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Describing Motion

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**1. **Describing Motion Mary Oldham
Norwood High School
2009-2010

**2. **Motion and Speed

**3. **Motion and Position Position is the term used to describe where an object is/was located
Motion describes how the object changed position
A reference point is an object used to tell how far an object has moved from its initial position

**4. **Motion and Time Speed is the rate of change in position, or the distance traveled per unit of time
Speed = distance traveled / travel time
Units are always: length / time
m/s km/hr mi/hr km/s
Instantaneous speed is the rate of motion at any given instant
Speedometer on a car

**5. **Motion and Time (cont’d) Constant speed when an object travels at the same rate for a certain time interval
Most objects travel at inconsistent rates so average speed is often used to help describe motion when speed is not constant
Average speed is the total distance traveled divided by the total time of travel

**6. **Calculating Average Speed v = d/t
v = speed
d = distance
t = time
Ex: Jim alternates between walking and jogging in a park. If his total time in the park is 2.5 hours and he covers a distance of 8 miles, what is his average speed?
v = d/t d = 8mi t = 2.5 hr
= 8mi/2.5hr
= 3.2 mi/hr

**7. **Practice Sandy drives in her car for 32 miles. If it takes her 1.5 hours to get to her destination, what was her average speed?
Bill wants to find the average speed of his rat as it travels through a maze. If the rate covers a distance of 18 meters and it takes him 90 seconds to travel the maze, what was the rat’s average speed?
Find the average speed of a bird migrating 2000 miles South for the winter if it takes the bird 84 hours to make the trip.

**8. **Answers to Practice Sandy drives in her car for 32 miles. If it takes her 1.5 hours to get to her destination, what was her average speed?
v = ?
d = 32 miles
t = 1.5 hours
v = d/t = 32 mi/1.5 hr = 21.3 mi/hr

**9. **Answers to Practice 2) Bill wants to find the average speed of his rat as it travels through a maze. If the rate covers a distance of 18 meters and it takes him 90 seconds to travel the maze, what was the rat’s average speed?
v= ?
d = 18 m
t = 90 s
v = d/t = 18 m/ 90 s = 0.2 m/s

**10. **Answers to Practice 3) Find the average speed of a bird migrating 2000 miles South for the winter if it takes the bird 84 hours to make the trip.
v = ?
d = 2000 mi
t = 84 hr
v = d/t = 2000 mi/ 84 hr = 23.8 mi/hr

**11. **Graphing Speed Position v. time graphs are used to represent an object’s change in position over a given time interval
Position is plotted on the y-axis
Time is plotted on the x-axis
The slope of the line on a position-time graph gives the speed of an object.

**12. **Graphing Practice Interval Motion
a-b constant speed in positive direction
b-c constant position no speed
c-d constant speed toward and then past origin
d-e constant speed in positive direction back to the origin

**13. **Earth’s Crust Some objects move too slowly to use units such as m/s or km/hr
The speed at which tectonic plates move is frequently measured in cm per year
The speed of the plates along the San Andreas fault in California average 2cm per year

**14. **3.2 Velocity and Acceleration

**15. **Velocity Velocity is similar to speed but includes a direction
70mph is just a speed, but once a direction is included it becomes a velocity
70mph West is a velocity

**16. **Speed or Velocity? 30m/s
240m/s East
12mph
70mph toward the beach
8cm/min up
406 mi/day

**17. **Acceleration Occurs when velocity is not constant
Acceleration is the rate of change in velocity
The rate an object is speeding up
The rate an object is slowing down

**18. **Acceleration (cont’d) If the acceleration is in the same direction of the velocity then the object is speeding up
If the acceleration is opposite the direction of velocity, then the object is slowing down

**19. **Acceleration (cont’d) Acceleration depends on
Change in velocity
Time period for the change
Formula
a = ?v/?t = vf – vi
tf - ti

**20. **Calculating Acceleration A car’s velocity changes from 0 m/s to 30 m/s in 10 seconds. Calculate the car’s average acceleration.
What is known? initial velocity = 0m/s; final velocity = 30 m/s; time = 10s
a = (vf – vi) / t = (30 m/s – 0 m/s) / 10 s
= (30 m/s) / 10 s = 3 m/s2

**21. **Acceleration Practice Find the average acceleration of a vehicle traveling at 30m/s that speeds up to 50m/s in 5 seconds.
Find the average acceleration of a vehicle traveling at a speed of 60m/s that comes to a stop light and stops in 10 seconds.
What is the acceleration of a jet that starts from rest and accelerates to 350mph in 0.5h?

**22. **Acceleration Practice 1) a = (vf – vi) / t
= (50 m/s – 30 m/s) / 5 s
= (20 m/s) / 5 s
= 4 m/s2

**23. **Acceleration Practice 1) a = (vf – vi) / t
= (0 m/s – 60 m/s) / 10 s
= (-60 m/s) / 10 s
= -6 m/s2

**24. **Acceleration Practice 1) a = (vf – vi) / t
= (350mph – 0mph) / 0.5h
= 350 mph / 0.5 h
= 700 mph2
or
= 700 mi/hr2