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MOTION

MOTION. Do you have to see something move to know that motion has taken place??. All you need to know is that its position has changed!. Motion occurs when an object changes its position RELATIVE TO A REFERENCE POINT.

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MOTION

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  1. MOTION

  2. Do you have to see something move to know that motion has taken place?? All you need to know is that its position has changed! Motion occurs when an object changes its position RELATIVE TO A REFERENCE POINT.

  3. The dog’s position has changed relative to the tree (reference point) so you know motion must have occurred.

  4. After you choose your reference point, you can create what is called a FRAME OF REFERENCE This is basically just a GRAPH!! Y axis X axis

  5. DESCRIBING MOTION 1. DISTANCE • Describes how far something has moved. QUICK REVIEW: Distance is measured in what unit when you use SI??

  6. DESCRIBING MOTION DISTANCE V/S DISPLACEMENT Distance is HOW FAR SOMETHING HAS TRAVELED from the reference point (starting position) Whereas 2. Displacement is DISTANCE and the DIRECTION from the reference point (starting position) something is.

  7. EXAMPLE Total Distance Traveled =80 m Displacement = 20 m North

  8. DESCRIBING MOTION 3. Speed = How far something travels (distance) in a given amount of time Speed (in meters/second) = distance (in meters) time (in seconds) (You could also state speed in km/h or km/s or cm/yr)

  9. EX. OF A SPEED PROBLEM A car traveling at a constant speed covers a distance of 750 m in 25 s. What is the car’s speed? Important info: Distance (d) = 750 m Time (t) = 25 s Speed = d/t Speed = 750 m = 25 s 30 m/s

  10. Different Types of Speed • Constant Speed = when the speed hardly changes. Like when you use cruise control on your car. • Changing Speed = when the speed changes over a given distance. Like when you ride a bike or drive through downtown. • Average Speed = this describes speed of motion when you have changing speed. It is the total distance traveled divided by the total time traveled.

  11. Different Types of Speed 4. Instantaneous Speed = The speed at a given point in time. Like going from 0 km/h to 60 km/h. Your speedometer shows you your instantaneous speed. -Instantaneous Speed changes when you increase or decrease your speed -Instantaneous Speed stays the same when your speed is constant

  12. How would these different types of speed look if your graphed them? CONSTANT SPEED Distance Distance CHANGING SPEED time time

  13. GRAPHING MOTION Use a DISTANCE/TIME GRAPH -Time = X axis -Distance = Y axis A graph of constant speed will represent the motion with a straight line To show two constant speeds which are different, you use SLOPE = steepness of the line = speed Steeper the slope = faster the speed

  14. GRAPHING MOTION A graph of changing speed will have a zig-zag or curved line

  15. Putting it all together FASTEST CONSTANT SPEED DISTANCE SLOWER CONSTANT SPEED What is the slope of this part of the line? CHANGING SPEED TIME

  16. Putting it all together What is the average speed (slope) during the 10 min to 20 min period? 1000 600 Change in distance = 1200-600 = 600 Amount of time = 20-10 = 10 min 200 Avg. Speed = change in distance (600 m) Amount of time (10 min) 0 10 20 30 = 60 m/min

  17. Putting it all together What is the instantaneous speed at t= 10 min? (it is the slope of the line at that point) 1000 600 600m – 0m = 600 m 10min – 0min = 10min 600m/10min = 60m/min 200 0 10 20 30

  18. VELOCITY Includes the SPEED of an object and the DIRECTION it is moving -Velocity of an object can change even though the object’s speed doesn’t – how?? -The velocity changes if the direction changes even though the speed is constant

  19. How are Velocity and Speed different?? Velocity includes direction/speed doesn’t!

  20. Example Problem with Velocity Describe the velocity of an object that travels north 6.9 m in 3 s, then turns and travels south 2.8 m in 4 s. 6.9 m / 3 s = 2.3m/s NORTH Work this just like you did for finding average speed, but now you include direction! Then 2.8 m / 4 s = 0.7m/s SOUTH

  21. ACCELERATION = Rate of change of velocity -When velocity changes = object is accelerating -So: Acceleration occurs when an object changes speed, changes direction, or changes both speed and direction

  22. Acceleration due to a Change in Speed • Increasing Speed = Positive Acceleration • Decreasing Speed = Negative Acceleration (Deceleration)

  23. Graphing Positive and Negative Acceleration Positive Slope = Positive Acceleration = like going downhill (speeding up) Negative Slope = Negative Acceleration = like climbing a hill (slowing down) Speed Speed Time Time

  24. Graphing Zero Acceleration Zero Slope = Zero Acceleration Speed This means No change in Velocity = No change in Speed or direction Time

  25. Putting them all together Speed 0 A - A + A Time

  26. Calculating Acceleration Acceleration (A) m/s2 = Change in Velocity (m/s) Time (s) Final Velocity = Vf Initial Velocity = Vi A (m/s2) = Vf-Vi (m/s) Time (s)

  27. Calculating Acceleration If there is no direction change and only speed changes, then a change in velocity = change in speed so you use: A (m/s2) =Final speed – Initial Speed Time

  28. Example Problems 1. A plane on a runway reaches 80 m/s in 20 s. What is the planes acceleration? Important Info: Final Speed = 80m/s Time = 20 s A = Vf-Vi = 80m/s – 0m/s = 80 = 4m/s2 t 20s 20

  29. Example Problem A skateboarder is going 3m/s and stops in 2 seconds. Important Info: Final Speed = 0m/s Initial Speed = 3m/s Time = 2 s A (m/s2) = Vf-Vi = 0m/s – 3m/s = -1.5 m/s2 t 2s

  30. Working Backwards The acceleration of a car that comes to a stop in 5 seconds is -4m/s2. What was the car’s speed when it started accelerating (decelerating)? -4m/s2 = 0 – Vi 5s A = -4m/s2 t = 5s 5s X X 5s A = Vf-Vi t -20 m/s = -Vi 20 m/s = Vi

  31. Get your book: Work these problems! pg. 51 # 6-7

  32. Force = A push or pull -Can cause the motion of an object to change (so it can change velocity) EX: kicking a soccer ball, playing pool, hitting a tennis ball

  33. Does the Atmosphere have force? Do you feel it? Does gravity have force? Do you feel it?

  34. Net Force -When two or more forces act on an object at the same time, the forces combine and we call this NET FORCE

  35. Balanced Forces -Forces on an object that are equal in size and opposite in direction -The Forces cancel each other out -Net Force = 0

  36. Unbalanced Forces -When two or more unequal forces act on an object Situation #1 -Forces are unequal and in opposite directions -Object is moved in the direction of the larger force -Net force = difference between the two forces

  37. Unbalanced Forces Situation #2 -Forces are applied in the same direction =Forces are combined (added together) -Net force = add two forces together

  38. Inertia = The tendency of an object to resist a change in its motion -The velocity (speed and direction) of an object remains constant unless a force changes it This said, if an object is at rest, it should stay at rest unless a force causes it to move. Time to test this out.

  39. Inertia and Mass -The greater an object’s mass = The greater its inertia (the more it resists moving) DUH! It takes more force to stop a bowling ball by swatting it with a paddle than it does to stop a tennis ball!

  40. Newton’s First Law of Motion -An object moving at a constant velocity(constant speed and direction) keeps moving at that velocity unless an unbalanced net force acts on it. -AKA: The law of Inertia

  41. Two students push on a box in the same direction, and one pushed in the opposite direction. What is the net force on the box if each pushes with a force of 50 N. Subtract the opposite force from your combined forces 50N 50N 50N Net Force = 100 N – 50 N = 50 N in the direction the two students are pushing Add your combined forces (50 + 50 = 100)

  42. The downward force of gravity and the upward force of air resistance on a ball are both 5N. What is the ball’s acceleration? (Remember acceleration = change in velocity divided by time) 5 N Net force = Difference between the two = 5 N – 5 N = 0 So is there any change in velocity (speed or direction of the object)? NOPE. Acceleration = 0 m/s2 5 N

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