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MOTION!
CHAPTER 11
To describe motion accurately, a frame of reference is
necessary
• These are objects which are NOT moving with respect to the “moving” object.
Some possible frames are -- the interior of the airliner,
-- the surface of the Earth, or -- the distant stars with respect to which the Earth rotates
and moves.
Relative motion….depends where you are…
• Relative Motion (frame of reference)
DISTANCE = length between 2 points
• In science, we use METRICS to measure everything…so start thinking in metrics…
• 1 m = 100 cm
• 1 km = 1000 m
• 10 mm = 1 cm
Displacement is the distance between start and finish…
• If you walk in a circle, your displacement is ZERO!
VECTORS…
• A vector is magnitude AND direction.
• Go north 4 km. Turn around and go south 2 km. Your total displacement is 2 km.
• Vectors are used to navigate in an airplane or ship.
• Sometimes you have to fly or sail “around” something.
• P. 331 has a nice illustration…
SPEED!
• Average speed is total distance/total time traveled.
• m/s
• Miles/hour
• Km/hour
• Etc……
Average speed =
total distance = d/t
total time
Let’s practice!
• A person jogs 4 km in 32 min. Then, he jogs 2 km in 22 min. Finally, after a drink of water, he jogs 1 km in 16 min. What is the jogger’s average speed?
• A train travels 190 km in 3 hours. Then the train speeds 120 km in 2 hours. What is the train’s average speed?
Instantaneous Speed…
• The speed you are going right this second!
• A cheetah can be clocked at 90 km/hr!
So, how do you GRAPH speed?
• The slope of the line on a distance time graph is speed.
• P. 334 figure 7
VELOCITY is a vector…
• Speed and direction is aka velocity.
p. 337
• Why do you need to know this?
• If you are on a boat or ski-doo, you can calculate true speed if you add your vectors. (same with an airplane)
• So, when you fly to LAX, it may take longer than when you fly back to DAY.
• It’s important for pilots to calculate this to estimate how much fuel they need.
Oh yeah, there’s a big “jet stream” that adds or subtracts true ground
speed of the plane.
ACCELERATION! (11.3)
• Changes in speed
• Changes in direction
• Or
• Changes in both
• Acceleration is a vector!
What is the acceleration of gravity?
• 9.8 m/s2
How do you calculate acceleration?
• Acceleration =
• change of velocity/total time
• a = (vf – vi)/t
MATH PRACTICE!!!
• A car goes 10 m/s starts to decelerate steadily. It comes to a complete stop in 20 seconds. What is its acceleration?
• An Airplane travels down a runway for 4 seconds with an acceleration of 9.0 m/s2. What is its change in velocity during this time?
• A kid drops a ball from a bridge. The ball strikes the water under the bridge 2.0 seconds later. What was the velocity of the ball when it strikes the water?
• A stupid boy throws a rock straight up into the air. It reaches the highest point of its flight after 2.5 seconds. How fast was it going when it left the boy’s hand?
• How fast will the rock hit the kid in the eye when it returns to earth?
Confucius say….
• Never spit into the wind!