 # UNIT #3 – CHAPTER #8.2 – AVERAGE VELOCITY. Objectives  Be able to calculate average velocity, displacement, and time interval for an object in uniform.

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UNIT #3 – CHAPTER #8.2 – AVERAGE VELOCITY

Objectives  Be able to calculate average velocity, displacement, and time interval for an object in uniform motion, given appropriate data

HOW COULD YOU DEFINE SPEED, OR VELOCITY?  What do you notice when you move fast or when you move slow?  What do you notice about objects that move fast or move slow?  TRY TO DESCRIBE IN TERMS OF POSITION AND TIME.

SPEED AND VELOCITY, WHATS THE DIFFERENCE?  Can two objects have the same speed and different velocities?  Velocity is a vector – has direction and magnitude  Speed is a scalar, it only has magnitude.

SPEED AND VELOCITY  A)B)  Is the speed the same for A and B?  Is the velocity the same for A and B?  Objects at the same speed can have different velocities.  Speed can be called the magnitude of the velocity.

AVERAGE VELOCITY  Copy these two formulas in your notes!  Average velocity is equal to the change in position (displacement) divided by the change in time.  This is why velocity has the units (m/s)

WHY DO WE SAY AVERAGE VELOCITY, INSTEAD OF JUST VELOCITY?

AVERAGE VELOCITY  In a way, average velocity is a simplification.  It tells us on average over a certain time interval how fast an object is moving.  It does not tell us exactly what velocity the object moved at for all instants in that interval.

DIFFERENCE BETWEEN AVERAGE VELOCITY AND AVERAGE SPEED.  I walk 10 m North in 5 seconds and then walk for 10 m South in the next 5 seconds.  What is my average velocity? My average speed?  Graphically  Mathematically

Calculating Average Velocity The relationship between average velocity, displacement, and time is given by: Use the above equation to answer the following questions. 1. What is the average velocity of a dog that takes 4.0 s to run forward 14 m? (3.5 m/s forward) 2. A boat travels 280 m east in a time of 120 s. What is the boat’s average velocity? (2.3 m/s east)

Calculating Displacement The relationship between displacement, average velocity, and time is given by: Use the above equation to answer the following questions. 1. What is the displacement of a bicycle that travels 8.0 m/s [N] for 15 s? (120 m [N]) 2. A person, originally at the starting line, runs west at 6.5 m/s. What is the runner’s displacement after 12 s? (78 m west)

Calculating Time The relationship between time, average velocity, and displacement is given by: Use the above equation to answer the following questions. 1. How long would it take a cat walking north at 0.80 m/s to travel 12 m north? (15 s) 2. A car is driving forward at 15 m/s. How long would it take this car to pass through an intersection that is 11 m long? (0.73 s)

CONVERTING UNITS: m/s Km/h  Remember unit analysis with Mr. Baron? Here it is in science!  For example, convert 75 km/h to m/s.

Converting between m/s and km/h Try the following unit conversion problems. 1.Convert 95 km/h to m/s. (26 m/s) 2.A truck’s displacement is 45 km north after driving for 1.3 h. What was the truck’s average velocity in km/h and m/s? (35 km/h [N], 9.6 m/s [N]) 3.What is the displacement of an airplane flying 480 km/h [E] during a 5.0 min time interval? (40 km [E] or 40, 000 m [E])

PROBLEM #1:  What is the average velocity of a skateboard that goes 50 m [W] over a time interval of 8 seconds?  Answer: Average Velocity = 6.25 m/s [W]

PROBLEM #2  If a cyclist can go the same distance in a time interval of 4 seconds, how much greater is her average velocity than the skateboarder’s average velocity?  Double the velocity of the skateboarder; Average velocity = 12.5 m/s [W]

PROBLEM #3  A deer is running forward at a speed of 46 km / h. How far does it travel in 30 seconds?  Displacement = 0.38 km or 380 meters [forward]

PROBLEM #4:  How many minutes would it take a car moving at an average velocity of 82 km / h to travel 125 km?  Time = 91 minutes

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