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Journal #28 10/16/09

Journal #28 10/16/09. A skydiver has reached a terminal velocity while falling. He then opens a parachute. Which of the following statements is FALSE? The velocity of the skydiver will always be downward

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Journal #28 10/16/09

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  1. Journal #28 10/16/09 • A skydiver has reached a terminal velocity while falling. He then opens a parachute. Which of the following statements is FALSE? • The velocity of the skydiver will always be downward • Just after the parachute is opened, the skydiver will have a net force that is upward • Just before pulling the parachute, the net force of the skydiver was zero • The acceleration of the skydiver will always be downward

  2. Journal #29 10/19/09 • Reflect on your current grade in physics. (glue your progress report into your journal under your writing)

  3. Journal #30 10/20/09 • Solve for the missing side of the triangles using the Pythagorean Theorem. a2 + b2 = c2 25.0 x 5.0 x x 11.2 20.3 7.8 4.0

  4. 25.0 x 5.0 x x 11.2 20.3 7.8 4.0 Journal #30 a2 + b2 = c2

  5. Chapter 5 Forces in Two Dimensions

  6. Vector Addition • Resultant – the result of adding 2 or more vectors together • If 2 or more vectors run along the same straight line in the same direction, ADD them together to find resultant. • If two vectors run along the same straight line in opposite directions, SUBTRACT them to find resultant.

  7. Vectors at Right Angles (90º) For 2 Vectors: • Rearrange (move) the placement of one vector so that they are drawn “tip-to-tail” • You can move either one and still get the same answer! • Draw the resultant from the tail of the first vector to the tip of the last vector. • If there are more than 2 vectors in a problem, you should be able to simplify them down to two directions before attempting this step!!!

  8. Resultant Tip to Tail Method Free Body Diagram Resultant Tip to Tail Method Moving Vectors Around

  9. Mathematically Solving Right Angle Problems • Mathematically, when vector A is at a right angle to vector B, use the Pythagorean theorem to find resultant: r2 = a2 + b2 • The sum of the squares of the magnitudes of the two component vectors is equal to the square of the resultant.

  10. Components of vectors • You can break a single vector into its 2 components - vectors which make it up. This process is called vector resolution • Any vector drawn on paper can be broken into the horizontal and vertical components which make it

  11. y component x component Components of a vector

  12. Example 1 • During a cheer, Jessica moves 3m forward and 5m to the right. What is her displacement?

  13. Example 2 • On the first play of the game, Austin ran the ball 10m forward then 3 meters to the right.

  14. Example 3 • To get home, Zac had a displacement of 5 miles to the northwest. If he traveled 4 miles directly to the north, how far did he travel to the west to finish his trip?

  15. Example 4 • A crate is being pushed forward by two boys with a force of 20N each while a third boy is pushing to the left with a force of 25N. What is the resulting force vector for the crate? (Start with a free-body diagram!)

  16. Homework • P. 141 • #79, 81, 82

  17. Lab Data Table

  18. 5.2: Friction • P. 126-130 in textbook

  19. Friction • Friction is a force that opposes motion. • There are two types: • Kinetic Friction • Static Friction

  20. Kinetic Friction • The friction exerted on one surface by another when the two surfaces rub against each other because one or both of them are moving • Examples: • Hands rubbing together • Pushing a box up a ramp • Dragging a crate

  21. Static Friction • The friction exerted on one surface by another when there is no motion between the two surfaces • Examples: • Leaning against a table but it doesn’t move • Trying to push a couch but you can’t move it

  22. Relating FN to Ff • Different surfaces cause different amounts of friction between objects. • If you were to plot a graph of Ff vs. FN for an object, the slope of the line is called the coefficient of friction (). This number is a constant, regardless of the weight of the object and can be found with the following formulas:

  23. Important notes about friction • When working with Ff, you will always have to consider the rules of calculating Fnet • Usually, you will have to consider that Fnet = F(forward motion) - Ff

  24. Common Coefficients

  25. FN Fthrust Ff Fg Example 1 - P. 128, #18 • You need to move a 105-kg sofa to a different location in the room. It takes a force of 102N to start it moving. What is the coefficient of static friction between the sofa and the carpet?

  26. FN FT Ff Fg Example 2 - P. 128, #20 • Suppose that a 52-N sled is resting on packed snow. The coefficient of kinetic friction is only 0.12. If a person weighing 650N sits on the sled, what force is needed to pull the sled across the snow at constant speed?

  27. Journal #34 10-26-09 • A 1.4-kg block slides across a rough surface such that it slows down with an acceleration of 1.25 m/s2. What is the coefficient of kinetic friction between the block and the surface?

  28. FN Ff Fg Journal #34 10-26-09 • A 1.4-kg block slides across a rough surface such that it slows down with an acceleration of 1.25 m/s2. What is the coefficient of kinetic friction between the block and the surface?

  29. Homework Assignment • P. 128, #17, 19 • P. 130, #23, 24

  30. Objects on an inclined surface

  31. Journal #35 10-27-09 • Draw the FBD of an object that is being pulled on but is still at rest because of static friction. • Draw the FBD of an object that is accelerating while being pulled across the floor.

  32. Journal #36 10-29-09 • Draw and color the plan for your Center of Mass Project. Do not draw any strings connecting the layers.

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