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Chapter 5 – Work and Energy Physics Coach Kelsoe Pages 159–191 Section 5–1: Work Physics Coach Kelsoe Pages 160–163 Objectives • Recognize the difference between the scientific and ordinary definitions of work. • Define work by relating it to force and displacement • Identify where work is being performed in a variety of situations. • Calculate the net work done when many forces are applied to an object. Definition of Work • Work is done on an object when a force causes a displacement of the object. • Work is done only when components of a force are parallel to a displacement. So Is This Work? • Consider the following scenarios: – Jared holds a heavy chair at arm’s length for several minutes. – Kim carries a bucket of water along a horizontal path while walking at a constant velocity. • Is work being done on the chair or the bucket in these situations? • The answer is no, even though effort is required in both cases. Definition of Work • Work is the product of the component of a force along the direction of displacement and the magnitude of the displacement. • The formula for work is W = Fd, where W = work, F = force, and d = displacement. Notice that we are substituting the variable “d” for our Δx. • Work is measured in units of Joules (J), which are equal to N·m. Joules • The joule is named for British physicist James Prescott Joule, who had major contributions in the fields of energy, heat, and electricity. • To put joules into perspective, the work it takes to lift an apple from your waist to your head is about 1 joule. Why Isn’t Work Done On The Chair? • The application of a force alone does not constitute work. • Even though Jared exerts a force to support the chair, the chair doesn’t move. • Jared’s arm muscles will go through many small displacements within his body, and therefore do work within his body, but NOT on the chair. Why Isn’t Work Done On The Bucket of Water? • Work is only done when COMPONENTS of a force are parallel to a displacement. • Since Kim exerts an upward force on the bucket of water, which is perpendicular to the displacement, there is no work done on the bucket of water. What About Angles? • When the force on an object and the object’s displacement are in different directions, only the component of the force that is parallel to the object’s displacement does work. • As long as there is a component parallel to the displacement (usually horizontally), work is taking place. More Than One Force • If there are more than one constant forces acting on an object, you can find the net work done on the object by finding the net force on the object: • Wnet = Fnetd cos θ Definition of Work Sample Problem • How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0º above the horizontal? Sample Problem Solution • 1. Identify givens and unknowns: – F = 50.0 N – θ = 30.0º – d = 3.0 m • 2. Choose equation that fits the problem – W = Fd cos θ • 3. Substitute for values. – W = (50.0 N)(3.0 m)(cos 30.0º) – W = 130 J Work Is A Scalar Quantity • Work is a scalar quantity and can be positive or negative. • Work is positive when the component of force is in the same direction as the displacement. – Example – lifting a box • Work is negative when the component of force is opposite of the displacement. – Example – the force of kinetic friction between a sliding box and the floor Work is a Scalar Quantity • If the work done on an object results only in a change in the object’s speed, the sign of the net work on the object tells you whether the object’s speed is increasing or decreasing. • If net work is positive, the object speeds up and work is done ON the object. • If net work is negative, the object slows down and work is done by the object on something else. Vocabulary • Work