Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
WORK & ENERGY Unit 6.1 1 WORK In the everyday sense, the term “work” means to do something that takes physical or mental effort. In physics, work has a distinctly different meaning. 2 WORK Work is the product of the magnitudes of the component of a force along the direction of displacement and the displacement. W = Fd * d represents displacement instead of ∆x 3 WORK Work is not done on an object unless the object is moved because of the action of a force. The application of force alone does not constitute work. 4 WORK So even if a person holds a chair at arm’s length, the person exerts a force to support the chair but since the chair does not move, no work is done. 5 WORK When the force on an object and the object’s displacement are in different directions, only the component of the force that is in the direction of the object’s displacement does work. 6 WORK Components of the force perpendicular to a displacement do not do work. 7 WORK For example, imagine pushing a crate along the ground. If the force you exert is horizontal, all of your effort moves the crate. 8 WORK If your force is other than horizontal, only the horizontal component of your applied force causes displacement and does work. 9 WORK If the angle between the force and the direction of displacement is θ, then W = Fd(cos θ) θ 10 WORK If θ = 0°, then cos 0 = 1 which results in W = Fd If θ = 90°, then cos 90 = 0 which results in W = 0 11 WORK Example: A bucket of water being carried by a student walking horizontally. The upward force exerted to support the bucket is perpendicular to the displacement of the bucket which results in no work done on the bucket. 12 WORK Work has dimensions of force times length N * m or J (joules) 13 WORK Work is a vector quantity and can be negative or positive. F F d d F d F d 14 WORK Work is positive when the component of force is in the same direction as the displacement. 15 WORK Work is negative when the component of force is in the direction opposite of the displacement. 16 WHICH OF THE FOLLOWING ARE EXAMPLES OF WORK? 1. 2. 3. A book falls off a table and free falls to the ground. A rocket accelerates through space. A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. 17 ANSWERS: 1. Yes, this is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i.e., "fall"). 2. Yes. This is an example of work. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space. 18 ANSWERS: 3. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room).Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement. 19 https://www.youtube.com/watch?v=w4QFJb9a8vo 20