Download - Cross Roads ISD

Primetime
 What do we use to measure distance and time?
 Name the steps in the scientific method.
TEKS 2B,C, 4A,B,C,E
Motion
 A mass or masses moving in different directions
with different speeds is motion
 One dimensional motion is the simplest form of
motion (object only travels in one direction); ex:
train traveling on the tracks
 Motion takes place over time and depends on the
frame of reference; ex: while the train moves on
the tracks, the earth is spinning on its axis and it’s
rotating around the sun
Frame of Reference
 When modeling a situation like the train moving,
choose a simple frame of reference like the two
stations the train is traveling between
 You can choose any frame of reference you want , but
you must be consistent
Primetime
 If you were to walk from here to the baseball field
would your displacement and distance be the same?
Why?
 If you were to walk to the lunch room and back would
your displacement and distance be the same? Why?
Distance vs. Displacement
 Distance is how far an object travels in total
 Displacement: length of the straight line drawn from
object’s initial position to its final position; how far the
object is from its starting point
 Ex: A gecko was initially at 22cm and moved to 85cm.
What was the gecko’s displacement?
Gecko’s Displacement
 xi = initial position
 xf = final position
 ∆x = displacement
 ∆ x = xf – xi
 ∆ x = 85cm – 22cm = 63cm
 Displacement is NOT always equal to distance
traveled; ex: room walk
Displacement
 Displacement can be positive or negative
 Sign tells you the direction
 Positive:object is moving to the right
 Negative: object is moving to the left
 Ex: Table 2-1
Velocity
 Velocity is like speed but also has a direction
associated with it
 Average velocity is displacement over time or d/t
 Another way to write the formula is as follows:
Formula Average Velocity
 vavg = average velocity
 ∆ x = displacement
 ∆ t = change in time
 vavg = ∆ x/ ∆ t = (xf – xi)/(tf – ti)
 Average velocity can be positive or negative and tells us
the direction
 Time will NOT be negative, ever
Velocity Example
 You travel to a friend’s house 370km to the west. If you
left your house at 10am and arrived at 3pm, what was
your average velocity?
Example, cont.
 Known
 Unknown
 xi = 0km
 ?vavg
 xf = 370km west
 ti= 10am
 tf = 3pm
 vavg = ∆ x/ ∆ t =
(xf – xi)/(tf – ti)
 vavg = (-370km – 0km)/(5hrs)
 vavg = -370km/5hr
 vavg = -74km/hr or 74km/hr west
Primetime
 Find the velocity of a person that traveled 5m North,
8m South, and 10m North in 64seconds.
Sample Problem 2A
 Known
 Unknown
 ∆ t = 137s
 ?∆x
 vavg= 6.02m/s east
 vavg = ∆ x/ ∆ t so
rearranging to get ∆ x by
itself we get
 ∆ x = vavg ∆ t
Sample, cont.
 ∆ x = (6.02m/s)(137s)
 Seconds cancel out because one is on top of fraction
and other on bottom, leaving meters
 ∆ x = 825m east
Speed vs. Velocity
 Velocity and speed are not the same
 Speed only has a magnitude (value)
 Velocity has both a magnitude and direction
 Velocity can be found graphically as on figure 2-5 in
your book
Instantaneous Velocity
 Defined as the exact velocity at a specific time
 Instantaneous velocity may not be the same as average
velocity
Assignment
 Practice 2A p.44 1-6
 Section Review p.47 1-6