Describing Motion Chapter 3

Objectives Quiz 2 Draw and use motion diagrams to describe motion Use a particle model to represent a moving object Choose coordinate systems for motion problems Differentiate between scalar and vector quantities

Objectives Quiz 2 Define a displacement vector and determine a time interval Recognize how the chosen coordinate system affects the signs of vectors quantities Define velocity and acceleration operationally Relate the direction and magnitude of velocity and acceleration vectors to the motion of objects Create pictorial and physical models for solving motion problems

Motion and Particle Model Diagrams Motion Diagram A series of images of a moving object after equal time periods If images are equal distance, constant speed If images get closer, slowing down If images get farther, speeding up Particle Model Using a dot instead of actual object Physics teachers are notorious bad artists 

Motion Diagrams What can you determine about the motion of the bird? The runner? The car?

Particle Diagram

Coordinate Systems Indicate direction

Time Intervals ∆t

Use a particle model or motion Runner moving at constant speed Runner starting from rest and picking up speed Car that starts from rest, speeds up to a constant speed, and then slows to a stop

Use a particle model or motion A wheel turning at constant speed

Coordinate Systems Choosing what equals zero Origin In high jump, ground is zero feet, and vertical is positive In long jump, line is zero feet, and horizontal is positive Usually chosen to make math easier

Scalar and Vector Quantities are quantities which are fully described by a magnitude (or numerical value) alone Vectors are quantities which are fully described by both a magnitude and a direction.

Scalar or Vector? 1) Elevation 22,000 ft 2) 60 mph 3) 5 miles North 4) 20 degrees Celsius 5) 4000 calories 6) 10 m/s East

Scalar Vector Answers Vector Scalar

Speed A non-directional rate at which something is covered. Distance over Time V=d/t

Instantaneous Speed Speed at any instant

Total distance divided by total time= Average Speed Quantity divided by time=Rate

Speed Related Terms Velocity (v)- Speed in a given direction This changes if either speed or direction change Acceleration (a)- Change in velocity/time interval

Positive and Negative Acceleration

Describe my motion A) SOOOOOOOOOOOOOOOOOOE B) S O O O O O O O O O O O O O O E C) E O O O O O O O O O O O O O O S D) S O O O O O O E E) S O O O O O O E F) E O O O O O O S G) SOO O O O O O O O E

Questions about acceleration A car is slowing down, is the acceleration positive or negative? A car driving down the highway maintains a constant speed around a curve. Does the car accelerate?

Scalar Vector Distance and Speed are Scalar Quantities How much ground an object has covered How fast the object is moving without regards to direction Displacement and Velocity are Vector Quantities It is the object's overall change in position. How fast the object moved in the right direction

Adding Vectors Line up vector arrows from tip to tail Add magnitudes to find resultant vector Reminder: Some are negative

Displacement ∆d = df - di

Displacement A physics teacher walks 4 meters East, 2 meters South, 4 meters West, and finally 2 meters North. Distance? Displacement?

Displacement Distance Covered = 12 meters Displacement = 0 meters South Cancels North East Cancels West

More Practice Distance the skier covered? Avg speed of skier? Avg velocity of skier?

More Practice Distance = 420 m Displacement = 140 m to the right

Follow up How do the answers change if the origin is the skier’s location at 1 min? How do the answers change if left is positive?

More Practice Distance the Coach moved? Avg velocity of coach for 10 min?

Crab Walk The slowest animal ever discovered was a crab found in the red sea. It traveled an average speed of 5.7 km/yr. How many seconds would it take this crab to sprint the 100 m dash?

Pictorial Models A problem solving strategy, just a sketch of what is going on Read the problem and sketch the situation May need multiple separate sketches Establish coordinate system if needed Reread problem and make a list of variables and assign a symbol to them Determine known and unknown quantities

Draw a model A dragster starting from rest accelerates at 49 meters per second squared. How fast is it going when it has traveled 325 meters?

Draw a model A speeding car is traveling at a constant speed of 30 m/s when it passes a stopped police car. The police car accelerates at 7 m/s/s. How fast will it be going when it catches up with the speeding car?

Draw a model A car is traveling 20 m/s when the driver sees a chidl standing in the road. He takes 0.8s to react, then steps on the brakes and accelerates at -7 m/s/s. How far does the car go before it stops?

Draw a model You throw a ball downward from a window at a speed of 2.0 m/s. The ball accelerates at 9.8 m/s/s. How fast is the ball moving when it hits the sidewalk? How does the model change if the ball is thrown up at -2.0 m/s?