Phys151_PPL8 - Not Picky about Work Forces can do"more than...

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Not Picky about Work! Forces can do “more” than affect motion. They can do work . Caveat: Definition of work in Physics is unusual. You lift up a suitcase, carry it at constant speed parallel to the ground for a distance and then lower it to the floor - you will have done no work! Hold up a barbell for 5 hours. Again, according to Physics you will have done absolutely no work . Lift up a suitcase, you will do positive work on it while simultaneously gravity does negative work.
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Working for the Man Why do physicists choose to make life so difficult with this “queer” view of work? Reasons will be obvious later. For now, accept that there are loads of benefits. Work: its technical definition. For work to be non-zero, it requires two “elements” to be present : a force and motion (constant-velocity or accelerated, any kind) along the direction of that force, so that there is displacement . For the diagram above , F 1 does work but not F 2; magnitude of the work done is the product: Force x distance along direction of force Here W 1 = F 1 . d, while W 2 = 0. F 1 F 2 motion d
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Working like a Dog Another way to decide if work is being done- set in rap lyrics: If you are not pushing/pulling or it’s not along the right direction moving, then you’re certainly not working . Sample calculation: I lift a 20- kg suitcase into the back of a truck that is 1 m above the road. How much work did I do during the process? Did I push or pull? Yes, I pulled with a force F just slightly larger than the weight (20 x 10 = 200 N). Was the suitcase moving? Yes, for it to rise from the street to the truck, it had to be moved. The height risen in the direction of F is 1 m.
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