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3.1 Spring Scales

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

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Springs, torsion spring

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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?

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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

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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 Hooke’s Law

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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

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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 “+” Spring compression

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

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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? A.The spring force at P is less than the force of gravity on the ball. B.The spring force at P is greater than the force of gravity on the ball. C.The spring force at P is equal to the force of gravity on the ball. Clicker question

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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?

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Spring Scales Use a spring to measure weight Scale

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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

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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 Scale on skates

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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) Scale on spring

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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

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Total Weight = sum of all four measurements Two scales

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A balance measures MASS Balance

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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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