1 / 39

Today’s lesson

Learn what “work” is! Learn how to calculate work See who can do the most work! Learn about power . Learn Hooke’s Law. Today’s lesson. Energy - the ability of a body or system of bodies to perform work . A body is given energy when a force does work on it. Energy and Work. Work.

jodie
Download Presentation

Today’s lesson

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Learn what “work” is! Learn how to calculate work See who can do the most work! Learn about power. Learn Hooke’s Law. Today’s lesson

  2. Energy - the ability of a body or system of bodies to perform work. A body is given energy when a force does work on it. Energy and Work

  3. Work In physics, work has a special meaning, different to “normal” English.

  4. A force does work on a body (and changes its energy) when it causes a displacement. If a force causes no displacement, it does no work. But What is Work?

  5. Riddle Me This If a man holds a 50 kg box at arms length for 2 hours as he stands still, how much work does he do on the box? Zip ZERO Zilch Nada NONE

  6. There is no work done by a force if it causes no displacement. • Forces perpendicular to displacement, such as the normal force, can do no work. • Likewise, centripetal forces never do work. Counterintuitive Results

  7. Work is the dot product of force and displacement. Work is a scalar resulting from the interaction of two vectors. Calculating Work

  8. Vector Multiplication • There are three ways to multiply vectors: • Scalar Multiplication • Dot Product • Cross Product

  9. Scalar Multiplication • Magnitude of vector changes. • Direction of vector does not change. If m = 5 kg a = 10 m·s-1 F = 50 N

  10. Dot Product Note that the dot product of two vectors gives a scalar . θ

  11. Dot Product Geometrically, the dot product is the projection of one vector on a second vector multiplied by the magnitude of the second vector. θ

  12. Calculating Work θ

  13. Which does more work? Two forces are acting on the box shown causing it to move across the floor. Which force does more work? F2 θ F1

  14. F Vectors and Work

  15. Vectors and Work F s W = F • s W = F s cos 0o W = F s Maximum positive work

  16. F Vectors and Work

  17. Vectors and Work F  s W = F • s W = F s cos  Only the component of force aligned with displacement does work. Work is less.

  18. F Vectors and Work

  19. s Vectors and Work F W = F • s W = F s cos 180o W = - F s Maximum negative work.

  20. F mg Gravity often does negative work. When the load goes up, gravity does negative work and the crane does positive work. When the load goes down, gravity does positive work and the crane does negative work.

  21. Positive, zero, or negative work? A box is being moved with a velocity v by a force P (parallel to v) along a level floor. The normal force is FN, the frictional force is fk, and the weight of the box is mg. Decide which forces do positive, zero, or negative work.

  22. Positive, zero, or negative work? v FN fk P s mg

  23. Units of Work Energy is measured in Joules (J). That’s me! J = N·m J = kg·m2·s-2

  24. F(x) xb W =  F(x) dx xa x xa xb The area under the curve of a graph of force vs displacement gives the work done by the force. Work and variable force

  25. Let’s look at some examples

  26. Work done (J) = Force (N) x distance (m) A woman pushes a car with a force of 400 N at an angle of 10° to the horizontal for a distance of 15m. How much work has she done?

  27. Work done (J) = Force (N) x distance (m) A woman pushes a car with a force of 400 N at an angle of 10° to the horizontal for a distance of 15m. How much work has she done? W = Fscosθ = 400x15x0.985 W = 5900 J

  28. Work done (J) = Force (N) x distance (m) A man lifts a mass of 120 kg to a height of 2.5m. How much work did he do?

  29. Work done (J) = Force (N) x distance (m) A man lifts a mass of 120 kg to a height of 2.5m. How much work did he do? Force = weight = 1200N Work = F x d = 1200 x 2.5 Work = 3000 J

  30. How much work can you do?

  31. Can you copy this please?

  32. Arm curls Force required = weight of object = mass (kg) x 10 distance

  33. Off you go!

  34. Power!

  35. Power! Power is the rate of doing work. Power is the amount of work done per unit time. Power is measured in Watts (1 Watt = 1 J/s)

  36. Power For each of the people in your table, can you calculate their power?

  37. HOOKE’S lAW When we stretch or compress a spring, a force arises that attempts to return the spring to its original length.

  38. Our First Hooke’s Law Problem A force of 125 N is required to extend a spring by 2.8 cm. What force is required to stretch the same spring by 3.2 cm? Step 2: Solve for the force Step 1: Solve for k

  39. Elastic Limit

More Related