1 / 46

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!.

sulwyn
Download Presentation

Mechanics: Kinematics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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).

More Related