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 Earth’s 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
Acceleration =
change of velocity
time interval
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 seconds: d = ½(10)(3)2 = 5x9 = 45 meters

12. Application: “Hang-time” of jumpers
Michael Jordan’s 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, that’s 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 plane’s 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 hours—longer 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/s v = gt d = ½gt2.