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Further Topics: Work & Energy

Further Topics: Work & Energy. Section 8.1. Reminders. Weekly Reflection #8 due on Tuesday evening. Online reading quiz due prior to the start of class on Thursday. LAB this week A7-CE : Conservation of Energy due in lab by Friday at 4 p.m.

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Further Topics: Work & Energy

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  1. Further Topics: Work & Energy Section 8.1

  2. Reminders • Weekly Reflection #8 due on Tuesday evening. • Online reading quiz due prior to the start of class on Thursday. • LAB this week A7-CE: Conservation of Energydue in lab by Friday at 4 p.m. • Quiz #4 on Thursday, October 23rd, addressing Chapter 8 (Work and Energy) and Chapter 9 (Gravitation).

  3. Work • Work is done only when energy is expended. • Physical work not same as physiological work. • Work is defined as force times distance, FΔd • Work causes a change in the energy of a body: • FΔd = ΔE (work-energy principle) • Impulse causes a change in momentum: • FΔt = mΔv (impulse-momentum principle)

  4. Mechanical Energy • Kinetic Energy = (½)mv2 • Gravitational potential energy = mgh • Elastic potential energy = (½)kx2 where k is the spring constant that has units of F/Δx • The unit of work or energy is the Nm or Joule, J • Like momentum, mechanical energy is always conserved. That is, Ei = Ef • It is “meaningfully” conserved only in “isolated” systems and in perfectly elastic collisions.

  5. Power • Power is the rate at which energy is produced or consumed. • P = energy transferred/unit time = E/t • The unit of power, Nm/s = J/s = Watt, W.

  6. An now some examples • Conservation of energy, Ei = Ef • A ball is tossed up into the air… • An amusement park cart collides with a spring… • A moving roller coaster reaches bottom… • Work FΔd= ΔE (work-energy principle) • A block slides under frictional force,f = μN= -μmg • Power = ΔE/t = W/t • How much power required to lift an elevator…

  7. Sample Problems #1 & #2 • A bullet is shot upward from with an initial speed of 55m/s. How high does it go? • #1: Use kinematics (equations of accelerated motion) to solve the problem. • #2: Use conservation of energy to solve the problem.

  8. Sample Problems #3 & #4 • A 0.005kg bullet moving horizontally with a speed of 330m/s hits a wooden block with a mass of 2.3kg that is suspended like a pendulum but with two V-strings. The bullet become imbedded in the wood. • #3: What is the speed of the block immediately after the bullet is embedded? • #4: How high do the block and bullet go?

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