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Understanding Energy, Work, and Power in Physics

Learn about work, energy, and power in physics, including calculations for forces, displacements, and different types of energy. Explore examples and concepts such as scalar products and the conservation of energy. Discover how varying forces and spring constants affect work done and power in a system.

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Understanding Energy, Work, and Power in Physics

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  1. Energy Work and Power

  2. In physics, work has a very specific definition. • Work is done when: • A force is exerted on an object • An object has a displacement in the same direction as the force W=Fdcosq • SI unit for work is Joule (J) • 1 J = 1N*m (=0.7376 ft*lb) Work

  3. A 15 kg block is dragged over a rough, horizontal surface by a 70 N force acting at 20 degrees above the horizontal. The block is displaced 5 m, and the coefficient of kinetic friction is 0.3. Find the work done by: • The 70 N force • The normal force • The force of gravity • The friction force Example 1

  4. We are only interested in the component of Force that acts in the same direction (along the same axis) as the displacement. • By using the equation: W=Fdcosq, we are finding the scalar product of the two vectors, force and displacement. • The scalar product is also called the dot product. Work

  5. The dot product is positive for angles between 0 and 90, negative if the angle is between 90 and 180 and zero if the vectors are perpendicular. Dot product

  6. Start with the scalar products of the unit vectors: Calculating Dot Product

  7. Calculating Dot Product

  8. A particle moving in the xy plane undergoes a displacement d=(2.0i+3.0j)m as a constant force F=(5.0i+2.0j)N acts on the particle. • Calculate the work done by the force. • Calculate the angle between the force and the displacement. Example 2

  9. Energy is defined as the ability to do work. Energy dealing with motion and forces is called mechanical: Kinetic Potential Energy

  10. Kinetic Energy is the energy of motion. Potential Energy is stored energy. We will primarily use gravitational potential energy. Kinetic and Potential

  11. Energy in a closed system cannot be gained or lost; it is transferred from one type to another. Non-conservative forces (or dissipative forces) can add or subtract to the mechanical energy of the system. Conservation of Energy

  12. If the force changes over time, we need to look at the force in small pieces to get a more accurate picture of the work that is done. W As we make the divisions smaller and smaller, our picture of the work being done gets more accurate. Which leads us to… Varying Force

  13. Spring force is linearly related to amount the spring is displaced (stretched or compressed). The spring force acts in the opposite direction of the displacement. Spring Forces

  14. The spring constant varies from spring to spring. It is dependent on the spring’s material, shape and size. (Newton/meter) Spring Constant

  15. (where x0 = 0 m) Work on/by a Spring

  16. Power is the rate at which work is done. …..measured in Watts Power

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