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Motion and Force

Motion and Force - 8th grade science

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Motion and Force

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  1. Motion

  2. Describing Motion • Motion – occurs when an object changes position relative to a reference point. • Position – separation between an object and the reference point • Frame of reference – coordinate system used to define motion. • Reference point– zero location in a frame of reference

  3. Distance versus Displacement • Distance – how far an object has traveled • Displacement – distance and direction of an object’s change in position from the starting point

  4. Distance versus Displacement What is the runner’s distance? What is the runner’s displacement?

  5. Speed • Speed – the distance an object travels per unit of time; rate of change in position • Calculating Speed: • Formula: distance/time. • Unit: meters per second (m/s).

  6. Calculating Speed At what speed did a plane fly if it traveled 1760 meters in 8 seconds?

  7. Speed Practice • Try these practice problems on your own: • A car travels 240 km in 3 hours. What is the speed of the car during that time? • The speed of a cruise ship is 50 km/hr. How far will the ship travel in 14 hours? • If speed is 30 miles per hour and time elapsed is 2 hours then distance traveled is what ?

  8. Types of Speed • Average Speed • Instantaneous Speed • Constant Speed

  9. Average Speed • Average speed is the total distance traveled divided by total time. • A cyclist travels 32 km during the first two hours of a race. The cyclist travels 13 km for the last hour of the race. What is the cyclist’s average speed?

  10. Instantaneous Speed • Instantaneous Speed: An object’s speed at a particular moment in time. • Speed the speedometer reads in a car when you look at it at any given time.

  11. Constant Speed • Constant speed occurs when an object travels at a steady rate with the same instantaneous speed for some period of time. • The cyclist travels at a speed of 12 m/s along a strait path for several minutes.

  12. QUICK QUIZ • You arrive in my class 45 seconds after leaving math which is 90 meters away. How fast did you travel? • The pitcher’s mound in baseball is 85 m from the plate. It takes 4 seconds for a pitch to reach the plate. How fast is the pitch? • An airplane travels at 850 m/s. It travels in the air for 2 minutes. How far does it travel?

  13. Distance- Time Graph • Motion can be graphed on a distance-time graph with time plotted on the horizontal axis and distance plotted on the vertical axis. • The steeper the line on a distance-time graph, the greater the speed. • A horizontal line on a distance-time graph indicates that no change in position is occurring, and the speed is zero.

  14. Distance- Time Graph

  15. Interpreting Graphs • The graph to the left is showing: • Constant speed • Increase in speed • Object is stationary

  16. Velocity • Velocity—speed of an object and its direction of motion; velocity changes if either, or both, of these changes. • You know the velocity of a storm that is traveling 30 mi/h eastward.

  17. Change in Velocity • Each time you take a step you are changing the velocity of your body. • You are probably most familiar with the velocity changes of a moving bus or car. • The rate at which velocity (speed or direction) changes occur is called acceleration.

  18. Change in Velocity • If an object is speeding up you give a positive value • Ex: A skater increases her velocity. Her acceleration would be positive. • If an object is slowing down, you give a negative value. • Ex: A Skater decreases his velocity. His acceleration would be negative.

  19. Calculating Acceleration Acceleration= final velocity- starting velocity time Change in velocity = final – starting velocity velocity Acceleration= change in velocity time

  20. Using the Formula The formula for calculating acceleration is: acceleration = final velocity- starting velocity time The units for acceleration are m/s2

  21. Acceleration Practice • A cyclist accelerates from 0 m/s to 8 m/s in 3 seconds. • What is his acceleration ? • Is this acceleration higher than that of a car which accelerates from 0 to 30 m/s in 8 seconds?

  22. Free Fall • The constant acceleration of an object moving only under the force of gravity is "g". • The acceleration caused by gravity is 9.8 m/s2 • If there was no air, all objects would fall at the same speed

  23. Falling • Air resistance will increase as it falls faster • An upward force on the object • Eventually gravity will balance with air resistance • Reaches terminal velocity - highest speed reached by a falling object.

  24. Acceleration Due to Gravity • 3 things must occur for us to use this type of acceleration. • The object must be free falling • Air resistance is neglected • The object must be close to the surface of Earth.

  25. Acceleration Due to Gravity Formula • V=initial velocity +(the acceleration due to gravity X time) =Vo + gt =Vo+ (-9.8 m/s2) (t) Gravity=-9.8 m/s2 Gravity is negative because direction is downward. Remember velocity is speed with direction.

  26. Acceleration Due to Gravity Example • How fast will a pebble be traveling 3 seconds after being dropped?

  27. Forces

  28. What is a Force? • A force a push or a pull. • The SI unit of force is the newton (N). • Forces are measure with a scale.

  29. Newton’s Laws • Newton’s first law of motion states that an object will continue to stay at rest or move at constant speed in a straight line unless a force acts upon it. • A force has a size and a direction.

  30. Newton’s Law • Newton’s second law of motion states that a force F acting upon a body of mass m causes an acceleration a of the body given by: F = ma

  31. Newton’s Laws • Newton’s third law of motion states for every action, there is an equal and opposite reaction.

  32. Weight • Weight· (W) the force due to gravity. • If you were on the surface of the moon you would have a smaller weight, but your mass would be unchanged.

  33. Examples of Forces • Your weight points straight down towards the center of the Earth. • The size of your weight is given by: • W = mg • Remember g= 9.8 m/s2

  34. Examples of Forces • Example The acceleration due to gravity on the surface of the moon is g = 1.6 m/s2. • What would be the weight of an astronaut with mass m = 60 kg?

  35. Examples of Forces • On a new planet the same astronaut measures her weight to be 528 N. What is the acceleration due to gravity on the planet?

  36. Normal Force • A book resting on a table stays at rest because of another force which balances the gravitational force. This force is the normal force. • Normalforce· (N) the force produced by a surface that stops the object penetrating the surface.

  37. Normal Force Normal Weight The normal force always acts at right angles to the surface.

  38. Normal Force Example • Example If a book with mass 1.2 kg rests on a desk, what normal reaction force is produced by the desk?

  39. Normal Force Example • If you push down on a tabletop with a force of 30N, what will be the magnitude of the normal reaction? • (a) 30 N • (b) 300 N • (c) 3N • (d) 0

  40. Friction • Friction • The force of friction resists attempts to move objects. • The frictional force opposes the other force.

  41. Contact Force • Contact (Applied) Force ·a force which is applied to an object by a person or another object

  42. Force as Vectors • The motion of objects can be described by words like going fast, stopped, slowing down, speeding up, and turning. • We also use words such as distance, displacement, speed, velocity, and acceleration to describe motion.

  43. Forces as Vectors • These words are associated with mathematical quantities and can be divided into two categories. • Scalars are quantities which are fully described by a magnitude (or numerical value) alone. • Vectors are quantities which are fully described by both a magnitude and a direction.

  44. Forces as Vectors Example • Categorize each quantity as being either a vector or a scalar • 5 m • 30 m/sec, East • 5 mi., North • 20 degrees Celsius • 256 bytes • 4000 Calories

  45. Force Vectors • A force is a vector quantity. • To fully describe the force acting upon an object, you must describe both the magnitude (size or numerical value) and the direction.

  46. Force Vectors Contact Force (10 N) • Thus, 10 N is not a full description of the force acting upon an object. 10 N, to the right is a complete description of the force acting upon an object.

  47. Drawing Force Diagrams • Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object.

  48. Drawing Force Diagrams

  49. Drawing Force Diagrams • The direction of the arrow shows the direction which the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force.

  50. Drawing Force Diagrams • Example A book is at rest on a table top. Diagram the forces acting on the book. F norm F frict F app F grav

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