chapter 2 : motion
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Chapter 2 : MOTION. p.16 in your book!. Aristotle (384-322 BC) Objects have a proper “place” And strive to get there. NATURAL MOTION - No force required ex: boulder “falls down” smoke “goes up” Thought heavier objects fall faster than lighter objects. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 2 : MOTION
p.16 in your book!
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Aristotle (384-322 BC)Objects have a proper “place” And strive to get there. NATURAL MOTION - No force required
ex: boulder “falls down” smoke “goes up”Thought heavier objects fall faster than lighter
objects
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UNNATURAL MOTION- Requires force
EX: push a book across table
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Galileo- Objects drop at same rate (except for
air friction)“Leaning Tower of Pisa “ experiment If no friction…no forces required to
keep moving objects moving. EX:Satellites
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As a ball rolls down an incline it speeds up up incline,slows Reduced angle, ball goes farther
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Inertia
Objects at rest tend to remain at rest.
Moving objects tend to remain moving.
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Speed
How fast something is moving: the rate at which distance is covered.
Speed= Distance
Time
EX: mph (mi/hr) , km/hr, cm/hr
/ = “per” = divided by ex: 100km/hr
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1. Instantaneous speed
Speed something has at any instant
Ex: speedometer
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Average speed= total distance covered
time interval
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Example: we drive 100 km in a time of 2 hrs.
Av sp= Total distance covered = 100km =50km
time interval 2hrs hr
Trip could have variations in speed -average speed!
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Another example: we walk to McDonalds : 2.0km away & it takes 40 minutes.
Av speed = Total distance covered Time interval
Av speed = 2.0km / 40 min = 0.05 km /min
But… stopped for traffic,tied shoe,ran across the road… YOU GET THE IDEA!!
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Velocity – includes speed & directionex: 60km/ hr North
This is a Vector Quantity- includes direction & magnitude.
What is the difference between constant speed & velocity?
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How can a racecar have constant speed but it’s velocity is changing?
Constant speed- doesn’t speed up or slow down.
Changing velocity because direction is changing.
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Other formulas:
V = D/T
D = V x T
T = D/V
V = velocity, D = distance, T=time
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Interpreting Distance vs. Time graphs:
See board: Speed vs. Velocity What is Slope? What is ______ doing?
Car “a” Car “b” Car “c” Car “d”
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Lets try some problems:
1. Av speed of bike that travels
150 m in 15 secs V = D / T V = 150 m /15 s V = 10m/s
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# 2 : You ran an av. Speed of 3 km/hr for 1 hr.
a. ) How far did you go? D=VxT 3km/hr • 1 hr
D = 3km
b. At this rate, how far in 2 hrs? 10 hrs?
3km/hr • 2 hr = 6km
3km/hr • 10 hr = 30km
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2.4 Motion Is relative
Right now :Your speed is zero relative to Earth,
But.. 30 km / s relative to the sun.
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Isaac Newton
P. 22 green boxNewton’s 1st Law “THE LAW OF INERTIA” Every object continues in a state of rest, or in a
state of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it.
“the table cloth trick”“penny & index card inquiry”
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Net Force – combination of all forces that act on an object.
See Board
Newton (N) – unit for force
An arrow represents Force as vector quantities.
Arrows length represents magnitude (how much) and direction (which way)
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Vector Addition:
1) 12 N + 8 N = _____
2) -20 N + 3 N = _____
3) 7 N + 8 N = _____
4) 15 N - 10 N = _____
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2.7 Equilibrium for objects at rest
Spring scale & block example on board
Attracted to the Earth with a force of __ N.
Weight of object (downward force)= tension in rope (upward force).
The block is at rest, so net force is Zero.
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Mechanical equilibrium : ∑ F = 0
∑ - sum
F- force
Objects at rest have equal & opposite forces acting on them.
Sum of upward vectors= sum of downward vectors
Static Equilibrium
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Why don’t we fall through the floor?
Support Force or “normal force”.- the upward force
EX: book on desk : weight & gravity
∑ F = 0
What is the net force on a bathroom scale when a 110 lb person stands on it?
A: Zero. Scale is at rest. Scale reads support force which has same magnitude as weight.
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Equilibrium for moving objects
Equilibrium- state of no change.
An object moving at constant velocity is in dynamic equilibrium.
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Some Questions for you…
Give an example of something moving when a net force of zero acts on it?
If we push a crate at a constant velocity, how do we know how much friction acts on the crate compared to our pushing force?
Harry the painter swings from his painter’s chair. His weight is 500 N and the rope has a breaking point of 300 N. Why doesn’t the rope break when he is supported as shown left on the board
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Some Questions for you…
Harry the painter swings from his painter’s chair. His weight is 500 N and the rope has a breaking point of 300 N. Why doesn’t the rope break when he is supported as shown left below? One day he decides to anchor his chair to a nearby flagpole – why did Harry end up taking vacation early?