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Chapter 3.1 Announcements: Homework and Springs Concepts

This announcement provides information about the due date for Homework 3.1 and covers concepts related to springs, scales, bending steel, Hooke's law, mass, weight, and gravitational force.

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Chapter 3.1 Announcements: Homework and Springs Concepts

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  1. Chapter 3.1 Announcements: Homework 3.1: due Thursday, Feb. 28 Chapter 3: Exercises: 2, 3, 6, 8 Problems: 1, 2, 3, 4 • All grades will continue to be posted at: http://www.wfu.edu/~gutholdm/Physics110/phy110.htm • Listed by last four digits of student ID • Reminder: • Team work is allowed, but don’t turn in identical homework • late homework: late, same day: 80% up to two days later: 50%

  2. Chapter 3.1 Springs Concepts Demos and Objects • springs • scales • bending steel • Hooke’s law: • Scales measure weight (force) not mass! • Many objects obey Hooke’s law

  3. i-clicker-1 A diver is standing motionless at the end of a spring board and the board bends downward. If her identical twin joins her at the end of the board, how far downward will the board bend? A. The same amount. B. Twice as far. C. Four times as far.

  4. Mass and Weight – different beasts • Mass, m • independent of measuring location • can be measured by measuring acceleration, a, as a function of force, F • difficult to measure directly • measured in kilogram (kg) • Weight, m·g • Weight is a force! • depends on g and thus on measuring location (earth or moon) • easier to measure than mass • measure with a spring balance • measured in Newtons (N) (or pounds)

  5. Force due to gravity = g Fundamental force law: For us, m2 is the mass of earth; r is the radius of earth G… Gravitational constant G = 6.673·10-11 N·m2/kg2 m1, m2…masses of particles 1 and 2 r… distance separating these particles … unit vector in r-direction m F = weight, m = mass Earth m2

  6. To measure the weight of an object we use a Spring balance How does it work?

  7. Hooke’s law: F… force on spring x… extension of spring k … spring constant of spring Stiffer springs have higher spring constants.

  8. Grocer’s balance The more weight (force) you put in the basket the further extended is the spring. • You put a 7 pound (31 N) melon into a grocer’s balance. The spring inside the balance extends 0.1 m. • What is the spring constant k of the balance? • If you put in 21 pounds how far will it extend? • If you put in 10 pounds how far will it extend? i-clicker-3 (question 2.) i-clicker-2 (question 1.) 1A. 310 N/m 1B. 70 N/m 1C. 31 N/m 1D. 7 N/m 1E. 3.1 N/m 2A. 0.1 m 2B. 0.2 m 2C. 0.3 m 2D. 0.4 m 2E. 0.5 m

  9. Spring Scales and Acceleration • Weight measurement requires equilibrium (net force on melon is zero). • 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!

  10. Springs are everywhere! Atomic Force Microscopy imaging. A tiny tip on a cantilever scans and senses the topography of a surface via force 50 mm 10 mm

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