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Ch 6. Work and Energy

Example 1 Suitcase-on-Wheels Find work done if force is 45.0-N, angle is 50.0 degrees , and displacement is 75.0 m. Ch 6. Work and Energy. Angle between displacement and normal force is 90 degrees. Angle between displacement and weight is 90 degrees.

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Ch 6. Work and Energy

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  1. Example 1 Suitcase-on-Wheels Find work done if force is 45.0-N, angle is 50.0 degrees, and displacement is 75.0 m. Ch 6. Work and Energy

  2. Angle between displacement and normal force is 90 degrees. Angle between displacement and weight is 90 degrees. 6.1 Work Done by a Constant Force Example 3 Accelerating a Crate A truck accelerating at +1.50 m/s2. Mass of crate is 120-kg. Displacement is 65 m. What is total work done on crate by all the forces acting on it? Angle between displacement and friction force is 0 degrees. When multiple forces are acting:

  3. WORK-ENERGY THEOREM When a net external force does work on an object, the kinetic energy of the object changes according to Work-Energy Theorem & Kinetic Energy KINETIC ENERGY Kinetic energy KE of mass mtraveling with speed v

  4. Example: Down-hill Skier Work-Energy Theorem & Kinetic Energy Example 4 Deep Space 1 Massof space probe is 474-kg and initial velocity is 275 m/s. A 56.0-mN force acts on the probe through a displacementof 2.42×109m, what is its finalspeed? s net force

  5. Example 7 Gymnast on a Trampoline Gymnast leaves trampoline at initial height of 1.20-m; reaches max height of 4.80 m. What was the initial speedof the gymnast? 6.3 Gravitational Potential Energy DEFINITION OF GRAVITATIONAL POTENTIAL ENERGY Units

  6. Conservative Force work done  independent of path between initial & final positions no work done  moving in a closed path

  7. Conservation of Mechanical Energy Nonconservative Force Mechanical Energy E = KE + PE Example = Friction If there is NO non-conservative force Total work = conservative (Wc) + Nonconservative (Wnc) WORK-ENERGY THEOREM

  8. Conservation of Mechanical Energy Example 8 Daredevil Motorcyclist Motorcyclist leaps across canyon  drives horizontally at 38.0 m/s. Find the speed with which the cycle strikes the ground on the other side.

  9. WORK-ENERGY THEOREM Example 11 Fireworks The nonconservative force generated by the burning propellant does 425 J of work. What is the final speed of the 200gm rocket.

  10. 6.9 Work Done by a Variable Force 6.7 Power Power = rate at which work is done Constant Force Variable Force 6.8 Conservation of Energy Energy can neither be created not destroyed. Only converted from one form to another.

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