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5-May-15 Physics 1 (Garcia) SJSU Chapter 3 Linear Motion (Motion in a straight line, such as falling straight downward)

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Presentation on theme: "5-May-15 Physics 1 (Garcia) SJSU Chapter 3 Linear Motion (Motion in a straight line, such as falling straight downward)"— Presentation transcript:

1 5-May-15 Physics 1 (Garcia) SJSU Chapter 3 Linear Motion (Motion in a straight line, such as falling straight downward)

2 5-May-15 Physics 1 (Garcia) SJSU Demo: Cracking Nuts What’s the best way to crack open a nut?

3 5-May-15 Physics 1 (Garcia) SJSU Tools you’ll learn today Speed and Velocity Acceleration Relationships among distance, velocity, and acceleration. Falling motion.

4 5-May-15 Physics 1 (Garcia) SJSU Speed Define speed of an object as For example, 30 miles per hour means object travels distance of 30 miles in an elapsed time of one hour. Write as, (SPEED) = (Distance traveled) (Time elapsed) 30 miles per hour = 30 miles hour

5 5-May-15 Physics 1 (Garcia) SJSU Speed and Distance From definition of speed, Example: If speed is 30 miles per hour and time elapsed is 2 hours then distance traveled is (30)X(2) = 60 miles. (Distance traveled) = (Speed) X (Time elapsed) Match units: This time elapsed is also 120 minutes but it’s not correct to compute distance traveled as (30)X(120) = 3600 miles {WRONG}.

6 5-May-15 Physics 1 (Garcia) SJSU Check Yourself What is the average speed of a cheetah that sprints 100 meters in 4 seconds? How about if it sprints 50 meters in 2 seconds? A car has an average speed of 100 kilometers per hour. How far does it travel in 60 minutes?

7 5-May-15 Physics 1 (Garcia) SJSU Average versus Instantaneous Sometimes consider average speed, other times we speak of instantaneous speed. For example, say it takes you one hour to drive the 30 miles from home to campus. Average speed is 30 miles per hour. Instantaneous speed (given by your speedometer) varies due to traffic, stop lights, morons driving in front of you, etc.

8 5-May-15 Physics 1 (Garcia) SJSU Demo: Ball Races Which ball wins the race, A or B? A B Finish Line Which ball has the larger average speed? Which has the larger instantaneous speed at each point.

9 5-May-15 Physics 1 (Garcia) SJSU Check Yourself A car has an average speed of 60 miles per hour. Is it possible for the instantaneous speed to always be less than 60 miles per hour?

10 5-May-15 Physics 1 (Garcia) SJSU Velocity Velocity is speed and direction of object’s motion. Examples: 30 miles per hour, Northward 25 meters per second, Downward 300 miles per hour, Coming towards you 25 m/s, upward 25 m/s, downward Same speeds Different velocities

11 5-May-15 Physics 1 (Garcia) SJSU Check Yourself The speedometer of a car moving east reads 100 km/h. It passes another car moving west at 100 km/h. Do they have same speed? Velocity? During a certain period of time, the speedometer of a car reads a constant 60 km/h. Does this indicate a constant speed? Constant velocity?

12 5-May-15 Physics 1 (Garcia) SJSU Changes in Velocity Velocity changes if speed or direction of motion change. 25 m/s, downward 10 m/s, downward 25 meters per second, 45 degrees upward 25 meters per second, 45 degrees downward Velocity changes in both these cases.

13 5-May-15 Physics 1 (Garcia) SJSU Acceleration Define acceleration as, (ACCELERATION) = (Change in Velocity) (Time interval) Note: An object accelerates anytime its velocity changes. Examples include: Object speeds up. Object slows down (speed decreases). Object speed constant but direction changes (curved path) Best example of acceleration is objects in free fall

14 5-May-15 Physics 1 (Garcia) SJSU Demo: String of Falling Balls Falling objects accelerate (speed increases). Listen for the sound as balls hit the ground. Time between “clicks” gets shorter & shorter (falling faster & faster). String does not pull; no tension while falling.

15 5-May-15 Physics 1 (Garcia) SJSU Velocity in Free Fall (Down) How fast do objects go when they fall? Acceleration of gravity is 10 meters per second per second. With each second of fall, speed increases by 10 meters/second Zero meters per sec. 10 meters per sec. 20 meters per sec. 30 meters per sec. 40 meters per sec. Release 1 second 2 seconds 3 seconds 4 seconds

16 5-May-15 Physics 1 (Garcia) SJSU Velocity in Free Fall (Up & Down) Moving upward, with each second the speed decreases by 10 meters/second. Going back down the motion exactly reverses itself. See Fig. 3.8 (pg. 47) Zero meters per sec. 10 meters per sec. 20 meters per sec. 30 meters per sec. 40 meters per sec.

17 5-May-15 Physics 1 (Garcia) SJSU Position in Free Fall How far do objects go when they fall? More complicated because speed is increasing. There’s a pattern & Galileo figured it out. But it wasn’t easy. 5 meters 20 meters 45 meters Release 1 second 2 seconds 3 seconds 4 seconds 80 meters Higher than King library Higher than this ceiling

18 5-May-15 Physics 1 (Garcia) SJSU Galileo’s Inclines Galileo realized that rolling down an incline and falling were very similar. It was much easier for him to study the slower motion of an incline.

19 5-May-15 Physics 1 (Garcia) SJSU Demo: Galileo’s Clicking Ramps Roll balls down notched, inclined ramps and listen for the clicks. Start =2x2 9=3x3 16=4x4 25=5x5 36=6x6 49=7x7 64=8x8

20 5-May-15 Physics 1 (Garcia) SJSU Position in Free Fall (cont.) 5 meters 20 meters 45 meters Release 1 second 2 seconds 3 seconds 4 seconds 80 meters (Distance Fallen) = ½ (Acceleration)(Time)(Time) Galileo realized that: Gravity acceleration is 10 meters per second per second so at a time of 3 seconds, (Distance Fallen) = ½ (10)(3)(3) = 45 meters 20=5x 2x2 45=5x 3x3 80 =5x 4x4 5=5x 1x1

21 5-May-15 Physics 1 (Garcia) SJSU Lab: Acceleration of Gravity Record position of falling object using spark timer and paper tape.

22 5-May-15 Physics 1 (Garcia) SJSU Demo: Dropping the Ball (Distance Fallen) = ½ (Acceleration)(Time)(Time) How long does it take a ball to fall 3 meters? Using the formula, Can check that it takes 0.77 seconds since (3) = ½ (10)(0.77)(0.77) Beauty of science: Predict, then verify by dropping balls! Note: Do similar measurement in first experiment in the Phys 1L lab.

23 5-May-15 Physics 1 (Garcia) SJSU Demo: Catch a Buck Put thumb and index fingers near Washington’s head. Can you react fast enough to catch the money?

24 5-May-15 Physics 1 (Garcia) SJSU Demo: Reaction Time Release Catch Distance (inches) Time (sec.)


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