3 agenda

1
3 agenda

• lecture
• practicing physics p8, p10
• lab measurement of constant velocity and
  constant acceleration

2
3 linear motion

• speed and velocity
• changing velocity
• distance traveled

3
motion

• measured with respect to Earths surface unless
  otherwise indicated
• unit meters/second m/s
• speed is the scalar of motion
• velocity is the vector of motion

4
speed

• speed rate of travel
• average speed (distance traveled)/time
• speed assumes time is small
• average speed assumes time is large (i.e.,
  speed may vary)
• velocity speed and direction

5
Velocity

• Velocity is speed in a given direction (velocity
  is a vector, speed is a scalar)

• frequently we use to mean upward or
  rightward and - to mean leftward or
  downward

• Note that an object may have constant speed but
  changing velocity

6
Displacement

• displacement change in position
• when velocity is constant (unchanging), the
  displacement vt, where v velocity and t
  time.
• Example velocity -3m/s, time 2s, the
  displacement is vt (-3m/s)(2s) -6m, i.e., has
  moved 6 meters in leftward direction ( - being
  left, being right)

7
Acceleration
SI unit m/s/s
The motorcycle rider experiences an acceleration
in each case shown. What are the directions for
each case?
8
Question

• Can an object have zero velocity but non-zero
  acceleration?

Answer Yes!
Eg. Throw a ball up in the air at the top of
its flight, as it turns around it has momentarily
zero speed but is changing its direction of
motion, so has non-zero acceleration
9
Free fall

• falling under influence of gravity alone (no air
  resistance, etc.)
• a g 10m/s/s
• independent of mass (free-fall)
• from rest v gt.

10
object thrown upward

• slows at a rate of g
• then has zero velocity as it changes its
  direction from up to down.
• then falls speeding up at a rate of g.
• equal elevations have same speed (but opposite
  direction)

11
Free-Fall Distance

• initial velocity 0
• final velocity gt
• average velocity ½ (0 gt) ½gt.
• distance d (average velocity)x(time) d ½gt
  x t ? d ½gt2.
• Example after 3.0 secondsd ½(10)(3)2 5x9
  45 meters

12
Application Hang-time of jumpers

• Michael Jordans best hang-time was 0.9 s
• Round this to 1 s. How high can he jump?
• Use d ½ g t2 . For 1 s hang-time, thats ½ s up
  and ½ s down.
• Substituting ½ 0.5 seconds into the distance
  equation
• d ½ (10) (0.5)2 1.25 m
• This is about 4 feet!

13
Question (to think about)
14
For example Cities are 600 km apart, and
planes airspeed is 300 km/h (relative to still
air). Time each way with no wind is 2 hours.
Round trip time is 4 hours. If a 100 km/h
tailwind is blowing, the groundspeed is 400 km/h
one way and 200 km/h the other. The times are
(600 km)/(400km/h) 1.5 h and
(600 km)/(200km/h) 3.0 h The round trip now
takes 4.5 hourslonger than with no wind at all.
15
3 summary

• speed is the rate of travel
• velocity is speed and direction
• acceleration is the rate of change of velocity
• for free-fall a g 10 m/s/sv gt d
  ½gt2.