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To understand energy and conservation of energy, we must first define some terms: work, kinetic energy (KE), and potential energy (PE). We’ll get to PE in the next Chapter. Let’s look at work and KE. Definition of work done by a force: consider an object moving while a constant force F is applied to the object.
Work: The Scientific Definition. Explain how an object must be displaced for a force on it to do work. Explain how relative directions of force and displacement determine whether the work done is positive, negative, or zero. 7.2. Kinetic Energy and the Work-Energy Theorem.
To calculate the work done on an object by a force during a displacement, we use only the force component along the object’s displacement. The force component perpendicular to the displacement does zero work. = F d = F cos φ ⋅ d = F ⋅ d ( 7.3) x. φ. d. - Assumptions: 1) F=cte, 2) Object particle-like. Units: 1 Joule = 1J = 1 kgm2/s2.
The body losing energy does work, the body gaining energy has work done on it. Work is given by the force multiplied by the displacement through which the force acts, or: Work = Change in Energy = Force × displacement. WW = ∆E = F × d. where F = force (N), d = displacement (m) Note: Work is a scalar quantity. The unit of work is the Joule (J)
Figure shows the position x of the lunchbox as a function of time t as the wind pushes on the lunchbox. From the graph, estimate the kinetic energy of the lunchbox at (a) t = 1.0 s and (b) t = 5.0 s. (b) How much work does the force from the wind do on the lunchbox from t = 1.0 s to t = 5.0 s.
Online Textbook. These notes were updated in 2022 to reflect corrections that readers have noticed. Chapter 1: Introduction to Classical Mechanics (PDF) Chapter 2: Units, Dimensional Analysis, Problem Solving, and Estimation (PDF - 4.5 MB) Chapter 3: Vectors (PDF - 4.4 MB)
When our physics problems involve forces for which we can have a potential energy function, we usually think about the change in potential energy of the objects rather than the work done by these forces.
