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No Strings Attached: Normal Forces, Force Vectors, Strings, Springs and Pulleys

No Strings Attached: Normal Forces, Force Vectors, Strings, Springs and Pulleys. Chapter 6.1-6.3. Important Vocabulary: Normal Force Contact Force Tension Coefficient of Friction. F. N. F f. F g. Friction: the most important everyday force, next to gravity!. The force of friction ..….

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No Strings Attached: Normal Forces, Force Vectors, Strings, Springs and Pulleys

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  1. No Strings Attached:Normal Forces, Force Vectors, Strings, Springs and Pulleys Chapter 6.1-6.3 • Important Vocabulary: • Normal Force • Contact Force • Tension • Coefficient of Friction

  2. F N Ff Fg Friction: the most important everyday force, next to gravity! The force of friction ..….. • Is the result of contact between two bodies. • Always acts to oppose (slow down) the motion. • Is proportional to the Normal force. • Does not depend on area of contact. Why not?

  3. Friction depends on whether or not the object is moving. • Static friction: friction for object at rest. • Kinetic friction: friction for moving object. Kinetic Static V>0 V=0 Two more notes about friction: -Coefficient of static friction is higher than that of kinetic friction (frictional force decreases when object begins to move). -Coefficient of kinetic friction does not change with speed And finally(?)….frictional force “laws” are an approximation, but a good one.

  4. A problem of friction A block of mass M = 1.5 kg sits on a hinged inclined plane. The coefficient of static friction is m=0.15. At what angle of the inclined plane does the block begin to slide? • Draw the picture showing the forces on the block. What are they? • Draw the free-body diagram. • Write down Newton’s law, Fnet = M a • Think about the problem. When does the acceleration become greater than zero?

  5. Science Friction. V0 m=0 m=.05 A block of mass M=1 kg slides with speed Vo over a frictionless surface. Then, it hits a rough surface with kinetic coefficient of friction m=0.05 . How much further does it slide before it stops?

  6. Have I got a tension headache! • Tension, T, is the “contact force” for pulling objects • Tension is a real force—you can measure it by cutting the string and inserting a force scale TENSION

  7. Tension is real—it can be measured.

  8. M2 T F M1 Case of NO FRICTION. F2=F-T F T F1=T Tension Problem • Given, M1, M2, and F • What is acceleration? • What is the Tension, T in the line? • What is the force on each block? IMPORTANT: Blocks move together, so each has the same acceleration “a” and speed “v”. Check work: Look a limits of large and small M1, M2.

  9. Pulleys: the beginnings of technology. A pulley changes the direction of Tension

  10. Compare the tension in the left and right cases. 0 / 100 • The left is higher. • The right is higher since the mass is double. • They are the same. Cross-Tab Label

  11. Simplest pulley system.

  12. What happens to the tension? 0 / 100 • It is the same in both cases, the bucket mass doesn’t change. • The tension doubles in the right side. • The tension is reduced by ½ in the right side. Cross-Tab Label

  13. The “bosun’s chair” problem.

  14. Accelerating blocks: more of a challenge Given M2, M1 and g. What is a? What is the tension? What are the forces? (ignore friction)

  15. Accelerated blocks and tension. CAUTION: The TOTAL force on M2 is NOT JUST THE WEIGHT!

  16. Freebody diagram a a NOTE: Why is the diagram for mass 2 correct? Isn’t it moving in the y axis?

  17. Accelerating tethered blocks. a Block on the table. Block dropping down. Does this make sense? Check it by looking at limits of M2.

  18. Tethered blocks: add one more! T1 T2 T2 M3g Do on “board” then reveal Add all three equations together.

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