motion and speed notes 9-1 & 9-2. an object is in motion if it changes position relative to a...
TRANSCRIPT
INTRODUCTION TO PHYSICS
Motion and SpeedNotes 9-1 & 9-2
Motion
An object is in motion if it changes position relative to a reference point
Stationary objects make good reference points
Relative Motion Whether or not
an object is in motion depends on the reference point you choose.
Distance and Displacement Distance is the total length of the actual
path between two points. Displacement is the length and direction of a straight line between starting and ending points.
What is the total distance this person traveled (in blocks)?
7 Blocks
What is the total displacement of this person?
5 Blocks Northeast
Vectors Quantities that have both a magnitude
and a direction Example: Displacement
Calculating Speed
If you know the distance an object travels in a certain amount of time, you can calculate the speed of the object.
Airplane Practice Problem
An airplane that moves 100 meters in two seconds has an average speed of …
Distance = 100 meters Time = 2 seconds
100 meters = 50 meters per second2 seconds
Average Speed
The speed of most moving objects is not constant
Instantaneous Speed
Rate at which object is moving at a given instant in time
Velocity
Speed in a given direction Velocity is a vector because it has both
magnitude and direction Changes in velocity may be due to
changes is speed, changes in direction, or both
Graphing Motion You can use distance-versus-time graphs
to interpret motion.
Metric Conversions
1 cm = 10 mm
1 m = 100 cm or 1,000 mm
1kilometer (think Sequoia 5K) – 1,000 m or 10,000 cm or 1,000,000
Let’s
Review!
Review Questions
1. Is a moving bus a good reference point from which to measure your position? a. No, because it is often late. b. No, because it is not a stationary object. c. Yes, because it is very large. d. Yes, because it can travel very far.
Review Questions
1. Is a moving bus a good reference point from which to measure your position? a. No, because it is often late. b. No, because it is not a stationary object. c. Yes, because it is very large. d. Yes, because it can travel very far.
Review Questions
2. To describe a friend’s position with respect to you, you need to know a. Your friend’s distance from you. b. The direction your friend is facing. c. Your friend’s distance and direction
from you. d. Your friend’s distance from a nearby
object.
Review Questions
2. To describe a friend’s position with respect to you, you need to know a. Your friend’s distance from you. b. The direction your friend is facing. c. Your friend’s distance and direction
from you. d. Your friend’s distance from a nearby
object.
Review Questions
3. Two cars traveling in the same direction pass you at exactly the same time. The car that is going faster a. moves farther in the same amount of
time. b. has more mass.
c. has the louder engine. d. has less momentum.
Review Questions
3. Two cars traveling in the same direction pass you at exactly the same time. The car that is going faster a. moves farther in the same amount of time.
b. has more mass. c. has the louder engine. d. has less momentum.
Review Questions
4. To describe an object’s motion, you need to know its a. position. b. change in position. c. distance. d. change in position over time.
Review Questions
4. To describe an object’s motion, you need to know its a. position. b. change in position. c. distance. d. change in position over time.
NOTES 9-3Acceleration
Acceleration
Rate velocity changes with time Vector quantity In science, acceleration refers to
increasing speed, decreasing speed, or changing direction
Decreasing speed = deceleration
Calculating Acceleration To determine the acceleration of an
object, you must calculate its change in velocity per unit of time.
Acceleration = Final Velocity – Initial Velocity
Time
Chapter 9 Motion and Energy
Let’s Try a Problem Calculate the
plane’s acceleration in the first 5 seconds of motion.
A= Vf – Vi
timeA = 40 m/s – 0 m/s5 s
A = 8 m/s2
Let’s try another…
A car traveling at 50 m/s speeds up to 80 m/s over a period of 15 seconds. The average acceleration of the car is
80-50 m/s = 30 m/s = 2 m/s/s or 2m/s² 15 s 15 s
Calculating Acceleration As a roller-coaster car starts down a slope, its velocity
is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration?
Read and Understand
What information have you been given? Initial velocity = 4 m/s Final velocity = 22 m/s Time = 3 s
Chapter 9 Motion and Energy
Calculating Acceleration As a roller-coaster car starts down a slope, its velocity
is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration?
Plan and Solve What quantity are you trying to calculate? The acceleration of the roller-coaster car = __ What formula contains the given quantities and the
unknown quantity? Acceleration = (Final velocity - Initial velocity)/Time Perform the calculation. Acceleration = (22 m/s - 4 m/s)/3 s = 18 m/s/3 s Acceleration = 6 m/s2
The acceleration is 6 m/s2 down the slope .
Chapter 9 Motion and Energy
Calculating Acceleration As a roller-coaster car starts down a slope, its velocity
is 4 m/s. But 3 seconds later, its velocity is 22 m/s in the same direction. What is its acceleration?
Look Back and Check
Does your answer make sense? The answer is reasonable. If the car’s velocity
increases by 6 m/s each second, its velocity will be 10 m/s after 1 second, 16 m/s after 2 seconds, and 22 m/s after 3 seconds.
Chapter 9 Motion and Energy
Calculating Acceleration Practice Problem
A falling raindrop accelerates from 10 m/s to 30 m/s in 2 seconds. What is the raindrop’s acceleration?
(30 m/s - 10 m/s) ÷ 2 seconds = 10 m/s2
Chapter 9 Motion and Energy
Calculating Acceleration Practice Problem
A certain car can accelerate from rest to 27 m/s in 9 seconds. Find the car’s acceleration.
(27 m/s - 0 m/s) ÷ 9 s = 27 m/s ÷ 9 s = 3 m/s2
Chapter 9 Motion and Energy
Graphing Acceleration
You can use both a speed-versus-time graph and a distance-versus-time graph to analyze the motion of an accelerating object.
Chapter 9 Motion and Energy
Sequoia 5K Runner Example
A student starts the 5K at 12 :15 and finishes at 12:35.
What can you calculate with this information?Time: 12:35 – 12:15 = 20 min.Distance: 5K
A. Speed = d/tB. Average Speed=Total distance/Total TimeC. Velocity=d/t accounting for changes in speed and/or
directionD. Acceleration= change in velocity/t
The Energy of Motion: 9-4
Energy of motion is kinetic energy. Stored energy is referred to as potential
energy. The potential energy of an object depends on
its weight and height. The formula for calculating mechanical energy is
potential energy + kinetic energy = total mechanical energy