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 Energy due in lab by Friday at 4 p.m. Quiz #4 on Thursday, October 23 rd, 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)mv 2 Gravitational potential energy = mgh Elastic potential energy = (½)kx 2 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, E i = E f 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, E i = E f – 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?