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Chapter 2 : MOTION

Chapter 2 : MOTION. p.16 in your book!. Aristotle (384-322 BC) Objects have a proper “place” And strive to get there. NATURAL MOTION - No force required ex: boulder “falls down” smoke “goes up” Thought heavier objects fall faster than lighter objects.

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Chapter 2 : MOTION

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  1. Chapter 2 : MOTION p.16 in your book!

  2. Aristotle (384-322 BC) Objects have a proper “place” And strive to get there. • NATURAL MOTION - No force required ex: boulder “falls down” smoke “goes up” Thought heavier objects fall faster than lighter objects

  3. UNNATURAL MOTION- Requires force EX: push a book across table

  4. Galileo- • Objects drop at same rate (except for air friction) “Leaning Tower of Pisa “ experiment • If no friction…no forces required to keep moving objects moving. EX:Satellites

  5. As a ball rolls down an incline it speeds up • up incline,slows • Reduced angle, ball goes farther

  6. Inertia Objects at rest tend to remain at rest. Moving objects tend to remain moving.

  7. Speed • How fast something is moving: the rate at which distance is covered. Speed= Distance Time EX: mph (mi/hr) , km/hr, cm/hr / = “per” = divided by ex: 100km/hr

  8. 1. Instantaneous speed • Speed something has at any instant • Ex: speedometer

  9. Average speed= total distance covered time interval

  10. Example: we drive 100 km in a time of 2 hrs. Av sp= Total distance covered = 100km =50km time interval 2hrs hr Trip could have variations in speed -average speed!

  11. Another example: we walk to McDonalds : 2.0km away & it takes 40 minutes. Av speed = Total distance covered Time interval Av speed = 2.0km / 40 min = 0.05 km /min But… stopped for traffic,tied shoe,ran across the road… YOU GET THE IDEA!!

  12. Velocity – includes speed & direction ex: 60km/ hr North This is a Vector Quantity- includes direction & magnitude. What is the difference between constant speed & velocity?

  13. How can a racecar have constant speed but it’s velocity is changing? • Constant speed- doesn’t speed up or slow down. • Changing velocity because direction is changing.

  14. Other formulas: V = D/T D = V x T T = D/V V = velocity, D = distance, T=time

  15. Interpreting Distance vs. Time graphs: See board: • Speed vs. Velocity • What is Slope? • What is ______ doing? • Car “a” • Car “b” • Car “c” • Car “d”

  16. Lets try some problems: • Av speed of bike that travels 150 m in 15 secs • V = D / T • V = 150 m /15 s • V = 10m/s

  17. # 2 : You ran an av. Speed of 3 km/hr for 1 hr. • ) How far did you go? • D=VxT 3km/hr • 1 hr D = 3km b. At this rate, how far in 2 hrs? 10 hrs? 3km/hr • 2 hr = 6km 3km/hr • 10 hr = 30km

  18. 2.4 Motion Is relative • Right now :Your speed is zero relative to Earth, But.. 30 km / s relative to the sun.

  19. Isaac Newton • P. 22 green box Newton’s 1st Law “THE LAW OF INERTIA” • Every object continues in a state of rest, or in a state of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it. “the table cloth trick” “penny & index card inquiry”

  20. Net Force – combination of all forces that act on an object. See Board Newton (N) – unit for force An arrow represents Force as vector quantities. • Arrows length represents magnitude (how much) and direction (which way)

  21. Vector Addition: • 12 N + 8 N = _____ • -20 N + 3 N = _____ • 7 N + 8 N = _____ • 15 N - 10 N = _____

  22. 2.7 Equilibrium for objects at rest Spring scale & block example on board Attracted to the Earth with a force of __ N. Weight of object (downward force)= tension in rope (upward force). The block is at rest, so net force is Zero.

  23. Mechanical equilibrium : ∑ F = 0 ∑ - sum F- force Objects at rest have equal & opposite forces acting on them. • Sum of upward vectors= sum of downward vectors • Static Equilibrium

  24. Why don’t we fall through the floor? Support Force or “normal force”.- the upward force EX: book on desk : weight & gravity ∑ F = 0 What is the net force on a bathroom scale when a 110 lb person stands on it? A: Zero. Scale is at rest. Scale reads support force which has same magnitude as weight.

  25. Equilibrium for moving objects Equilibrium- state of no change. An object moving at constant velocity is in dynamic equilibrium.

  26. Some Questions for you… • Give an example of something moving when a net force of zero acts on it? • If we push a crate at a constant velocity, how do we know how much friction acts on the crate compared to our pushing force? • Harry the painter swings from his painter’s chair. His weight is 500 N and the rope has a breaking point of 300 N. Why doesn’t the rope break when he is supported as shown left on the board

  27. Some Questions for you… • Harry the painter swings from his painter’s chair. His weight is 500 N and the rope has a breaking point of 300 N. Why doesn’t the rope break when he is supported as shown left below? One day he decides to anchor his chair to a nearby flagpole – why did Harry end up taking vacation early?

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