# Motion

Document Sample

```					An Introduction To Physics
.

Physics is the study of the
relationships between matter
and energy.
Physics deals with the real world
Versus………
Physics deals with the interactions of
matter and energy, both PE vs KE!!!!
   Beginning Physics is called
Newtonian Physics.
Newtonian Physics
 Named for Sir Isaac Newton, it deals
mainly with the interactions of visible
objects around us
 Physics explains the universe in
mathematical terms.
Motion
MOTION:
a change in position,
measured by distance (also
called DISPLACEMENT) and
time.
REFERENCE POINT
   Reference point:
 thepoint from which movement is
determined, and which is assumed to be
nonmoving itself.
What is motion?

• If you are standing in one place, and your friend
walks by you, are you moving relative to your
friend?
– Is your friend moving relative to you?
– Is either of you moving relative to the earth?

• You are moving relative to your friend, and
your friend is moving relative to you!
• You (the Joker) are not moving relative to
the earth, but your friend is. You are both
moving relative to the sun!
Who is moving
relative to the
computer
screen?
What is motion?

• If you and your friend are walking down the hall
together at the same speed, in the same direction,
are you moving relative to your friend?
– Is your friend moving relative
to you?
– Are either of you moving
relative to the earth?
• You are NOT moving relative to your
friend, and your friend is NOT moving
relative to you. You both are moving
relative to the earth.
Frame of REFERENCE
• EXAMPLE: Tossing a ball
• Describe the motion of the ball."
• "If we were going down the road in a school
bus, me standing in the isle and you sitting in
the seats, would the motion of the ball be
different?"
•   "If someone was standing beside the road as we
passed, what would they see the ball doing?"
Frames of Reference
To measure movement, some point
must be considered as nonmoving.
• Most common is the
earth
• Motion is a change
in position, relative to
a frame of reference
“Hey Bob!!!
…Did I scare Ya?”
• Earth is the most common frame of
reference, however:
•   Earth rotates on its axis at almost 1100
miles/hour.
•   Earth moves around the sun at over 68,000
miles/hour.
•   The whole galaxy is rotating at about 490,000
miles/hour.
•   Is there a universal frame of reference we can
use to define the motions of all things?
Here’s a problem (or two)…..

• If the earth spins on its axis at 1100
miles/hour, what is the speed of the
Earth's rotation in feet per second?
• The earth travels at 68,000 miles/hour as
it moves around the sun. How many miles
does the earth travel in one trip around
the sun?
Speed

• Speed = Distance ÷ Time

S = D/T

Example: A car travels 300km in 6 hours.
What is the speed of the car?
SPEED!!!!
• Instantaneous Speed: the rate of
speed at any particular moment in
time
• Average speed: the speed of moving
objects is not always constant:
– Average speed = total distance / total
time
MORE SPEED!!!!
• Average Speed Practice Problem:
You drive 300 kilometers in 3 hours before
stopping for 30 minutes for lunch and gas.
After lunch you travel 150 kilometers in an
hour and a half. What was your average
speed for the trip?

• Speed = distance ÷ time
• Speed = 450 Km ÷ 5 hours
• Speed = 90km/hr
More practice

• 1. How far can a plane travel if it flies
800km/hr for 9 hours?

• 2. How long does it take a ship to go
500 km if it travels at a speed of
50km/hr?
If: S = D/T

Then: D = S x T

800km ▪ 9hrs = 7200km
hr

And then: (since S = D/T)

T = D/S

500km ÷ 50km = 10 hrs
hr
RANDOM
FAR SIDE!!
Velocity

• Speed in a given direction.
• What is the velocity of a boat that
travels from St. Peter to Mankato
(10 miles) in 15 minutes?

• Speed = distance ÷ time
• Speed = 10 miles ÷ 15 minutes
• Speed = 0.67 mi/min
• Velocity = 0.67 mi/min South

• 0.67mi/min x 60min/hr =

• 40 mi/hr
Distance-time graphs
• On your paper, graph the following:
–   D (m)      T (sec)
0           0
5            7
10          14
15          21
Distance - Time Graph:

• Time is the independent variable and
always plotted on the horizontal axis.
Distance is the dependent variable
and always plotted on the vertical axis.
The slope of this line indicates the
speed.
Was your graph a straight line?

• A distance-time graph which is a
straight line indicates constant
speed.
• In constant speed, the object
does not speed up or slow down.
The acceleration is zero.
SPEED!!!! (one more thing)
• Instantaneous Speed: the rate of
speed at any particular moment in
time
– What device measures instantaneous
speed on you car???
VELOCITY
-- speed in a given direction.

• Speed only gives distance and time.
• Velocity gives distance, time, and the
direction of travel.
Velocity is known as a vector
quantity because it has both
speed and direction.
Consider the flight of an airplane: The first
arrow (“A arrow”) shows the speed and
heading of the plane. The second arrow
(“Another arrow”) shows the speed and
direction the wind is blowing. Since the wind
is changing the speed and direction of the
to determine the actual speed and direction
traveled. If the length of the blue vectors is
drawn to scale, the length of the resultant
vector will indicate the actual velocity of the
plane.

When you go around a curve at a
Constant speed, are you changing
Direction???
More VELOCITY NOTES
• Constant Velocity = constant speed and
constant direction ( the object is moving in a
straight path)
•   Changing velocity means either a decrease or
increase in speed, motion along a curved path,
or both
•   Cars have 3 controls to change velocity:
•       1) gas pedal
2) brake
3) steering wheel
ACCELERATION

• Acceleration: The rate of change in
velocity.

• Acceleration = (Final Velocity) - (Original Velocity) / Time
a = (Vf – Vi) / t
ACCELERATION!!!

• Problem: Suppose a car moving in a
speed each second. First from 35 to
40 km/hr, then from 45 to 50 km/hr.
What is its acceleration?
ACCELERATION!!!
• Problem too: In 5 seconds a car
moving in a straight line increases its
speed from 50 km/h to 65 km/h
while a truck goes from rest to 15
km/h in a straight line
 Which undergoes greater
acceleration?
 What is the acceleration of each
vehicle?
ACCELERATION!!!
• Acceleration also applies to changes in
direction
•      EX: going around a curve at a constant
speed of 50 km/hr, what do you notice or
feel?
•   motion is changing every instant because
velocity is changing every instant
•   YOU ARE ACCELERATING!!!!
ACCELERATION
• Deceleration ( “negative”
acceleration):
• A term commonly used to mean a
decrease in speed.
• Acceleration in a direction opposite to the
direction of travel.
• Deceleration is negative acceleration and
has a negative value to indicate direction.
Graph the following on a distance-
time graph:
•   D (m)       T (s)
0          0
5          1
20         2
45          3
80          4
125         5
0 1 2 3 4 5

• A graph that curves on a
distance-time graph shows that
the object is accelerating
• A graph of acceleration, always
has shape!!
Distance-time graphs

• Describe the motion of the object as
shown in the
graph.
From 0-8 sec,
constant speed:
(25 m/sec);
From 8-12 sec,
no motion;
From 12-16 sec,
acceleration;
From 16-20 sec,
constant speed
Speed-time graphs

• Using the distance-time graph from the
last frame, draw a speed time graph. First
fill in the table below:
Average Speed (m/s)     Time (sec)
____
25           0 to 8
____
0           8 to 12
____
37.5         12 to 20
What does your graph look like?

• Constant speed will be a
horizontal line on a speed time
graph.
• If the speed decreases, the line
will slant down.
• If the speed increases, the line
will slant up.
What do the following speed-time
graphs depict?
Acceleration problem
• A roller coaster’s
velocity at the top of
a hill is 10m/s. Two
seconds later it
reaches the bottom of
the hill with a velocity
of 26m/s. What is
the acceleration of
the roller coaster?

• Acceleration = ∆V/ ∆T
• a = 26m/s – 10m/s
2s
a = 16m/s
2s
a = 8m/s/s or 8m/s2
More acceleration problems

• 1. A car accelerates at a rate of
20mi/hr/s. How long does it take to reach
a speed of 80 mi/hr?
• 2. A car travels at 60 miles per hour
around a curve. Is the car accelerating?
• 3. A car travels in a straight line at
60mi/hr. Is the car accelerating?

1. 4sec = t
2. yes! Because it’s changing direction!
3. no! It’s not changing speed or direction!
• Acceleration = ∆V/ ∆T
• Acceleration = Vf – Vi
t
• a = 20mi/hr – 60mi/hr
4s
a = -40mi/hr
4s
a = -10mi/hr/s
Review: Distance-time graph of
acceleration
Distance-time graph of deceleration
Review:Speed-time graph of
acceleration
Review: Speed-time graph of
deceleration
Review: Distance-time graph of
constant speed
GO……………….
Momentum
 Momentum     = Mass x Velocity
 Which has more momentum:
a 300lb football player moving
at 5m/s or a 200lb
quarterback moving at
10m/s?
 Momentum of the 300lb player is
300lbs x 5m/s = 1500lb-m/s
 Momentum of the quarterback is
200lbs x 10m/s = 2000lb-m/s
 The quarterback has a greater momentum!
Momentum problems
   2 cars are heading east, car A is
traveling 30mi/hr, car B is traveling
60mi/hr. Each car weighs 2000lbs.
 What is the momentum of car A?

 What is the momentum of car B?

 If car B crashes into car A, what is
the total momentum?
   P=mv
   Car A’s momentum = 30mi/hr x 2000lbs
PA = 60,000 mi-lbs/hr east
   Car B’s momentum = 60mi/hr x 2000lbs
PB = 120,000 mi-lbs/hr east
   Total momentum = PA + PB
= 60,000 + 120,000
= 180,000 mi-lbs/hr east
Inelastic collision
Elastic collision
Two dimensional collision
Another momentum problem!
   Car X is traveling 30mi/hr east, car Y is
traveling 60mi/hr west. Each car
weighs 2000lbs.
 What is the momentum of car X?

 What is the momentum of car Y?

 If car X crashes into car Y, what is
the total momentum?
   P=mv
   Car X’s momentum = 30mi/hr x 2000lbs
PA = 60,000 mi-lbs/hr east
   Car Y’s momentum = 60mi/hr x 2000lbs
PY = 120,000 mi-lbs/hr west
   Total momentum = PY - PX
= 120,000 - 60,000
= 60,000 mi-lbs/hr west
Which has more momentum?

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 11 posted: 8/9/2012 language: English pages: 75