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Chapter 10 Motion

Chapter 10 Motion. Measuring Motion. Motion —when an object changes its position relative to a reference point Distance —how far an object has moved Displacement —distance and direction of an object’s change of position from a starting point. Motion. Problem: Is your desk moving?

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Chapter 10 Motion

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  1. Chapter 10 Motion

  2. Measuring Motion • Motion—when an object changes its position relative to a reference point • Distance—how far an object has moved • Displacement—distance and direction of an object’s change of position from a starting point

  3. Motion • Problem: • Is your desk moving? • We need a reference point... • nonmoving point from which motion is measured

  4. Reference point Motion Motion • Motion • Change in position in relation to a reference point.

  5. Motion Problem: • You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward. • You have mistakenly set yourself as the reference point.

  6. Measuring Motion • Speed—distance an object travels per unit of time • Rate—any change over time • Calculation for speed: speed = distance/time • Speed that doesn’t change over time—constant speed • Speed is usually not constant; usually an object has changing speed. • Average speed—speed of motion when speed is changing: speed = total distance/total travel time • Instantaneous speed—speed at any given point in time

  7. d v t Speed • Speed • rate of motion • distance traveled per unit time

  8. Speed & Velocity • Instantaneous Speed • speed at a given instant • Average Speed

  9. Measuring Motion • A distance-time graph displays motion of an object over time. • Plot distance on a(n) vertical axis. • Plot time on a(n) horizontal axis. • Velocity—speed and direction of an object’s motion • Motion of Earth’s crust—so slow we don’t notice

  10. Speed & Velocity • Problem: • A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried? • It depends on the storm’s direction!

  11. Speed & Velocity • Velocity • speed in a given direction • can change even when the speed is constant!

  12. Acceleration • Acceleration—change in velocity’s rate • Positive acceleration—speed is increasing. • Negative acceleration—speed is decreasing. • When an object changes speed or direction, it is accelerating.

  13. Acceleration • Calculating acceleration • Acceleration = change invelocity/time • Change in velocity = final velocity – initial velocity • Unit for acceleration—meters per second squared • Positive acceleration—positive number with a positive slope on a velocity-time graph • Negative acceleration—negative number with a negative slope on a velocity-time graph

  14. Acceleration • Amusement park acceleration—Roller coasters • Changes in speed cause acceleration. • Changes in direction cause acceleration.

  15. vf - vi t a Acceleration • Acceleration • the rate of change of velocity • change in speed or direction a: acceleration vf: final velocity vi: initial velocity t: time

  16. Motion and Force • Force—a push or pull that one body applies to another • A force can cause an object’s motion to change. • When two or more forces combine at the same time, they create a net force. • Balanced forces are equal in size and opposite in direction. • Unbalanced forces are unequal in size and / or are not in the same direction.

  17. F a m Force F m F = ma F: force (N) m: mass (kg) a: accel (m/s2) 1 N = 1 kg ·m/s2

  18. Force What forces are being exerted on the football? Fkick Fgrav

  19. Balanced Forces forces acting on an object that are opposite in direction and equal in size no change in velocity Force

  20. N N Force • Net Force • unbalanced forces that are not opposite and equal • velocity changes (object accelerates) Fnet Ffriction Fpull W

  21. Motion and Force • Inertia and Mass • Inertia—an object’s resistance to any change in motion • Objects with greater mass have greater inertia. • Newton’s first law of motion—an object moving at a constant velocity keeps moving at that velocity unless a net force acts on it; an object at rest will stay at rest unless a net force acts on it.

  22. Motion and Force • Auto crashes—the law of inertia at work • A passenger not wearing a seat belt keeps moving forward at the car’s speed even after the car stops. • A passenger wearing a seat belt slows down as the car slows down and stops.

  23. F a m Calculations • What force would be required to accelerate a 40 kg mass by 4 m/s2? GIVEN: F = ? m = 40 kg a = 4 m/s2 WORK: F = ma F = (40 kg)(4 m/s2) F = 160 N

  24. F a m Calculations • A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? WORK: a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s2

  25. F a m Calculations • Mrs. J. weighs 557 N. What is her mass? GIVEN: F(W) = 557 N m = ? a(g) = 9.8 m/s2 WORK: m = F ÷ a m = (557 N) ÷ (9.8 m/s2) m = 56.8 kg

  26. d t v Calculations • Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: d = 100 m t = 20 s v = ? WORK: v = d ÷ t v = (100 m) ÷ (20 s) v = 5 m/s You skate faster!

  27. vf - vi t a Calculations • A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration? GIVEN: vi = 10 m/s t = 3 s vf = 32 m/s a = ? WORK: a = (vf- vi) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a= 7.3 m/s2

  28. d t v Calculations • Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it? GIVEN: v = 330 m/s d = 1km = 1000m t = ? WORK: t = d ÷ v t = (1000 m) ÷ (330 m/s) t = 3.03 s

  29. vf - vi t a Calculations • How long will it take a car traveling 30 m/s to come to a stop if its acceleration is -3 m/s2? GIVEN: t = ? vi = 30 m/s vf = 0 m/s a = -3 m/s2 WORK: t = (vf- vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2) t = -30 m/s ÷ -3m/s2 t = 10 s

  30. Distance-Time Graph A B Graphing Motion speed • slope = • steeper slope = • straight line = • flat line = faster speed constant speed no motion

  31. Distance-Time Graph A B Graphing Motion • Who started out faster? • A (steeper slope) • Who had a constant speed? • A • Describe B from 10-20 min. • B stopped moving • Find their average speeds. • A = (2400m) ÷ (30min) A = 80 m/min • B = (1200m) ÷ (30min) B = 40 m/min

  32. Distance-Time Graph Graphing Motion • Acceleration is indicated by a curve on a Distance-Time graph. • Changing slope = changing velocity

  33. Speed-Time Graph Graphing Motion acceleration • +ve = speeds up • -ve = slows down • slope = • straight line = • flat line = constant accel. no accel. (constant velocity)

  34. Speed-Time Graph Graphing Motion Specify the time period when the object was... • slowing down • 5 to 10 seconds • speeding up • 0 to 3 seconds • moving at a constant speed • 3 to 5 seconds • not moving • 0 & 10 seconds

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