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”

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!