1 velocity and acceleration. 2 1. frame of reference
DESCRIPTION
3 Frame of Reference – how you determine the position. We normally use N-S-E-W. Our Reference: Positive, (+) Moving RIGHT or Moving UP Negative, (-) Moving LEFT or Moving DOWN If you are not sure of the direction, use Positive(+) Choose your starting position as Zero (0), unless the problem tells you otherwise!TRANSCRIPT
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Velocity and
Acceleration
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1. Frame of Reference
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Frame of Reference – how you determine the position. We normally use N-S-E-W.
Our Reference: Positive, (+) Moving RIGHT or Moving UP Negative, (-) Moving LEFT or Moving DOWN If you are not sure of the direction, use Positive(+)Choose your starting position as Zero (0), unless
the problem tells you otherwise!
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Scalar Quantity - Magnitude - “How much”Direction does not matterMass, temperature, and time are scalar
quantities
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Vector Quantity –Magnitude AND DirectionHow much and which wayVelocity, force and
displacement are vector quantities.
Use “vector diagrams” to represent these quantities.
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The arrow shows the directionLength of arrow is proportional to the magnitude.High velocity to the right Low velocity to the left
Position – your location using the frame of reference. Ships use longitude and latitude. Symbol is d for horizontal motion. (your book
uses x)Vector quantity – magnitude and direction. I am located 5 km south of home.
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Displacement – change in position“How far out of place an object is”Symbol is d (x-horizontal, y-vertical) “delta” is “change in” d = df - di
Vector Quantity: Magnitude and DirectionUnits include m (meter), km, cm, mmYou travel 10 km EastDisplacement = zero (0) when you return to
the original position. You are not “out of place”
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You travel 10 km East, stop and then turn around and travel 10 km West. Displacement, d = 0 10 km East 10 km West Displacement = d = 0
You travel 10 km East, stop and then turn around and travel 15 km West. 10 km East
15 km West Displacement = d = -5km
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Distance How far you traveledScalar quantityMagnitude only I drove 20 km today.10 km East 10 km West Distance Traveled = 20 km10 km East 15 km West Distance Traveled = 25 km
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TimeClock readingScalar QuantitySymbol is tUnits include seconds, minutes, hours, days
Time IntervalElapsed time, change in timeSymbol tScalar Quantityt = tf – ti (final time – initial time)Units include seconds, minutes, hours, days
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2. Velocity Motion – Change in position Average Velocity
Rate of change in position =
IntervalTimeTotalntDisplacemeTotal
TimeinChangePositioninChangev
hrkm
sm
tdv
v
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Vector Quantity – Magnitude and Direction We will use positive (+) and negative (-) to show
direction + right or up - left or down + moving towards you (you are the reference) - moving away from you Average velocity does not indicate what happens
during the time interval. You could have stopped, sped up, or even slowed down.
vvv
v
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Instantaneous velocity – the velocity at any given instant in time.
Constant velocity - all the instantaneous velocities are equal. “Cruise Control”
Only true judgment of velocity is a reference point that is not moving.
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Overall Velocity Moving Sidewalk +3m/s You walk on it +1 m/s Overall velocity +4m/sMoving sidewalk +3m/s You walk backwards -1m/s Overall velocity +2m/s
Speed – scalar quantity, distance traveled over a period of time. No indication of direction!
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3. Acceleration Average Acceleration
Rate of change in velocity
Toughest time to walk on the bus? When it is speeding up or slowing down!
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Average acceleration =
vf = final velocity
vi = initial velocity
Units :
NOT!!
IntervalTimeVelocityinChange
tvv
tva if
tva
2sm
ssm
a
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Vector Quantity – magnitude and direction If you are moving right or up and
Speeding Up : +a (positive acceleration) Slowing Down : -a (negative acceleration) This is normal conditions!
If you are moving left or down and Speeding Up : -a (negative acceleration) Slowing Down : +a (positive acceleration)
If you do not know direction left or right use Speeding Up : +a Slowing Down : -a
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Just remember :The direction of the acceleration vector
depends on : Whether the object is speeding up
or slowing down. Whether the object is moving in the
positive (+) or negative (-) direction.Rule of Thumb –
If an object is slowing down, then its acceleration is in the opposite direction of its motion.
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If you are moving from rest vi = 0 (zero)
If you are coming to a stop vf = 0 (zero)
If you are moving at constant velocity = 0 (cruise control)
If you are not moving : v = 0 Acceleration is not discussed! (aka a 0)
a