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It’s a lot of WORK...

It’s a lot of WORK. … or is it?. Objectives – Students will be able to:. Define work , and recognize the difference between scientific and ordinary definitions of work Identify where work is being performed, in a variety of situations Calculate work, with appropriate units

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It’s a lot of WORK...

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  1. It’s a lot of WORK... … or is it?

  2. Objectives – Students will be able to: Define work , and recognize the difference between scientific and ordinary definitions of work Identify where work is being performed, in a variety of situations Calculate work, with appropriate units Define Joule, the unit of work

  3. What does it mean to work? • Consider each of the following situations – is work being performed, or not? • A student holds a heavy chair at arm’s length for several minutes • A man sits at a computer for 8 hours, programming a video game • A pair of people each pull on a rope in a game of tug-of-war, but since they are evenly matched, neither one moves • A student carries a bucket of water along a horizontal path while walking at constant velocity • A student thinks about a difficult math problem • A person pushes on a stalled car, but is not able to move it

  4. Work • Definition: • Formal: The product of the component of a force along the direction of displacement and the magnitude of the displacement. • In plainer language: Work is done on an object when a force causes a displacement of the object. Is the lift performing work on the man? Why or why not?

  5. Are these people doing work, according to the physics definition? Why or why not?

  6. Can we come up with a mathematical model to describe work? • Look at the definition again: The product of the component of a force along the direction of displacement and the magnitude of the displacement. 4 minutes with an informal group around you

  7. Equation and units for work • W = F ∙ d • Force has units of Newtons (N) • Displacement has units of meters (m) • So work has units of … N ∙ m, or Joule (J)

  8. Try a calculation: • How much work is done on a box pushed 10.0 m across a frictionless floor with a force of 35.0 N? W = F ∙ d = (35.0 N) ∙ (10.0 m) = 350 N ∙ m or 350 J

  9. What about the sign of work? • Is there a difference between positive and negative work? • Work is positive when the force is in the same direction as the displacement. • Work is negative when the force is in the opposite direction as the displacement.

  10. Practice • Predict whether the work done on the second object in each example will have a positive or negative value: • The road exerts a friction force on a speeding car skidding to a stop. • A rope exerts a force on a bucket as the bucket is raised up a well. • Gravity exerts a force on the same bucket as the bucket is raised up a well. • Air exerts a force on a parachute as the parachutist falls to Earth.

  11. REMINDER We will be using protractors in class on Monday – please bring one to class

  12. Exit Ticket: • On a half sheet of paper, put your name and period in the top right. • Answer the following question. Is work being done on the object in each of the following situations? Explain why or why not. • A tugboat pulls a disabled ship into harbor. • A cross-country skier skis along flat ground. • A waiter carries a tray of dishes across a busy restaurant dining room.

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