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3.1 Spring Scales Slide 2 Weight vs. Mass Springs Scales New ideas for today http://www.youtube.com/watch?v=MFXLhN3YNmg Slide 3 Springs, torsion spring Slide 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 weightsometimes How is all of this related? Slide 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 Slide 6 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 Hookes Law Slide 7 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 Slide 8 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 Slide 9 Springs Always produce a force that opposes the distorting forceMetal sheet Slide 10 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 Slide 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? Slide 12 Spring Scales Use a spring to measure weight Scale Slide 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 Slide 14 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 Slide 15 Spring Scales and Acceleration Weight measurement requires equilibrium Without equilibrium, spring force doesnt 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 Slide 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 Slide 17 Total Weight = sum of all four measurements Two scales Slide 18 A balance measures MASS Balance Slide 19 The international mass standard is the only old school standard left Theres a big effort to redefine the kilogram: Watt balance Extremely round silicon sphere