Post on 02-Jan-2016
transcript
Motion and ForceChapter 10-11
Motion
What are some examples of Motion?
Large scale Small scale Fast vs. slow Frame of reference?
What is motion?
Objects change in position relative to a reference point.› Reference point should be stationary
Motio
n
Dista
nce
D
ispla
cem
ent
Total length which an object moves
Can be a straight line, but doesn’t have to be
Doesn’t have a particular direction
Change in position of an object Straight line from the starting point to the
final point MUST BE A STRAIGHT LINE!!! Shorter or equal to distance, never more Must have a particular direction
(toward/away)
Example 2A+B= DISTANCEC = DISPLACEMENT
John’s house
Jane’s house
School
A
C
B
How do we determine the rate of movement?
Speed – the distance traveled in a given amount of time
To calculate speed, must know both distance and time› Speed = distance/time speed = d/t› (SI Units)
Distance – meters (m) Time – seconds (s) m/s
CHECKER AUTO PARTS 500
SPEED = DISTANCE
TIME
DISTANCE = 315 laps
TIME = 3 hoursSPEED = ?
Track length = 1 mile
V
s
t
Constant speed – object covers equal distances in equal amounts of time (doesn’t change)› Example: earth’s rotation
Most objects don’t have constant speed
Distance – Time Graph
X-axis – independent variable Y-axis – dependent variable Time is usually independent b/c it will pass
whether distance is traveled or not Constant speed is a straight line The slope (rise/run) give the speed of the object Steeper the slope, the faster the speed
Average speed = distance / time Instantaneous speed – speed at that
very instant› Example - speedometer
Constant speed – doesn’t change› Example – Earth’s rotation
Velocity
Speed in a certain direction Used for navigation, weather Direction can be N, S, E, or W of a fixed
point Negative or positive along the line of
motion Velocity = displacement (m) / time (s) SI units
› m/s
The speed of these racers may not have changed… Explain why their velocity has changed.
Can add 2 velocities that are going same direction to get resultant velocities
If moving in opposite directions, you subtract to get the resultant velocity
V
d
t
Acceleration
Any change in velocity
Change in direction causes acceleration› Examples – moon,
race track
Positive acceleration – object is speeding up› Examples: a car goes from 0mi/hr to 60
mi/hr in 3 seconds. Negative acceleration – object is
slowing down› Example: skier stopping from 20m/s in .5s
Acceleration can be determined by the change in velocity over a change in time
ACCELERATION = FINAL VELOCITY – INITIAL VELOCITY
TIME
a = (vf - vi)/(tf - ti) SI units:
› (m/s)-(m/s) / s = m/s2
ZERO ACCELLERATION Identify two conditions
that must be met for these joggers to be at zero acceleration.
1. No change of direction.2. No change
(increase/decrease) in speed.
Acceleration can be determined by a velocity-time graph› The slope of the line gives you the value of
acceleration› Positive slope – object is speeding up (positive
acceleration)
› Negative slope – object is slowing down (negative acceleration)
› Horizontal line – velocity is not changing, thus the acceleration is 0m/s2
Force
Force – an action which changes an objects state of rest or motion› Has magnitude and direction
Examples of FORCES:› Gravity› Friction › Engines
Net force – combination of all the forces acting on an object› If net force =0, the object is balanced
Doesn’t move› If net force is unbalanced, object
accelerates in the direction which force is greater
Friction
Friction – force that acts against a motion in progress› Constant force has to be applied to an
object to keep it moving Example: car will eventually stop, if gas is
not applied› Friction also affects stationary object
Example: truck parked on hill – friction of brakes provides a force against gravity
Sta
tic Friction
Kin
etic Frictio
n
Friction b/w 2 stationary surfaces Greater than kinetic
Friction b/w 2 moving surfaces Less than static b/c it takes more force to make
an object start moving than to keep it moving Sliding friction – 2 objects slide past each other Rolling friction – round object rolls over a flat
surface› Usually less than sliding
Air resistance opposes motion› Example: as a car moves, it must
push the air out of the way› The easier the air is pushed out of
the way, the faster it will go› Designing the shape of the car so
that less air must be displaced is called streamlining
Fric
tion
can
be h
arm
ful
or h
elp
ful
Harm
ful
Help
ful
Road rash Carpet burn Wear and tear on car tires Racing Air hockey
B/w road and tires makes driving possible
Sand on icy roads Stopping a car Racing
Laws of Motion
Sir Isaac Newton described the relationship b/w motion and force in 3 laws
1st Law of Motion - Inertia An object at rest remains
at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.› Example: as car stops
suddenly, body keeps moving
Inertia – tendency of an object to resist a change in velocity (speed & direction) until acted upon
Inertia is related to mass of an object› Object with small mass has
less inertia than object with large mass Example: softball & bowling
ball Other examples of inertia:
› Lean toward side around curves
› Seatbelts keep you from continuing to move
› Car seat has a more equal distribution of weight keeping baby safe
2nd Law of Motion
The unbalanced force acting on an object equals the objects mass times its acceleration
Force=mass x accelerationF=ma
SI unit› Newtons (N) = Kg x m/s2
Which is harder to push?
Example: empty grocery cart vs. full cart
Law of Universal Gravitation – all objects in the universe attract each other through gravitational force
Force = G (m1 x m2/d2)
G=6.673 x 10-11 N x m2/kg2
Gravitational force increases as 1 or both masses increase
Gravitational force decreases as distance b/w masses increase
Acceleration depends on the mass of the object and the unbalanced force applied› More mass – harder to accelerate› More force – faster acceleration
When gravity is the only force acting on an object it is in free fall› If there were no air resistance, all objects would
fall to the Earth at the same speed: 9.8m/s2 (acceleration due to gravity)
Weight is the force of gravity on an objectWeight = mass x gravity w = mg
SI unit is Newton (N) › Kg x m/s2
The mass of an object is always the same, but weight changes as gravity changes.› Example: an astronaut has a mass of 66kg.
What is his weight on Earth and on the moon? On the moon gravity is 1.6 m/s2
The force of gravity is constant:› Air resistance
increases as you fall› It eventually equals
force of gravity› Equal force of gravity
& air = 0 acceleration
› No acceleration means constant velocity (max) called terminal velocity
Orbiting objects are in free fall:› Move forward then free
fall toward other object Projectile motion –
curved path that an object follows when thrown, launched, or projected› Combination of forward
horizontal motion and downward horizontal motion
Newton’s 3rd Law – action/reaction For every action force, there is an
equal and opposite reaction force. Can occur when there is no motion:
› Example: sitting in chair Or can occur in motion:
› Example: rocket
B A L A N C E D F O R C E S
IN E R T IA
F irs t L aw
U N B A L A N C E D F O R C E S
A C C L E R A T IO N
S e co n d L aw
F O R C E S A C T IN P A IR S
A C T IO N = R E A C T IO N
T h ird L aw
N e w to n s L a w s o f M o tion
Momentum Momentum- the product of the mass &
velocity of an objectmomentum = mass x velocity
p = mv Si Units
Kg x m/s Has direction because velocity has
direction When force changes motion, momentum
changes as well
Example: calculate the momentum of a 6 kg bowling ball moving at 10m/s down the alley toward the pins.
Law of Conservation of Momentum:› The total amount of momentum is a
system is conserved› Used to predict motion of cars after a
collision