L06 - Physics 111 Lecture 6"Energy is eternal delight William Blake 1 17 Energy s Energy Work Question Kinetic energy Question Example Potential Energy

1 / 17 Physics 111 Lecture 6 “Energy is eternal delight.” –William Blake

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Energy Energy Work Question Kinetic energy Question Example Potential Energy Conservative forces Mechanical energy Question Example Nonconservative Example Power 2 / 17 ■ The problem with forces is that vectors can be difficult ■ It is far more preferable to work with a scalar values that do not have direction ■ So we turn to energy (a scalar) to solve problems ■ Energy is an internal property of an object ■ Energy can be transferred from one object to another (or from one type to another, as in dams that produce electricity) ■ The agent that transfers energy between objects is force ■ The amount of energy that is transferred into our out of an object is termed work

Work Energy Work Question Kinetic energy Question Example Potential Energy Conservative forces Mechanical energy Question Example Nonconservative Example Power 3 / 17 ■ For a constant force F displacing an object in the x -direction, we say that work is W = F x Δ x = F Δ x cos( φ ) (1) ■ The SI unit of work (and energy) is the Joule: [J] = [N] · [m] ■ Example: A 20 N force is applied to a 5 -kg block, displacing the block 20 m. ◆ What is the work done by the force if it acts parallel to the ground? ◆ And if vector F forms an angle of 20 o with the horizontal? ◆ And if vector F forms an angle of 20 o with the horizontal while pulling the block on a 20 o incline?

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Work Energy Work Question Kinetic energy Question Example Potential Energy Conservative forces Mechanical energy Question Example Nonconservative Example Power 4 / 17 ■ The sign convention for work is as follows: ◆ If φ < 90 o , then cos( φ ) > 0 , and W > 0 ◆ If φ > 90 o , then cos( φ ) < 0 , and W < 0 ◆ If φ = 90 o , then cos( φ ) = 0 , and W = 0

Question 5 / 17 Which of the following forces performs the greatest amount of work on the object (which moves in x )? (See figure) a) The 7N force b) The 8N force c) The 6N force d) All three forces perform the same amount of work on the object