Transcript F r i c t i o n - Southgate Community School District

CH4 Linear Motion with Uniform
Acceleration
Physics (A) Winter, 2010-2011
1
Relative Motion
An object is moving if its position relative
to a fixed point is changing.
Ex. Are you moving when:
• you are sitting in a moving car?
• ANS. YES, relative to ground
NO, relative to the driver’s seat
• running down the street?
• ANS. YES, relative to the ground
NO, relative to your shoes
• you are watching a city bus drive by?
• ANS.
NO, but the bus is relative to you/ground
NO, but the passengers are relative to
you/ground
2
Speed
•
•
•
•
•
•
Speed = change in distance/change in time
Symbol is v
Units: m/s “meters per second” / sign = per
Scalar Quantity: a # with NO DIRECTION
SAME AS VELOCITY BUT IGNORE DIRECTION!
Ex. You drove 40 mi/hr this morning on Eureka Rd
3
Velocity
•
•
•
•
Velocity = change in distance/change in time
Symbol is v
Units: m/s “meters per second” / sign = per
Vector Quantity: value has magnitude AND
direction (N,S, up, down, etc.)
• SAME AS SPEED BUT DIRECTION MATTERS!
• Ex. You drove 40 mi/hr eastbound this morning on
Eureka Rd
4
Speed & Velocity
• Speed/Velocity - the rate of change in
position over time (distance/time)
• Instantaneous speed: Your speedometer
• Average speed:
vavg: Δ d
(Eq. 5)
Δt
• Ex. You traveled from Southgate to
Westland (18 mi) in 1 hour, for an avg.
speed of 18 mi/hr
• How is velocity different from speed?
5
Change in Velocity
•
What causes an object to change velocity?
• 3 THINGS CAN CAUSE IT:
1. Change in Speed
2. Change in Direction
3. Change in BOTH
6
•
•
•
•
Acceleration
Acceleration = change in velocity/change in time
Symbol is a
Units: m/s2 “meters per second squared”
Vector Quantity: value has magnitude AND
direction (N,S, up, down, etc.)
• Ex. You accelerated at 4 m/s2 eastbound this
morning on Eureka Rd
• Can be positive or negative, accels = positive,
decels = negative
• Uniform Acceleration=constant acceleration
Ex. g=9.8 m/s2
7
Kinematic Equations for
Uniform Acceleration
vf = vi + a∙t
(Eq. 6)
∆d = ½(vf+vi)t
(Eq. 7)
∆d=vi∙t + ½∙a
2
2
vf =vi +
2∙a∙d
2
∙t
(Eq. 8)
(Eq. 9)
8
Delta
∆ is “Delta” on greek
alphabet. It means
“difference”
∆ gives you the right to
subtract. In Science,
∆ = Final - Initial
9
Free Fall
• Free Fall = object is under the
influence of force of gravity ONLY
• Elapsed Time (ET) = Δt=time an object
starts to fall/move to time object stops
to fall/move
• All objects fall on Earth at a=g=9.8 m/s2
• NEVER NEVER NEVER
ESTIMATE/APPROXIMATE g!!!!!!!
10
Air Resistance
• When an object falls, is the force of
gravity, the only force acting on the
object?
• NO! Air resistance, a form of
friction, is acting in OPPOSITE
direction of force of gravity
11
Graphs of MotionVelocity Plot
12
Graphs of MotionVelocity Plot
• Graph is Linear (straight line)
• Slope = m = rise
run
• Slope of Speed/Velocity Graph =
CONSTANT ACCELERATION, a
13
Graphs of MotionDistance Plot
14
Graphs of MotionDistance Plot
• Graph is NON-linear (parabola)
• Slope = m = rise
run
• Slope of Distance Graph =
INSTANTANEOUS SPEED, v
15
Math Practice
Ex. If your average speed is 40 km per
hour and you have traveled for 0.5 hour,
what distance have you traveled?
GIVEN:
Vavg = 40 km/h
Δt = 0.5 h
UNKNOWN: Δd = ?
EQUATION: Vavg = Δ d
Δt
PLUG-IN: 40 km/h = Δ d
0.5h
SOLVE: Δd = 20 (km)
16
Problem Solving Flashcard!
1. Circle what the question is asking you to find (less
than 3 words!). Put the symbol for this phrase
under Unknown: section.
2. Read problem and list all of your GIVENS as their
symbols NOT words under GIVEN: section.
3. Go to your formula sheet and pick a formula that
has ALL GIVENS AND UNKNOWN in it. Write it
under FORMULA: section.
4. Under PLUG-IN: section, plug-in what you know
(from GIVENS and UNKNOWN) EXACTLY WHERE
YOU FIND IT IN FORMULA.
5. Using Algebra, solve for missing variable with
units, under the SOLVE: section.
6. Circle your answer.
17