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Mechanics: Kinematics

Describing Motion. Mechanics: Kinematics. Mechanics. Modern study of motion 3 Types: Kinematics – how things move Dynamics – what causes things to move Statics – how stationary items react to pushes and pulls We will study Kinematics in this chapter and Dynamics in Chapter 5!.

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Mechanics: Kinematics

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  1. Describing Motion Mechanics: Kinematics

  2. Mechanics • Modern study of motion • 3 Types: • Kinematics – how things move • Dynamics – what causes things to move • Statics – how stationary items react to pushes and pulls • We will study Kinematics in this chapter and Dynamics in Chapter 5!

  3. Physical Systems • System – all processes, forces and measurable properties within some boundary • Surroundings – everything outside the system

  4. Is it in motion? • Look at the TV • 1. Is the TV in motion relative to you and your classmates? • 2. Is the TV in motion at all? • 3. Does the TV’s motion depend on where I look at it from? • 4. If the TV is moving through space why doesn’t it appear to be moving to us?

  5. Motion is a change in position So reference points are necessary

  6. Frame of Reference • Reference Point - the thing we use to measure movement • This point generally considered to be nonmoving • What is the most common frame of reference? • Earth ! • (Sometimes misleading, e.g. sun rising and setting)

  7. Sports use reference points • …To determine motion • E.g. • Football: sidelines, yard marker, and end zone markers • Basketball: half court, out of bounds, and three point shot • Baseball: foul ball, bases, and home run

  8. 3 Types of Reference Frames • 1. Fixed reference frame – “observer” is stationary (most common) • E.g watch a car drive by • 2. Accelerated reference frame – the “observer” accelerates with the system • E.g. riding in a car • 3. Rotational reference frame – “observer” rotates with system • E.g riding on a merry-go-round

  9. Coordinate Axis • Imaginary line marked off in distance units • Origin is marked with zero value • Motion occurs in positive direction • Movement in opposite direction is negative

  10. Time Intervals • A span of time

  11. Scalars and Vectors • Scalar – measurable quantity. Can be described with single piece of information • E.g. 9:05 am • 2nd period • Room 211 • Vector – measurable quantity that has a directional component • E.g. 15 miles due West • 150 lbsforce directed at center of Earth

  12. Some Physics Quantities • Vectors: • Displacement • Velocity • Acceleration • Momentum • Force • Scalars: • Distance • Speed • Time • Mass • Energy

  13. Mass vs. Weight • Mass • Scalar (no direction) • Measures the amount of matter in an object • Weight • Vector (points toward center of Earth) • Force of gravity on an object On the moon, your mass would be the same, but the magnitude of your weight would be less.

  14. Sec 4B – Kinematics: Describing Motion • Kinematics = motion • It is a measure of: • Position • Speed or velocity • acceleration • How things move!

  15. Motion • Motion = change in position

  16. Motion

  17. Distance • = how far an object has moved from a starting point • The SI unit of length is the meter (m). • Longer distances are usually measured in kilometers (km). • 1 km=1,000 m • Shorter distances are measured in cm. • 1 m=100 cm

  18. Displacement • = change in position from a starting point • Includes both distance and direction of an object • Distance and displacement may be equal when measuring in a straight line • Ex. Runner running from one end of the gym to the other, and then running back to the starting point. • Displacement=0 m • Distance=60 m

  19. How does Distance compare to Displacement? • http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Flash/ClassMechanics/DisplaceDistance/DisplaceDistance.html

  20. Speed • = rate of motion (how fast an object is moving) • Speed is described as the distance an object travels per unit of time. • A car traveling at 65 mph.

  21. Calculating Speed • Speed=Distance / Time or • Ex. Suppose you ran 2 km in 10 min. • = 0.2 km/min

  22. Motion with Constant Speed • Motion with constant speed - an object moving at a certain speed in which that speed does not increase or decrease. • Ex. A car traveling at 70 mph on an empty highway without accelerating or decelerating.

  23. Changing Speed • Much of the time the speed is not constant. • Ex. When driving a car, the car starts at 0 mph and increases in speed until you reach the speed limit. As you approach a stop sign, the car slows in speed until you come to a halt. During this time, the speed of the car is always changing.

  24. Average Speed • = speed of motion when speed is changing • = total distance traveled divided by the total time of travel. • Ex. For a bicycle trip, the total distance traveled by two friends was 5 km and the total time was ¼ of an hour, or .25 h. The average speed was: • S=d/t or 5 km/.25 h=20 km/h

  25. Average Speed

  26. Instantaneous Speed • Instantaneous speed is the speed at a given point in time. • Ex. At any point during a car ride, the speed that you read on the speedometer is the instantaneous speed.

  27. Velocity • Speed tells how fast something is moving, but not what direction it is moving • Velocity - speed of an object and the direction of its motion • So velocity can change even if speed remains constant! • Calculating:

  28. Velocity • In the news the last few years there has been mention of a few hurricanes. • When the speed of the hurricane is described, you hear that it is moving at 25 mph and is located 25 miles east of the United States. • Should we be worried?

  29. Is it possible for something to move so slowly that you cannot detect its motion? • If you think the answer is yes, how will you know that it has moved?

  30. Acceleration, Speed, and Velocity • You are sitting at a stop light in the car, as the light turns green, the driver steps on the pedal and the car starts to move faster and faster. • This change in speed is known as acceleration. • Acceleration - the rate of change of velocity

  31. Acceleration, Speed, and Velocity • Remember…velocity includes both speed and direction • A change in velocity can be either a change in speed or direction • Acceleration occurs when an object changes its speed, its direction, or both

  32. What does a change in acceleration look like? • http://faraday.physics.utoronto.ca/PVB/Harrison/Flash/ClassMechanics/MotionDiagram/MotionDiagram.html

  33. Speeding Up and Slowing Down • When you think of acceleration, what do you think of? • We think of something speeding up, however acceleration also occurs when something slows down. • When a car slows down, its speed is decreasing and it has a negative acceleration. • When a car speeds up, its speed is increasing and it has a positive acceleration.

  34. Changing Direction • A change in velocity can either be a change in how fast something is moving, or a change in direction. • Any time an object changes direction, its velocity changes and it is accelerating. • Can you think of any examples of an object traveling at a constant speed, but constantly changing direction?

  35. Calculating Acceleration • Acceleration= Change in Velocity Time • Change in velocity=final velocity-initial velocity The SI Unit for acceleration is m/s2

  36. Calculating Positive Acceleration • A jet takes off from rest on the runway and climbs to a speed of 80 m/s in 20 s. How do we calculate the acceleration? • A= (80 m/s- 0 m/s)= 4 m/s2 20 s The jet is speeding up, so the final speed is greater than the initial, and the acceleration is positive.

  37. Calculating Negative Acceleration • A skateboarder is moving in a straight line at a speed of 3 m/s and comes to a stop in 2 s. How do we calculate the acceleration? • A = (0 m/s- 3 m/s) = -1.5 m/s2 2 s The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative.

  38. http://www.physicsclassroom.com/mmedia/newtlaws/il.cfm • http://www.tutorvista.com/content/physics/physics-iii/motion-laws/inertia-animation.php • http://www.absorblearning.com/media/item.action?quick=4z • http://www.bigs.de/BLH/en/index.php?option=com_content&view=category&layout=blog&id=78&Itemid=246

  39. What Happens in a Crash • In a car crash, a car traveling at 50 mph hits something solid head on. • When this happens, the car crumples, slows down, and stops within a tenth of a second. • The passenger in the car continues moving at that same speed until they slam into the dashboard, steering wheel, windshield, or the back of the front seat.

  40. Seat Belts • Seat belts prevent that person from hitting objects in the car. • The belt loosens a bit, giving the person more time to slow down. • Car safety experts estimate that half of the people who die in car crashes would survive if they were wearing safety belts (71% for infants).

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