conceptual physics notes on chapter 2 motion in one direction
TRANSCRIPT
Conceptual Conceptual PhysicsPhysics
Notes on Chapter 2Notes on Chapter 2
Motion in Motion in
One DirectionOne Direction
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
MotionMotion in one plane / direction in one plane / direction
Motion is relative! Motion is relative! – relative to the Earth, Sun, Galaxy, Universerelative to the Earth, Sun, Galaxy, Universe
All moving objects have All moving objects have speedspeed
Speed is a measure of Speed is a measure of how fast something is how fast something is movingmoving or or rate in which rate in which distance is covereddistance is covered
Conceptual Physics Conceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
Speed Speed is defined as the is defined as the distance distance covered per unit of time.covered per unit of time.
Two types of speed: Two types of speed:
AverageAverage & & InstantaneousInstantaneous– Average speed is the Average speed is the total distance total distance
covered divided by total time covered divided by total time intervalinterval
– Instantaneous speed is the Instantaneous speed is the speed speed you are going at any given timeyou are going at any given time
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
VelocityVelocity
VelocityVelocity is speed in is speed in a given directiona given direction
– SpeedSpeed & & VelocityVelocity are are different in Physicsdifferent in Physics
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
EquationEquation
vv == d d // tt
vv = speed or velocity = speed or velocity
dd = distance = distance
tt = time = time
UnitsUnits
m/s km/s mm/sm/s km/s mm/s
metersmeters
secondsseconds
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
EXAMPLE:EXAMPLE:
If a cheetah can If a cheetah can run a constant speed of 25 m/srun a constant speed of 25 m/s, , how farhow far will it run will it run in 10 secondsin 10 seconds??
vv = 25 m/s = 25 m/sdd = ? = ?tt = 10 s = 10 s
vv == dd // t t
((25 m/s25 m/s) = ) = dd / ( / (10 s10 s))
((25 m/s25 m/s)()(10 s10 s) = ) = dd
d d = = 250 m250 m
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
Constant velocityConstant velocity vs vs Changing Changing velocityvelocity
Constant velocityConstant velocity is constant speed is constant speed and directionand direction
Changing velocityChanging velocity is changing EITHER is changing EITHER speed or directionspeed or direction
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
EXAMPLE:EXAMPLE:The speedometer in a moving southward The speedometer in a moving southward car reads 60 km/hr. It passes another car car reads 60 km/hr. It passes another car moving northward at 60 km/hr. moving northward at 60 km/hr.
Do both cars have Do both cars have
the same speed?the same speed?
Do they have the Do they have the
same velocity?same velocity?
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
AccelerationAcceleration is the is the rate rate
in which velocity in which velocity changes with time.changes with time.
a = Δ v / ta = Δ v / t
Δ means Δ means a change ina change in
UnitsUnits
Δ v = m/sΔ v = m/s t = secondst = seconds a = m/s/s or a = m/s/s or
m/sm/s22
AcceleratioAccelerationn
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FASTFAST
EXAMPLE:EXAMPLE:
Pg 16 Pg 16 Question #2Question #2
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FARFAR
Free Fall : How Far?Free Fall : How Far? Gravity causes objects to fall and Gravity causes objects to fall and
accelerate toward the Earth. Such accelerate toward the Earth. Such objects are said to be in objects are said to be in FREE FALLFREE FALL..
Gravity is 9.8 m/sGravity is 9.8 m/s22
We will use 10 m/sWe will use 10 m/s22
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FARFAR
Recall objects let go start at rest or ZERO Recall objects let go start at rest or ZERO velocityvelocity
a = Δv / ta = Δv / t Δv = atΔv = atif the acceleration is toward the earth, if the acceleration is toward the earth,
a = g : a = g : g is gravityg is gravity which is equal to which is equal to 10 m/s10 m/s22
Δv = Δv = ggtt
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FARFAR
EXAMPLE:EXAMPLE: What would the What would the
speed bespeed be of a rock of a rock be be after 4.5 secafter 4.5 sec after it is after it is dropped dropped from restfrom rest??
After After 8 sec8 sec.?.? After After 15 sec15 sec.?.?
SOLUTION:SOLUTION:
vv = g = gtt
v = (10 m/s/s)(4.5 s) = 45 m/sv = (10 m/s/s)(4.5 s) = 45 m/s
v = (10 m/s/s)(8 s) = 80 m/sv = (10 m/s/s)(8 s) = 80 m/s
v = (10 m/s/s)(15 s) =150 m/sv = (10 m/s/s)(15 s) =150 m/s
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FARFAR
We are trying to find a distance a object falls We are trying to find a distance a object falls and the average of that fall. Remember we and the average of that fall. Remember we drop a(n) object from rest! drop a(n) object from rest!
So we take the start velocity and the end So we take the start velocity and the end velocity and divide by two (2)velocity and divide by two (2)
dd == vvtt but but v = v = gtgt from above from above
dd = = ((gg/2/2 tt))tt or or
d = ½ gtd = ½ gt22
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FARFAR
Example:Example: A apple drops from a tree A apple drops from a tree
and hits the ground in one and hits the ground in one second.second.
What was the speed at What was the speed at impact? impact?
What was the distance the What was the distance the apple fell? apple fell?
What if the apple fell for 3 What if the apple fell for 3 seconds, how far above seconds, how far above the ground was the apple?the ground was the apple?
Conceptual PhysicsConceptual Physics Chapter 2 Chapter 2 HOW HOW FARFAR
Air ResistanceAir ResistanceIn the absence of air, ALL objects fall at In the absence of air, ALL objects fall at the same rate here on Earththe same rate here on Earth
g =g = 10 m/s10 m/s22