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Journal # 23

Journal # 23. (TRY NOT TO USE YOUR NOTES) What are the 3 rules of adding vectors?. JOURNAL # 23 ANSWER. If vectors have the SAME direction, ADD the values and keep that direction. If vectors have OPPOSITE directions, SUBTRACT the values and keep the direction of the larger value.

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Journal # 23

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  1. Journal #23 (TRY NOT TO USE YOUR NOTES) • What are the 3 rules of adding vectors?

  2. JOURNAL #23 ANSWER • If vectors have the SAME direction, ADD the values and keep that direction. • If vectors have OPPOSITE directions, SUBTRACT the values and keep the direction of the larger value. • If vectors are at RIGHT ANGLES to each other, use the Pythagorean Theorem to solve for the resultant.

  3. 5.2: Friction • P. 126-130 in textbook

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

  5. 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

  6. 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

  7. 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:

  8. 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

  9. Common Coefficients

  10. 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?

  11. 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?

  12. FN Ff Fg Example 3 - P. 130, #22 • 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?

  13. Homework Assignment • P. 128, #17-21 • P. 130, # 22-24

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