (50 + 50 = 100)
Net Force = 100 N – 50 N
= 50 N in the direction the
two students are pushing
The downward force of gravity and the
upward force of air resistance on a ball
are both 5N. What is the ball’s
acceleration? (Remember acceleration =
change in velocity divided by time)
5N
Net force = Difference between the
two = 5 N – 5 N = 0
So is there any change in velocity
(speed or direction of the object)?
5N
NOPE. Acceleration = 0 m/s2