Chapter2Section2

=__**Chapter 2 Section 2**__= toc
 * Constant Speed and Acceleration: Measuring Motion**

__**Learning Outcomes**__

 * Give examples of distance, time, speed, and acceleration.
 * Differentiate between instantaneous speed and average speed.
 * Recognize when motion is accelerated.
 * Calculate average speed and acceleration.

__**What Do You See?**__

 * the boy is walking slowing and all of his footprints are close together
 * the other boy is running and his foot steps are far apart
 * like strobe photos
 * farther=faster
 * closer=slower
 * faster you go you cover more ground with each step

__**What Do You Think?**__
Some major-league pitchers can throw a baseball 100mi/h, about 45 m/s. The ball reaches home plate in less than half a second. In that time, the batter must decide whether or not to swing at the pitch. If the batter decides to swing, he then must be able to react quickly enough to the pitch. It is no surprise that so few athletes are capable of competing in the major leagues.
 * 1) In your own words, explain the meaning of 100mi/h and 45 m/s.

__**Investigate**__
Predictions a.) You move at a constant speed. -the dots are evenly spread apart b.) You move at a faster constant speed. -the dots are farther apart, but the same amount c.) You move at a slower constant speed. -the dots are still evenly spaced out, but they are closer together. d. Predict how you think the distance between the dots will change if you walk at a constant speed, and then walk faster and faster. -the dots will be evenly spaced out and then begin to have more and more room in between them 6a.

7 a. You would expect it to be equal because we are pulling the paper at a constant speed. b. The speed was constant because the dots were evenly spaced.

8a.

b. What does the length of the paper segments tell you about the speed during each time interval? It tells you how fast you are pulling the paper. The faster you pull the further apart the dots are. c. Is there a trend in the lengths of the paper segments of your graph? The distance between the dots went from small to big to smaller to bigger.

9. a. 1.4 cm for the first 2 cm for the second

2-1.4/.2=.6/.2 .6/.2=3 3

b. Yes the tape had a constant acceleration because the distances in the spaces between the dots were equal to each other.

**__Physics Talk__**
-c**onstant speed:** the distance between the ticks were all equal in length
 * Measuring Motion: Constant Speed and Acceleration Using a Ticker Timer**
 * Acceleration**: a change in the velocity of an object over time
 * -slow constant speed:** the distance between the ticks is shorter when you travel
 * -fast constant speed:** you cover a greater distance then a constant medium and slow speed


 * -person accelerating:** the distance between the dots on the tape are not equal
 * -positive acceleration:** the person gradually increases speed and the distance between the dots gradually got longer
 * -negative acceleration**: the person gradually decreases in speed and the distance between the dots gradually gets shorter
 * (deceleration means negative acceleration)**

(unit for speed is written as distance per unit of time) average speed=distance traveled/time elapsed v=d/t Acceleration=change in speed/time interval A=v/t (m/s)/s or (km/h)/s
 * Calculating Speed**
 * Average speed:** the distance traveled divided by the time it took to travel that distance
 * Instantaneous Speed:** the speed measured during an instant; the speed as the time interval approaches, but does not become zero
 * Calculating Acceleration**
 * Units for Acceleration (m/s^2)**

v=distance/time=(delta d)/(delta t)= df-di / tf-ti=

a= (delta v)/(delta t) = vf-vi / tf-ti

instantaneous speed: example is a car (speedometer gives an instantaneous speed)

__**Checking Up**__
1. Describe the pattern of dots on a ticker tape for each of the following situations:
 * -person accelerating (constant):** the distance between the dots on the tape are not equal
 * -positive acceleration:** the person gradually increases speed and the distance between the dots gradually got longer
 * -negative acceleration**: the person gradually decreases in speed and the distance between the dots gradually gets shorter

2. An athlete runs 400m in 50s. What is the runner's average speed? v=d/t v=400/50 v=8

a=v/t a=8/50 a= 0.16m/s/s

3. What is the difference between instantaneous speed and average speed?
 * Average speed:** the distance traveled divided by the time it took to travel that distance
 * Instantaneous Speed:** the speed measured during an instant; the speed as the time interval approaches, but does not become zero

4. A vehicle accelerates from 0km/h to 100km/h in 10s. What is its average acceleration? a=100/10 a=10km/h/s

vi= 2m/s vf= -2m/s (other direction) velocity=magnitude and direction t= .01 a=? a=vf-vi/tf-ti a=-2-2/.01 a=-4/.01 a= -400m/s^2

a=vf-vi/t a=-2-2/2 a=.4/.2 a= -20m/s^2

__**Physics To Go**__
1. **Average speed:** the distance traveled divided by the time it took to travel that distance 2. Calculate the average speed v=d/t v=1000/15 v=66.7
 * Instantaneous Speed:** the speed measured during an instant; the speed as the time interval approaches, but does not become zero
 * a. a horse runs 1km in 15s**

a=v/t a=66.7/15 a=4.4m/s^2

v=d/t v=84/6 v=14
 * b. a skier travels 84m in 6s**

a=v/t a=14/6 a=2.3m/s^2

v=d/t v=9600/7200 v=1.3
 * c. you walk 9.6km in 2h**

a=v/t a=1.3/7200 a=1.9m/s^2

v=d/t v=400000/16200 v=24.7
 * d. a car travels 400km in 4.5h**

a=v/t a=24.7/16200 a=.0015m/s^2
 * answer= 1.33 m/s

3. Where is the acceleration occurring? (positive or negative) a. a runner falls down -negative acceleration

b. a runner takes off from a starting block -positive acceleration

c. you walk down a straight hall at a steady pace -no acceleration

d. a soccer ball is caught by the goalie -negative acceleration

e. a bowling ball rolls along the gutter at a constant speed -no acceleration

f. a parachutist falls at a constant speed -negative acceleration

4. a. a student moving with a constant increase in speed
 * A and D

b. a student moving with a constant speed
 * B

c. the greatest change in speed each second
 * A

d. the motion of a student whose speed first increased but later decreased
 * C

e. acceleration of the student in... A: positive acceleration B: zero acceleration C: positive acceleration then negative acceleration

6. 45km/h stop in 9s (stop= 0km/h) a. a=vf-vi/t a=0-45/9 a=-45/9 a=-5km/h/s b. This acceleration has a negative value because the speed is decreasing

7. a. constant b. positive acceleration c. constant acceleration then constant deceleration d. decelerate then accelerate

8. 100mi 2 hours v=d/t v=100/2 v= 50

a=v/t a=50/2 a=25 25mi/h^2

9. average speed= 15m/s That does not mean that the person's instantaneous speed is always 15m/s because average speed is the average of how far the person went divided by how long it took them. While instantaneous speed is the speed they were going at any instant.

10. The dots would start out right next to each other, but very quickly be very far apart then stop right away.

11. a= 4m/s after 5 s?

a=v/t 4=v/t v=20m

12. bicycle=average speed= 6m/s sprinter=100m in 10s v=d/t v=100/10 v=10

a=v/t a=10/10 a=1m/s The sprinter has a faster average speed