Transcript II. Describing Motion

Ch. 9
Motion
Describing Motion
Motion
 Speed & Velocity
 Acceleration

Newton’s First Law
 Newton’s
First Law of Motion
 An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net force
force.
A. Motion
 Problem:
 Is your desk moving?
 We
need a reference point...
 nonmoving point from which
motion is measured
A. Motion
 Motion
 Change in position in relation to
a reference point.
Reference point
Motion
A. Motion
Problem:
 You are a passenger in a car
stopped at a stop sign. Out of the
corner of your eye, you notice a
tree on the side of the road begin
to move forward.
 You have mistakenly set yourself
as the reference point.
B. Speed & Velocity
 Speed
d
 rate of motion
v t
 distance traveled per unit time
distance
speed 
time
B. Speed & Velocity
 Instantaneous
Speed
 speed at a given instant
 Average
Speed
total distance
avg. speed 
total time
B. Speed & Velocity
 Problem:
 A storm is 10 km away and is
moving at a speed of 60 km/h.
Should you be worried?
 It depends
on the
storm’s
direction!
B. Speed & Velocity
 Velocity
 speed in a given direction
 can change even when the
speed is constant!
C. Acceleration
vf - vi
a t
 Acceleration
 the rate of change of velocity
 change in speed or direction
a
v f  vi
t
a:
vf:
vi:
t:
acceleration
final velocity
initial velocity
time
C. Acceleration
 Positive
acceleration
 “speeding up”
 Negative
acceleration
 “slowing down”
D. Calculations
Your neighbor skates at a speed of 4 m/s.
You can skate 100 m in 20 s. Who skates
faster?
GIVEN:
WORK:

d = 100 m
t = 20 s
v=?
d
v t
v=d÷t
v = (100 m) ÷ (20 s)
v = 5 m/s
You skate faster!
D. Calculations
Sound travels 330 m/s. If a lightning bolt
strikes the ground 1 km away from you,
how long will it take for you to hear it?
GIVEN:
WORK:

v = 330 m/s
t=d÷v
d = 1km = 1000m
t = (1000 m) ÷ (330 m/s)
t=?
t
=
3.03
s
d
v t
D. Calculations
A roller coaster starts down a hill at 10 m/s.
Three seconds later, its speed is 32 m/s.
What is the roller coaster’s acceleration?
GIVEN:
WORK:

vi = 10 m/s
t=3s
vf = 32 m/s
vf - vi
a=?
a t
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2
D. Calculations
How long will it take a car traveling 30 m/s
to come to a stop if its acceleration is
-3 m/s2?
GIVEN:
WORK:

t=?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
vf - vi
a t
t = -30 m/s ÷ -3m/s2
t = 10 s
E. Graphing Motion
Distance-Time Graph
A
B

slope = speed

steeper slope =
faster speed

straight line =
constant speed

flat line =
no motion
E. Graphing Motion
Distance-Time Graph
A



B

Who started out faster?
 A (steeper slope)
Who had a constant speed?
 A
Describe B from 10-20 min.
 B stopped moving
Find their average speeds.
 A = (2400m) ÷ (30min)
A = 80 m/min
 B = (1200m) ÷ (30min)
B = 40 m/min
E. Graphing Motion
Distance-Time Graph
400

Acceleration is
indicated by a
curve on a
Distance-Time
graph.

Changing slope =
changing velocity
Distance (m)
300
200
100
0
0
5
10
Time (s)
15
20
E. Graphing Motion
Speed-Time Graph
3

slope = acceleration
 +ve = speeds up
 -ve = slows down

straight line =
constant accel.

flat line = no accel.
(constant velocity)
Speed (m/s)
2
1
0
0
2
4
6
Time (s)
8
10
E. Graphing Motion
Speed-Time Graph
Specify the time period
when the object was...
 slowing down
 5 to 10 seconds
 speeding up
 0 to 3 seconds
3
Speed (m/s)
2

1
0
0
2
4
6
Time (s)
8
10

moving at a constant
speed
 3 to 5 seconds
not moving
 0 & 10 seconds
VECTOR:



Vectors measure using arrows to show direction
and magnitude.
Shows direction of the object's motion.
 Ex: velocity, acceleration, force. (all involve a
direction)
Can you build the perfect paper airplane?







Principles:
A glider moves through the air without
the help of a motor or engine.
A glider can move through the air and
descend gradually when it is well
designed and built.
Think about aerodynamics to build a
glider that will fly well. "Drag" "Thrust"
"Lift" "Gravity“
Facts:
The design of your glider's body and
wings has a lot to do with how well it
will sail in the air.
Adding some weight to parts of your
glider will help it stay up in the air,
have lift, and travel in a straight path
instead of spinning or nosediving.