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PHYS 1443 – Section 001 Lecture #11

PHYS 1443 – Section 001 Lecture #11. Monday June 20, 2005 Dr. Andrew Brandt. Test problems Gravitational Potential Energy Power. Announcements. Test avg 75—no curve Interim grades posted Homework: HW6 on ch 7 due Monday 6/20 at 8pm HW7 on ch 8 due Tuesday 6/21 at 8pm

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PHYS 1443 – Section 001 Lecture #11

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  1. PHYS 1443 – Section 001Lecture #11 Monday June 20, 2005 Dr. Andrew Brandt • Test problems • Gravitational Potential Energy • Power PHYS 1443-001, Summer I 2005 Dr. Andrew Brandt

  2. Announcements • Test avg 75—no curve • Interim grades posted • Homework: • HW6 on ch 7 due Monday 6/20 at 8pm • HW7 on ch 8 due Tuesday 6/21 at 8pm • Keep up! Some of you are slipping! PHYS 1443-001, Summer I 2005 Dr. Andrew Brandt

  3. Example of Gravitational Potential Energy A particle of mass m is displaced through a small vertical distance Dy near the Earth’s surface. Show that in this situation the general expression for the change in gravitational potential energy is reduced to DU=mgDy. Taking the general expression of gravitational potential energy The above equation becomes Since the situation is close to the surface of the Earth Therefore, DU becomes Since on the surface of the Earth the gravitational field is The potential energy becomes PHYS 1443-001, Summer I 2005 Dr. Andrew Brandt

  4. vf=0 at h=rmax m h vi ME RE Escape Speed An object of mass m is projected vertically from the surface of the Earth with an initial speed vi and eventually comes to a stop (vf=0) at a distance rmax. Since the total mechanical energy is conserved Solving the above equation for vi, one obtains Therefore if the initial speed vi is known, one can use this formula to compute the final height h of the object. In order for the object to escape Earth’s gravitational field completely, the initial speed needs to be This is called the escape speed. This formula is valid for any planet or large mass object. How does this depend on the mass of the escaping object? Independent of the mass of the escaping object PHYS 1443-001, Summer I 2005 Dr. Andrew Brandt

  5. Power • Rate at which the work is done or the energy is transferred • What is the difference for the same car with two different engines (4 cylinder and 8 cylinder) climbing the same hill? •  8 cylinder car climbs faster NO Is the total amount of work done by the engines different? The rate at which the same amount of work is performed is higher for 8 cylinders than 4. Then what is different? Average power Instantaneous power Unit? What do power companies sell? Energy PHYS 1443-001, Summer I 2005 Dr. Andrew Brandt

  6. Energy Loss in Automobile An automobiles typically uses only 13% of its fuel to propel the vehicle. 16% in friction of mechanical parts Why? 67% in the engine: • Incomplete burning • Heat • Sound 4% in operating other crucial parts such as oil and fuel pumps, etc 13% used for balancing energy loss related to moving vehicle, like air resistance and road friction on tires, etc. Two frictional forces involved in moving vehicles Coefficient of Rolling Friction; m=0.016 Total Resistance Air Drag Total power to keep speed v=26.8m/s=60mi/h Power to overcome each component of resistance PHYS 1443-001, Summer I 2005 Dr. Andrew Brandt

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