chapter 2
accelerated motion
vocab
Sy s t e m a t i c e r ro r, r a n d o m e r ro r, h a l t ,
stationar y, initial, final, steadily, apply,
uniform, gradient, multiflash, free fall, circuit,
magnet, value, scatter, uncertainty, friction, air
resistance, twice, stop watch, parabola, angle,
t h ro w, p ro j e c t i l e , ve r t i c a l , h o r i z o n t a l ,
separately, resolve, cosine, sine, tangent,
graph
velocity is
constant, zero, positive, negative, or changing ?
acceleration is
constant, zero, positive, negative, or changing ?
V
0
t
graph
velocity is constant, zero, increasing, decreasing or
negative?
decreasing
acceleration is constant, zero, positive, negative, or
changing ? constant and negative
V
0
t
chart
velocity
acceleration
constant
zero
positive
negative
v
changing
V
0
t
v
graph
constant
Could you draw:
zero
positive
negative
V
changing
t
velocity
acceleration
v
v
v
v
v
v
v
v
v
measuring acceleration
measuring acceleration
How many measure points we need to find:
velocity,
acceleration
measuring velocity
2 measuring points
V = displacement / time = (s2-s1)/(t2-t1)
S
(t2, s2)
(t1, s1)
t
measuring acceleration
at least 3 measuring points
a = change in velocity / time = (v2-v1)/(t2-t1)
v1 = (s2-s1)/(t2-t1)
v2 = (s3-s2)/(t3-t2)
S
(t3, s3)
(t2, s2)
(t1, s1)
t
measurements using light gates
4 measure points: t1, t2, t3, t4
t1
t2
l1
t4
t3
l2
light gate
average speed at t1 is u
average speed at t3 is v
t1
t2
l1
t4
t3
l2
light gate
u = l1/(t2-t1)
v = l2/(t4-t3)
a = (v-u)/(t3-t1)
t1
t2
l1
t4
t3
l2
measuring using ticker-timer
from slow to fast
measuring using ticker-timer
cut each section
start
slow
fast
measuring using ticker-timer
place them side by side
draw a line from slow to fast
find slope of a line
slope is acceleration
fast
slow
start
equations of motions
introduction
should you run or stop at yellow light?
5 variables
s = displacement
a = acceleration
u = initial velocity
v = final velocity
t = time taken
equations of motions
how to use equations of
motions
find what we know
number for variables u, a, t, v, s
find what we want to know
which variable?
find the right equations of motions
substitude and calculate
problem
A street is 10 m wide. Yellow light last for
about 5 s. You know your top speed is 6m/s.
You stand at one end of the street when
traffic light turn yellow. Should you run or
stop?
problem
A street is 10 m wide. Yellow light last for
about 5 s. You stand at one end of the street
when traffic light turn yellow. How much you
need to accelerate to cross?
a=?
s = 10m
t = 5s
u=0
v - don't care
problem
A street is 10 m wide. Yellow light last for
about 5 s. You stand at one end of the street
when traffic light turn yellow. How much you
need to accelerate to cross?
a=?
s = 10m
t = 5s
u=0
v - don't care
use equation
problem
a = ?, s = 10m
t = 5s, u = 0
v - don't care
10 = a*25/2
a = 2m/s^2
As long as you can accelerate at 2m/s^2 you could
cross the street.
problem
a train accelerates steadily from 4.0m/s to
20m/s in 100s.
a. calculate the acceleration of the train.
b. from its initail and final velocitites, calculate
the average velocity of the train.
problem
a train accelerates steadily from 4.0m/s to
20m/s in 100s.
a. calculate the acceleration of the train.
problem
a train accelerates steadily from 4.0m/s to
20m/s in 100s.
a. calculate the acceleration of the train.
given: u = 4.0, v = 4.0, t = 100,
ask: a = ?
use equation: v=u+at
rearrange equation
problem
a train accelerates steadily from 4.0m/s to
20m/s in 100s.
b. from its initail and final velocitites, calculate
the average velocity of the train.
use equation: v average = (v-u)/2
problem
a train accelerates steadily from 4.0m/s to
20m/s in 100s.
c. calculate the distance travelled by the train in
this time of 100s.
problem
a train accelerates steadily from 4.0m/s to
20m/s in 100s.
c. calculate the distance travelled by the train in
this time of 100s.
know: u = 4.0, v = 4.0, t = 100,
want: s = ?
use answer for part a
use equation: s = ut +at^2/2