Speed, velocity and acceleration
Physics 100 Chapter 2
speed
speed = v =distance traveled
elapsed time
50km
v = 50km1 hr = 50km/hr
Hawaii Kai HaleiwaIn one hour
This is the averagespeed over 1 hour.For shorter time intervals it can be higher or lower.
Earth’s motion around the Sun
r=1.5x1011m
V = distance
elapsed time = 2 r 2 x 3.14 x 1.5 x 1011m
365 days x 24 hrs/day =
1year
= 9.4 x 1011 m8760 hrs
=9.4 x 1011 m
8.76 x103 hrs= 1.1x108 m/hr
= 1.1x105 km/hr 70,000 mi/hr
Velocity = speed + direction
r=1.5x1011m
6 months laterspeed = same
different directionv
velocity is a “vector”: a quantity that has both magnitude and direction
Length of the arrow = speedDirection of arrow same asdirection of the motion
Scalars and Vectors
Simple numbers:
Speed vTemperature T
Number + direction
Velocity vrelative positions rForce FAcceleration a
PSB
r CampusCenter
Acceleration ( changes in v)
acceleration =change in velocity
elapsed time
a =change in velapsed time
Change in V = 100km/hr
Elapsed time = 3 sec
“This baby goes from 0 to100km/hr in only 3 seconds”
a =change in velapsed time
=100km/hr
3 s = 33 km/hr s
103 m
3600 s=3.6x103s
=33x103m
3.6x103 sxs = 9.1 m/s2
Different ways to change V
v v
Car speeds up
v
Car slows up
vscreech!
a
a
Accelerations (continued)
v
Car turnsv
In all three cases, v changes. Therefore these are all examples of accelerations
a
Free Fall
5m
t=0 v0=0
t=1s v1=?
vavg = disttime
5m1 s
= = 5m/s
vavg = v0 + v1
2
0 + v1
2=
v1
2=
v1 = 2vavg = 10 m/s
V1 = 10 m/s
Free-fall acceleration
acceleration =change in velocity
elapsed time
a = 10m/s1s = 10 m/s2
10m/s
1s
This is called the “acceleration due to gravity” and given the special symbol:
g=10m/s2
Actually g=9.8m/s2. But 10 m/s2 is close enough for us.
g
Free fall from greater heights
V0 = 0
t = 0s
V1 = 10m/s
t = 1s
5m
V2 = 20m/s
t = 2s
V3 = 30m/s
t = 3s
V4 = 40m/s
t = 4s
15m
25m
35m
5m
20m
45m
80m
Total distance
12
gt2
Upward tossV4 = 0t = 4s
V3 = 10m/st = 3s5m
V2 = 20m/st = 2s
V1 = 30m/st = 1s
V0 = 40m/st = 0
15m
25m
35m
75m
60m
35m
0m
Total height
80m
gt212
v0t-
Simple rule for free fallaka: projectile motion
When Earth’s gravity is the only force involved:
actual height = height for no gravity – ½gt2
Horizontal tosst = 0s
t = 1s
5m
t = 2s t = 3s t = 4s
20m
45m
80m
upward toss
t = 0s
t = 1s
5m
t = 2s
t = 3st = 4s
20m
45m80m
Shoot the monkeycommunistdead whiteEuropean male
Shoot the DWEM: Slower bullet
Very fast horizontal toss
t = 0s
t = 1sx= 8km
5m
t = 2sx=16km
t = 3sx=24km
20m 45m
V=8km/s
Orbital motion is free fall
Artificial satellite
Turning car
An object free to slide on the dashboard,tries to follow a straight line path