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Further Topics: Work & Energy

Further Topics: Work & Energy. Section 8.1. Reminders. Weekly Reflection due on Tuesday evening after break, March 18. Reflection will be sent out this afternoon along with reminders. Online reading quiz due prior to the start of class on Tuesday after break, March 18.

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Further Topics: Work & Energy

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  1. Further Topics: Work & Energy Section 8.1

  2. Reminders • Weekly Reflection due on Tuesday evening after break, March 18. Reflection will be sent out this afternoon along with reminders. • Online reading quiz due prior to the start of class on Tuesday after break, March 18. • Extra credit problems will be provided today worth a total of 2 points and due at the start of class on Tuesday after break, March 18.

  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 (as well as its momentum). • FΔd = ΔE (work-energy principle) • 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 or 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… • FΔd= ΔE (work-energy principle) • A block slides under force of friction, f = μN • Power • How much power required to lift an elevator…

  7. Extra Credit 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. (1/2 point) • #2: Use conservation of energy to solve the problem. (1/2 point)

  8. Extra Credit 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 strings. The bullet become imbedded in the wood. • #3: What is the speed of the block right after the bullet is embedded in it? (1/2 point) • #4: How high does block & bullet go? (1/2 pt)

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