Transcript Speed and Velocity

THE NEED FOR SPEED!!!
Motion, Speed, and
Velocity
http://www.youtube.com/watch?v=
dKDDrnx-Fgw
Motion
• When an object changes its position,
motion has occurred.
– Distance- How far an object has moved.
– Displacement- How far an object has moved in
relation to its starting point.
– Consider direction
Example: Two runners travel along the same
straight path in a straight line for 500 meters.
At the end of the run their distances are the
same but their displacements are different.
How can this be so?
Speed- Distance an object
travels per unit of time
Relationships between speed, distance, and
time:
Speed = Distance/ Time = d/ t
» Constant Speed- speed does not change
over time
» Average Speed- speed of motion when
speed is changing
Avg Speed = Total Distance/ Total Time
» Instantaneous Speed- speed at any given
moment in time (speedometer)
Graphing Motion
• Graph
distance
on the yaxis and
time on the
x-axis
Slope = rise = distance = speed
run
time
Distance - Time Graph
• If something is not
moving, a horizontal
line is drawn.
• If something starts out
slow and then speeds
up, its change in
speed can look like
this.
Learning Checkpoint
This graph shows several
stages of motion:
•Stage 1: 100 m in 10 s
•Stage 2: 50 m in 10 s
•Stage 3: 150 m in 20 s
Calculate the speed as indicated by
each of the colors.
Calculate the average speed.
What is the total distance?
What is the displacement?
Solution
Stage 1: S= d/ t
100 m/ 10 s= 10 m/s
Stage 2: S= d/t
50 m/ 10 s= 5 m/s
Stage 3: S= d/t
150 m/ 20 s= 7.5 m/s
Ave Speed= Tot d/ Tot t
300 m/ 40 s= 7.5 m/s
Distance = 300 meters
Displacement = 0 meters
3.1 Calculating speed
-Since speed is a ratio of distance over
time, the units for speed are a ratio of
distance units over time units.
3.1 Calculating speed
• The change in position is a
distance traveled in a
given amount of time.
• To calculate the speed of
an object, you need to
know two things:
– the distance traveled by the
object
– the time it took to travel the
distance
Interpreting a distance versus
time graph
This distance versus time graph shows a boat traveling
through a long canal. The boat has to stop at locks for
changes in water level.
1.
How many stops does it make?
2.
What is the boat’s average
speed for the whole trip?
3.
What is the highest speed the
boat reaches?
1.
2.
3.
The boat makes three stops
because there are three
horizontal sections on the
graph.
The average speed is 10
km/h (100 km ÷ 10 h).
The highest speed is 20
km/h. The position
changes by 20 km in one
hour for the first, third, and
fifth hours of the trip.
Velocity
• The speed and direction of an object’s
motion.
– This car is moving 88 km / hr south
Velocity MUST include a
reference direction!
3.3 Working with Equations
» Equations can also be
rearranged to show how
any one variable depends
on all the others.
If a car moves with an average
speed of 60 km/hr for an hour,
it will travel a distance of 60
km. How far will it travel if it
continues this average rate for
4 hrs?
A runner makes one lap around
a 200 m track in a time of
25.0s. What was the runner's
average speed?
A motorist travels 406 km during
a 7.0 hr period. What was the
average speed in km/hr and m/s?
A bullet is shot from a rifle with
a speed of 720 m/s. What time
is required for the bullet to
strike a target 3240 m away?
What is the average speed
of a cheetah that sprints
100 m in 4 s? How about
if it sprints 50 m in 2 s?
REVIEW
• F=MxA
• Force=Mass X
Acceleration
Review
• A man hits a golf ball (0.2 kg) which accelerates at
a rate of 20 m/s2. What amount of force acted on
the ball?
Review
• You give a shopping
cart a shove down the
isle. A cart is full of
groceries and has a
mass of 18 kg. The
cart accelerates at a
rate of 3 m/s2. How
much force did you
exert on the cart?
Review
• What is the
acceleration of a 18 kg
mass pushed by a 9 N
force?
Review
• Given a force of 88 N
and a acceleration of 4
m/s2, what is the
mass?