Physical Science

Unit 5 - Motion

Scalars and Vectors

• Scalars are quantities that are fully described by a magnitude (or numerical value)– Ex. 1 ft., 27 miles, 1 cup

• Vectors are quantities that are fully described by both a magnitude and a direction– Ex. 2 ft to the left, 81 miles North

Check for Understanding

Quantity Category

a. 5 m

b. 30 m/sec, East

c. 5 mi., North

d. 20 degrees Celsius

e. 256 bytes

f. 4000 Calories

Categorize each quantity as either scalar or vector.

Distance vs. Displacement

Distance is a scalar quantity that refers to how much ground an abject has covered during its motion.

Displacement is a vector quantity that refers to how far out of position an object is; it refers to the object’s overall change in position.

Distance vs. Displacement

Distance = Displacement =

Distance and DisplacementDetermine the resulting displacement and distance travelled by this skier during these 3 minutes. (Hint: Skier moves from ABCD)

Distance vs. DisplacementCoach is pacing the sidelines. What is the coach’s resulting displacement and distance of travel? (Hint: he moves ABCD)

Check for Understanding

What is the displacement of the cross-country team if they begin at the school, run 10 miles and finish back at the school?

What is the distance and displacement of the race car drivers in the Indy 500?

Speed and Velocity

• Speed is a scalar quantity that refers to how fast an object is moving.– Ex. A Tectonic plate moves 2 cm in 1 year– An object at rest has a speed of zero

• Velocity is a vector quantity that refers to the rate at which an object changes its position.– Ex. A car is going 55 mph East– An object that does not change it’s position as a

result of it’s motion has a velocity of zero

Speed and Velocity

• Question: If a person is rocking back and forth from one foot to the other and does this for 30 minutes, what is his velocity?

Speed and Velocity

• An object can change speeds many times during its motion. (Think about a car slowing down in a curve.) We calculate an object’s average speed and average velocity for simplicity.

• Ave Speed = distance traveled/time traveled• Ave Velocity=change in position=displacement

time timeΔ is the symbol for change. Ex. Δ position = displacement

Speed and Velocity

• While on vacation, Lisa traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed?

Constant Speed vs. Changing Speed

• Instantaneous Speed is the speed of an object at any given instant in time.– Your car speedometer reads 55 mi/hr. You are

traveling at 55 mi/hr at that instant.

• Average Speed is the average of all instantaneous speeds.– Your trip was 100 miles long and it took you 2 hours

to get there. Your average speed was 50 mi/hr

Average Speed and Average Velocity

The motion above took 24 seconds. Determine the average speed and the average velocity

Average Speed & Average Velocity

Use the diagram to determine the average speed and the average velocity of the skier during these three minutes.

Average Speed & Average Velocity

What is the coach's average speed and average velocity?