Chapter 12 Chapter 12 What is Motion? What is Motion? Describing Motion Frame of Reference Motion Speed & Velocity Acceleration Momentum
Dec 18, 2015
Chapter 12Chapter 12What is Motion?What is Motion?
Describing Motion Frame of Reference Motion Speed & Velocity Acceleration Momentum
MotionMotion
Problem: Is your desk moving?
We need a frame (point) of reference...a nonmoving point from which
motion is measured
MotionMotion
MotionChange in position in relation to
a point of reference.
Reference point
Motion
MotionMotion
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.
SpeedSpeed
Speed rate at which an object moves distance traveled per unit time
time
distancespeed
sd
t
SpeedSpeed
Instantaneous Speedspeed at a given instant
•ex - the car is going 35 mph right now
Average Speed The average speed of the entire journey
time total
distance totalspeed avg.
VelocityVelocity
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!
VelocityVelocity
Velocityspeed in a given directionVelocity can change even when
the speed is constant!
Calculations of Calculations of Speed & VelocitySpeed & Velocity
Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster?
GIVEN:
d = 100 m
t = 20 s
s = ?
WORK:
s = d ÷ t
s = (100 m) ÷ (20 s)
s = 5 m/s
You skate faster!vd
t
Calculations of Calculations of Speed & VelocitySpeed & Velocity
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:
v = 330 m/s
d = 1km = 1000m
t = ?
WORK:
t = d ÷ v
t = (1000 m) ÷ (330 m/s)
t = 3.03 s
vd
t
Graphing Speed & VelocityGraphing Speed & Velocitydistance vs. time graphthe slope is = tospeed steeper slope meansfaster speedstraight line meansconstant speedflat line meansno motion
Distance-Time Graph
A
B
Graphing Speed & VelocityGraphing Speed & Velocity
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
Distance-Time Graph
A
B
0
100
200
300
400
0 5 10 15 20
Time (s)
Dis
tan
ce (
m)
Distance-Time Graph
Graphing Speed & VelocityGraphing Speed & Velocity
Acceleration is indicated by a curve on a Distance-Time graph.
Changing slope = changing velocity
AccelerationAcceleration
Acceleration the rate of change of velocitychange in speed or direction
t
vva if
a: acceleration
vf: final velocity
vi: initial velocity
t: time
a
vf - vi
t
AccelerationAcceleration
Positive acceleration “speeding up”
Negative acceleration “slowing down”
Acceleration CalculationsAcceleration CalculationsA 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:
vi = 10 m/s
t = 3 s
vf = 32 m/s
a = ?
WORK:
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2a
vf - vi
t
Acceleration CalculationsAcceleration CalculationsHow long will it take a car traveling 30 m/s
to come to a stop if its acceleration is - 3 m/s2?
GIVEN:
t = ?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
WORK:
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
t = -30 m/s ÷ -3m/s2
t = 10 sa
vf - vi
t
Graphing AccelerationGraphing Acceleration
0
1
2
3
0 2 4 6 8 10
Time (s)
Sp
ee
d (
m/s
)
Speed-Time Graph
slope =
straight line =
flat line =
acceleration +vel = speeds up -vel = slows down
constant accel.
no accel. (constant velocity)
Graphing AccelerationGraphing Acceleration
0
1
2
3
0 2 4 6 8 10
Time (s)
Sp
ee
d (
m/s
)
Speed-Time GraphSpecify the time period
when the object was... slowing down
5 to 10 seconds speeding up
0 to 3 seconds
moving at a constant speed 3 to 5 seconds
not moving 0 & 10 seconds
MomentumMomentum
All moving objects have momentum Momentum depends on the mass of the
object and the velocity with which it is traveling
Formula for calculating momentum
momentum = mass x velocity
m
p
v
Conservation of MomentumConservation of Momentum
The total momentum of any group of objects remains the same unless outside forces act on the objects.
Ex. 2 billiard balls hit into one another.
The total momentum of the 2 balls before they collide is the same as the total momentum of the two balls after they collide.
Momentum CalculationsMomentum CalculationsWhat is the momentum of a 0.3 kg bluejay
flying at 17 m/sec?
GIVEN:
p = ?
v = 17 m/s
m = 0.3 kg
WORK:
p = m x v
p = (0.3 kg)x(17 m/s)
p = 5.1 kg x m/sec
mp
v