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Announcements

Announcements. Homework for tomorrow… (Ch . 9, Prob 8, 16, & 18) 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. Chapter 9. Kinetic Energy & Work

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Announcements

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  1. Announcements • Homework for tomorrow… (Ch. 9, Prob 8, 16, & 18) • 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

  2. Chapter 9 Kinetic Energy & Work (Work done by a Gravitational & Spring Force)

  3. Review… • Work for a constant force (in 1D) • Work for a variable force (in 1D) • Kinetic energy (in 1D) • Net Work-Kinetic Energy Theorem

  4. Reading Exercise 9.1 A particle moves along an x axis. Does the kinetic energy of the particle increase, decrease, or remain the same if the particle’s velocity changes • from -3 m/s to -2 m/s • from -2 m/s to 2 m/s? • In each situation, is the net work done on the particle positive, negative, or zero?

  5. Quiz Question 1 The work done by gravity during the descent of a particle in free fall: • is positive • is negative • is zero • depends on the direction of the y-axis • depends on the direction of both the x- and y-axes.

  6. Section 9.4:Work Done by a Gravitational Force A rock is dropped off of a cliff from a heighth. Calculate the work done by gravity by using the • Definition of Work and • NetWork-Kinetic Energy Theorem. h v2

  7. Section 9.5:Work Done by a Spring Force Hooke’s Law gives the force of a 1D ideal spring where k is the spring constant. • Fxspringis a restoring force • k depends on the properties of the spring

  8. Work Done by a Spring Force Hooke’s Law for x = 0 in the relaxed state. Notice: • If x > 0, then Fxspring(x) < 0 • If x < 0, then Fxspring(x) > 0

  9. Work Done by a Spring Force The work done by the spring force on the block is the integral yields…

  10. Touchstone Example 9.3 A cumin canister of mass m = 0.40 kg slides across a horizontal frictionless counter with velocity It then runs into and compresses a spring of spring constant k = 750 N/m. When the canister is momentarily stopped by the spring, by what amount x is the spring compressed?

  11. Problem 9.24 A 250 g block is dropped onto a relaxed vertical spring that has a spring constant ofk= 2.5 N/cm (Fig. 9-29). The block becomes attached to the spring and compresses the spring 12 cm before turning around. While the spring is being compressed, what work is done on the block by a) the gravitational force on it and b) the spring force? c) What is the speed of the block just before it hits the spring? (Assume that friction is negligible.) d) If the speed at impact is doubled, what is the maximum compression of the spring?

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