Chapter 11: Motion

1. Chapter 11: Motion Section 1: Measuring Motion

2. Observing Motion • How do you determine if something is moving? • Is a person sitting in a driving car moving? • What about a character in a video game that is apparently running across the TV? • If the earth is orbiting the sun, are you moving now?

3. Observing Motion • The motion of an object must be observed in relation to objects that stay in place, or reference points • Frame of Reference: used to describe the motion of an object relative to reference points • When an object changes position with respect to a frame of reference, the object is in motion • Mythbusters

4. Observing Motion • Consider the person sitting in the car… • Describe their motion from each frame of reference: • Inside the car • Moving very fast • A car driving past • The person is slowly moving backwards

5. Observing Motion • Distance: measures the path taken • Displacement: the change of an object’s position • always includes direction Bill Nye Frame Of Reference and Law of Inertia

6. Speed and Velocity • Speed: the distance traveled divided by the time interval in which the motion occurred • Velocity: the speed of an object in a particular direction • Speed tells us how fast an object moves, and velocity tells us how fast the object moved (speed) AND the direction the object moved

7. Velocity • Velocity is described relative to a reference point • North, west, east, south, etc. • It can also be described as motion from a fixed line • Direction is described as positive or negative along the line of motion • Up and right are usually positive velocity • Down and left are usually negative velocity

8. Velocity • Could something have a negative velocity? • Yes, that means it is going backwards • Is it possible for something to have a negative speed? • No, there is no specific direction • Is it possible for something to have a constant speed but a changing velocity? • Yes, all the object has to do is turn • Remember, velocity includes direction, speed does not

9. Calculating Speed • Average speed is calculated as distance divided by time • Constant Speed: equal distances in equal amounts of time • Instantaneous Speed: the speed at any given time

10. Units • The SI unit for speed is meters per second (m/s) • Remember! We use the metric system in science • Instead of using miles per hour (mph), we use meters per second (m/s) or kilometers per hour (km/h) • The smaller the unit, the more precise the measurement • You will need to make conversions in your math problems!

11. Math skills: Velocity Metal stakes are sometimes placed in glaciers to help measure a glacier’s movement. For several days in 1936, Alaska’s Black Rapids glacier surged as swiftly as 89 meters per day down the valley. Find the glacier’s velocity in m/s. Remember to include direction. • List the given and the unknown values. Given:time -t = 1 day distance -d = 89 m down the valley Unknown: velocity - v = ? (m/s and direction)

12. Math skills: Velocity 2a. Perform any necessary conversions. To find the velocity in meters per second, the value for time must be in seconds. t = 1 day = 24 h x 60 min x 60 s 1 hr 1 min t = 1 day = 24 x 60 x 60 t = 86,400 s = 8.64 x 104 s

13. Math skills: velocity 2b. Write the equation for speed 3. Insert the known values and solve.

14. Section 2: acceleration Chapter 11: Motion

15. Acceleration • Acceleration: the rate at which velocity changes over time • Acceleration can be a change in speed • An increase or decrease in speed is an acceleration • Acceleration can be a change in direction • A motorcyclist who rides around the inside of a barrel is constantly accelerating • Acceleration can be BOTH a change in speed AND direction NFL Science of Football: kinematics

16. Acceleration • For straight line motion, a positive acceleration means the object’s velocity is increasing • The object is speeding up • A negative acceleration means that the object’s velocity is decreasing • The object is slowing down • Centripetal Acceleration: acceleration that occurs in circular motion

17. Acceleration • A constant (unchanging) velocity means no acceleration • A constant acceleration means a constantly changing velocity

18. Acceleration • To calculate acceleration subtract the initial velocity (vi) from the final velocity (vf) and divide by time a = vf – vi or a = v t t • The SI unit for acceleration is m/s2 (m/s/s)

19. Math skills: acceleration A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot. 1. List the given and the unknown values. Given:time -t = 1.5 s initial velocity -vi = 0 m/s final velocity -vf = 14.7 m/s down Unknown: acceleration - a = ? (m/s2 and direction)

20. Math skills: Acceleration 2. Write the equation for acceleration. 3. Insert the known values into the equation, and solve.

21. Graphing motion • Speed is shown with a distance vs. time graph of its motion • x-axis: time (ind. variable) • y-axis: distance (dep. variable) • The slope of the line represents the speed • The speed can be found by calculating the slope • A straight line means a constant speed How to read and interpret graphs

23. Graphing Motion • Acceleration is shown with a speed vs. time graph • x-axis: time (ind.variable) • y-axis: speed (dep. variable) • The slope of the line represents the acceleration • A positive slope means an object is speeding up • A negative slope means an object is slowing down

25. What’s Going on? at rest (0 velocity) Acceleration Negative velocity (backwards) Constant Velocity