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Chapter 7: Work and Kinetic Energy-1

Chapter 7: Work and Kinetic Energy-1. Reading assignment: Chapter 8.1-8.4 Homework: due Wednesday, Sept. 28, 2005 Chapter 8: Q23, 4, 19, 24, 34, 36. The concept of energy (and the conservation of energy) is one of the most important topics in physics. Work Kinetic energy

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Chapter 7: Work and Kinetic Energy-1

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  1. Chapter 7: Work and Kinetic Energy-1 Reading assignment: Chapter 8.1-8.4 Homework: due Wednesday, Sept. 28, 2005 Chapter 8: Q23, 4, 19, 24, 34, 36 • The concept of energy (and the conservation of energy) is one of the most important topics in physics. • Work • Kinetic energy • Energy approach is simpler than Newton’s second law.

  2. What if the force varies? We have to __________ the force along x Work done by a varying force: Thus, the work is equal to the ________ under the F(x) vs. x curve.

  3. Black board example 7.3 A force acting on a particle varies as shown in the Figure. What is the total work done on the particle as it is moved from x = 0 to x = 8 m? Hint: It is the area under the curve.

  4. Consider a spring law: (Force required to stretch or compress a spring by x): k is the _________________ constant of a spring. Stiff springs have a ____________ k value.

  5. Work done by a spring xi xf

  6. Black board example 7.3 A 0.500 kg mass is hung from a spring extending the spring by a distance x = 0.2 m • What is the spring constant of the spring? • How much work was done on the mass by the gravitational force • How much work was done on the mass by the spring force?

  7. The kinetic energy of a particle is:

  8. A bullet of mass m = 0.020 kg moves at 500 m/s. A truck of mass m = 1000 kg moves at 5 m/s Which has more kinetic energy?

  9. Work due to friction If friction is involved in moving objects, work has to be done against the kinetic frictional force. This work is:

  10. Black board example 7.4 • Angus is pulling a 10,000 kg truck with all his might (2000N) on a frictionless surface for 10.0 m. • How much work is the man doing? • What is the speed of the truck after 10 m. • What is the speed of the truck after 10 m if there is friction? • (friction coefficient: 0.0153)

  11. Power Power is the at which work is done: Average power: (work done per time interval Dt)

  12. The power can also be expressed as: (Dot product) The units of power are __________ (__/__) = watt (W)

  13. Black board example 7.7 • An elevator having a total mass of 3000 kg moves upward against the gravitational force at a constant speed of 9.13 m/s. • What is the power delivered by the motor?

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