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Linear Motion

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Title: Linear Motion


1
Linear Motion
  • Mr. Callan
  • Physics
  • Assignment Read Chapter 2
  • Do all even questions on pages 25-27 (except
    activities) chapter review.
  • Do problems 1-8 in Problem-Solving work book
    on pages 1-14 (WB 1-14)

2
Equations
3
QuizHow Fast are You Moving When You Are Sitting
Still?
  • What is your observation?
  • What do you need to know?

4
Hint 1
  • At the equator the surface of the earth rotates
    around the center at about 1000 mph
  • At our latitude the movement is about 650 mph.

5
Hint 2
  • You are revolving around the sun at about 66
    thousand miles per hour,

6
Hint 3
  • You are traveling around the center of the Milky
    Way galaxy at about 2 million miles per hour.

7
Linear Motion
  • Motion in a straight line. 1 dimensional
  • Everything is in motion.
  • Motion is a rate.
  • Rate is a quantity divided by time.
  • Since everything moves, motion is relative not
    absolute. A space shuttle in orbit moves at about
    8 km/sec.
  • But it also move very slow.
  • Review How fast is this in miles/hour?

8
Speed
  • A measure of distance / time.
  • The rate could be mi/hr, km/sec, ft/sec etc.
  • Definition Speed is the distance covered per
    unit of time. Per means divided by.

9
Instantaneous Speed
  • A car does not move at one speed. It changes from
    zero to its maximum speed.
  • Your speedometer measures the cars instantaneous
    speed.
  • It is the speed at any one point in time.

10
Average Speed
  • Average speed is used to plan trips.
  • It is the total distance traveled divided by the
    time it took to travel the distance.
  • Average Speed total distance/time interval

11
Velocity
  • Velocity and Speed are used interchangeably in
    everyday language.
  • But THEY ARE NOT THE SAME.
  • Velocity is speed in a given direction.
  • Such as 60 mile per hour North.

12
Constant Velocity
  • No change in speed
  • No change in direction
  • Motion is a straight line

13
Changing Velocity
  • Speed could be changing
  • OR
  • Direction could be changing.
  • OR Both

14
Acceleration
  • The rate of change of velocity.
  • The change in velocity/time interval.
  • OR the rate of change of speed.
  • The change in speed/time interval.
  • deceleration is the acceleration in the negative
    direction.

15
Additional Assignment
  • Always look at the questions in the yellow boxes.

16
2.5 Free Fall How fast
  • An apple falls from a tree. Does it accelerate
    while falling?
  • What do we know?
  • It starts from rest (velocity and acceleration
    0) then gains speed.
  • The gain in speed indicates that the apple does
    indeed accelerate.

17
Free Fall Cont.
  • Why does the apple accelerate? -Gravity
  • In this course we generally neglect air
    resistance (friction).
  • Therefore, we say that the apple is freely
    falling or is in free fall.
  • Objects in free fall increase their their
    velocity by about 10 m/s each second.
  • What does this mean?

18
Free Fall
  • a D v/ Dt 10 m/s per sec. or 10 m/s/s
  • For objects in free fall a g
  • More accurately g 9.8 m/s2
  • Since g a D v/ Dt
  • D v g Dt for an object starting at rest the
    the initial velocity (vi) zero, so the
    equation becomes v g Dt .
  • If the initial time interval (ti) is set to zero,
    the the equation becomes vgt

19
Final Velocity with Initial Velocity
  • If someone throws a ball, it has an initial
    velocity. Vi initial velocity
  • To calculate the final velocity Vf, the initial
    velocity is added into the equation.
  • Vf g Dt Vi
  • Lets say a ball is thrown straight down at 2m/s
    from the top of Sear Tower. How fast is it going
    after 3 seconds?

20
2.6 Free Fall How far?
  • An object falling from rest is moving at 10 m/sec
    at one sec. But what is its average velocity?
    10/2
  • Time (sec) Instant Speed (m/s) Ave. Speed
  • 0 0 0
  • 1 10
  • 2 20
  • 3 30
  • 4 40

21
Free Fall How Far
  • The average velocity is the velocity we need for
    finding the distance. (vivf)/2vave
  • d 1/2 gt2 Look at the units!
  • meters1/2 m/(sec x sec) x sec sec

22
Free Fall How far?
  • Time (sec) Instant Ave. Distance
  • 0 0 0 0
  • 1 10 5 5
  • 2 20 15 20
  • 3 30 25 45
  • 4 40 35 1/2gt2

23
Reaction Time
  • A bill is 15.5 cm long so if your table mate
    places his/her thumb and finger over the center
    and you let go of the bill can your table mate
    catch the bill?
  • No, the bill falls 8 cm in 1/8 of a sec. Your
    reaction time is much greater.

24
Lets Measure your Reaction Time
  • Lab 2
  • Find One and only one partner.

25
Hang Time
  • Your hang time is related to how high you jump
  • d ½ gt2
  • Solve the vertical distance equation for time and
    then have your tablemate measure how high you can
    jump.

26
Graph Matching
27
2.8 Air Resistance Falling Objects
  • http//www.hq.nasa.gov/alsj/a15/a15v.1672206.mov

28
Equations and Units
  • vat, av/t, and ta/v
  • vgt, gv/t and t v/g
  • d1/2at2
  • Reaction time (2d/a)1/2 , where ag9.8m/s2

29
Chapter Review Due Thursday
  • Do the even problems on pages 25-27
  • WB Wednesday
  • Test Friday
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