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Announcements

Announcements. *3 rd Exam on Thursday, November 3 rd * Homework for tomorrow… (Ch. 10, Probs 10 & 14) 9.38: 25 J Office hours… M 3-4 pm TWR 9-10 am F 1-2 pm Tutorial Learning Center – Houston Hall 113 MWR 8 am-6 pm T 8 am-7 pm

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Announcements

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  1. Announcements • *3rd Exam on Thursday, November 3rd * • Homework for tomorrow… (Ch. 10, Probs 10 & 14) 9.38: 25 J • Office hours… M 3-4 pm TWR 9-10 am F 1-2 pm • Tutorial Learning Center – Houston Hall 113 MWR 8 am-6 pm T 8 am-7 pm F 8 am-5 pm • 6th Annual Smashing Pumpkins Event - today @ noon, outside of Tomlinsion Library • Extra credit due tomorrow

  2. Chapter 10 Potential Energy & Energy Conservation (Conservation of Mechanical Energy )

  3. Last time… • Conservative vs. Nonconservative Forces • Potential Energy change • Gravitational Potential Energy • Spring Potential Energy

  4. Touchstone Example 10.2 A 2.0 kg sloth clings to a limb that is 5.0 m above the ground What is the gravitational potential energy of the sloth-Earth system if we take the reference point y1 = 0 to be • At the ground • At a balcony floor that is 3.0 m above the ground • At the limb • 1.0 m above the limb? Take the gravitational energy to be zero at y1 = 0 and denote y2 as y. The sloth drops to the ground. For each choice of reference point, what is the change in the potential energy of the sloth-Earth system due to the fall?

  5. Section 10.4:Mechanical Energy Conservation In any isolated system of objects interacting only through conservative forces, the total mechanical energy E = K + U, of the system, remains the same at all times

  6. Conservation of Energy Energy is neither created nor destroyed, it can only change form.

  7. How to solve ‘Conservation of Mechanical Energy’ problems… • Pick a body • Pick initial and final points of motion

  8. Quiz Question 1 Three identical balls are thrown from the top of the building, all with the same initial speed. The first ball is thrown horizontally, the second at some angle above the horizontal, and the third at some angle below the horizontal. Neglecting air resistance, rank the speeds of the balls as they reach the ground, from fastest to slowest. • 1, 2, 3 • 2, 1, 3 • 3, 1, 2 • All three balls strike the ground at the same speed

  9. Quiz Question 2 Bob, of mass m, drops from a diving platform at the same time that Alice, also of mass m, begins her descent down a frictionless slide. If they both start at the same height above the ground, which of the following is true about their kinetic energies as they reach the ground? • Bob’s KE is greater than Alice’s. • Alice’s KE is greater than Bob’s. • They have the same KE. • The answer depends on the shape of the slide.

  10. Touchstone Example 10.3: Bungee Jumper A 61.0 kg bungee-cord jumper is on a bridge 45.0 m above the river. The elastic bungee cord has a relaxed length of L = 25.0 m. Assume that the cord obeys Hooke’s law, with a spring constant of 160 N/m. If the jumper stops before reaching the water, what is the height h of her feet above the water at her lowest point?

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