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3.1 Spring Scales

3.1 Spring Scales. New ideas for today. Weight vs. Mass Springs Scales. http://www.youtube.com/watch?v=MFXLhN3YNmg. Springs, torsion spring. Observations. Lots of useful and fun things have springs in them Different types of materials act as springs

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3.1 Spring Scales

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  1. 3.1 Spring Scales

  2. New ideas for today Weight vs. Mass Springs Scales http://www.youtube.com/watch?v=MFXLhN3YNmg

  3. Springs, torsion spring

  4. Observations Lots of useful and fun things have springs in them Different types of materials act as springs Springs always seem to exert a force on you Scales measure your weight…sometimes How is all of this related?

  5. A Free Spring • A free spring adopts a certain length • Its ends experience zero net force • Its ends are in equilibrium • The spring is at its equilibrium length

  6. Hooke’s Law A Distorted Spring • If you distort a spring, forces act on its ends • These forces • act to restore the spring to equilibrium length • are called “restoring forces” • are proportional to the distortion

  7. Hooke’s Law The restoring force on the end of a spring is equal to a spring constant times the distance the spring is distorted. That force is directed opposite the distortion. Restoring Force = – Spring constant x Distortion

  8. Spring compression F = -k x “+” F = -k x = -k 0 = 0 F = -1 N/cm· 1 cm = -1 N F = -1 N/cm· 2 cm = -2 N F = -1 N/cm· (-1) cm = 1 N

  9. Metal sheet Springs Always produce a force that opposes the distorting force                                                          <>

  10. Clicker question The figure shows a strobed photograph of a small ball being shot upward by a spring. The spring, with the ball on top of it, was initially compressed to P (its equilibrium position is Q). The ball left the spring at point Q and flew up to point R. Neglect air resistance. Which is a true statement? • The spring force at P is less than the force of gravity on the ball. • The spring force at P is greater than the force of gravity on the ball. • The spring force at P is equal to the force of gravity on the ball.

  11. Follow the money work. What is the difference between the FLYBAR and a normal POGO stick? How do diving boards and trampolines allow you to jump higher? Why can you jump higher on the FLYBAR?

  12. Scale Spring Scales Use a spring to measure weight

  13. Equilibrium • An object in equilibrium • experiences zero net force • is not accelerating • At equilibrium, • individual forces balance one another perfectly • an object at rest remains at rest • an object in motion coasts

  14. Scale on skates Clicker Question A scale sits on a skate. A mass hangs over a pulley on one end, and the other end is attached to a pole (fixed). Read the scale. Now, remove the fixed end and attach it to an identical mass hung over a pulley. Compared to the first case, does the scale read: A) Twice B) Half C) The same

  15. Scale on spring Spring Scales and Acceleration • Weight measurement requires equilibrium • Without equilibrium, • spring force doesn’t balance weight • “measurement” is meaningless and inaccurate • You must not bounce on a scale! (wait for the scale to settle before reading)

  16. Clicker question: If her weight is 500 Newtons, what does the scale read (in Newtons)? A) 500 B) 1000 C) 550 D) 450 Mass= 50 kg

  17. Two scales Total Weight = sum of all four measurements

  18. Balance A balance measures MASS

  19. The international mass standard is the only “old school” standard left There’s a big effort to redefine the kilogram: • Watt balance • Extremely round silicon sphere

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