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Work

Work. Definition of Work. In its everyday sense, the term work means to do something that takes physical or mental effort. However, in Physics , the term work has a different meaning.

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Work

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  1. Work

  2. Definition of Work • In its everyday sense, the term work means to do something that takes physical or mental effort. • However, in Physics, the term work has a different meaning. • Work is done on an object when a force causes a displacement of the object in the direction of the force.

  3. Calculating Work • W = Fd where W is Work F is Force, and d is distance. • All examples of work have one thing in common – a force acts through a distance. • Work is ONLY done when an object moves some distance due to an applied force. • Work is the product of the applied force and the distance through which an object moves in the direction of the force.

  4. Units of Work • If force is measured in Newtons (kg∙m/s2), and distance is measured in meters, then work W = Fd must be in Newton-meters which is often called a Joule (J) … the SI unit for work 1 kg∙m2/s2 = 1 N∙m = 1 J

  5. Calculating Work • Example:A student pushes on a desk with a horizontal force of 120 N and moves the desk 2.0 m directly across the floor. How much work was done on the desk? F = 120 N d = 2.0 m W = ? W = Fd = (120 N)(2.0 m) = 240 N∙m W = 240 J

  6. Check Your Understanding • A force of 8.00 x 102 N is needed to push a car across a parking lot. Two students push the car 40.0 m. How much work is done to move the car? • How much work is done in lifting a 60.0 kg crate a vertical distance of 10.0 m? • A person carries a 34.0 N package from the ground floor up to the fifth floor of an office building (15.0 m upward). How much work does the person do to move the package? • What work is done to lift a 49.0 kg crate a distance of 10.0 m? • A worker carries cement blocks, weighing 1.50 x 102 N each, up a ladder onto a scaffold 8.00 m high. The worker carries them at a rate of 2 blocks per minute. How much work is done by the worker in: a) 10.0 minutes b) 1.00 hour

  7. Check Your Understanding • A force of 8.00 x 102 N is needed to push a car across a parking lot. Two students push the car 40.0 m. How much work is done to move the car? W = Fd = (8.00x102N)(40.0m) = 32000 N∙m = W ≈ 3.20x 104 J 2. How much work is done in lifting a 60.0 kg crate a vertical distance of 10.0 m? W = Fd = mgd = (60.0kg)(9.81m/s2)(10.0m) = W = 5886 N∙m W ≈ 5890 J

  8. Check Your Understanding • A person carries a 34.0 N package from the ground floor up to the fifth floor of an office building (15.0 m upward). How much work does the person do to move the package? W = Fd = (34.0N)(15.0m) = 510 N∙m = W ≈ 510 J 4. What work is done to lift a 49.0 kg crate a distance of 10.0 m? W = Fd = mgd = (49.0kg)(9.81m/s2)(10.0m) = W = 4806.9 N∙m = W ≈ 4.81 x 103 J

  9. Check Your Understanding 5. A worker carries cement blocks, weighing 1.50 x 102 N each, up a ladder onto a scaffold 8.00 m high. The worker carries them at a rate of 2 blocks per minute. How much work is done by the worker in: a) 10.0 minutes W = Fd = (1.50x102N)(8.00m)/block = 1200J/block (1200J/block)(2blocks/min)(10.0 min) = W ≈ 2.40x104 J b) 1.00 hour W = Fd = (1200J/block)(2 blocks/min)(60 min) = W ≈ 1.44x105 J

  10. Work and Direction of Force • If a net force is applied to an object at an angle with the direction of motion, the component of the net force that acts in the direction of motion is the force that does work! W = Fd cos θ

  11. Work: Force & Displacement at an Angle Example: A boy is cutting the grass by pushing a lawn mower using a net force of 125 N applied at an angle of 25.0o above the horizontal. How much work is the boy doing if he pushes the lawn mower 20.2 m? W = Fdcosθ= (125N)(20.2m)(cos 25.0o) = W = 2288.4 N∙m = W = 2290 J

  12. Check Your Understanding • A force of 6.00x102 N is applied to a metal box to pull it 15.0m across the floor. The rope used to pull the box is held at an angle of 46.0o with the floor. How much work is done? • A person uses a rope to pull a 1.00x103kg boat 50.0m along a wharf. The rope makes an angle of 45.0o with the horizontal. If a force of 40.0N is used to move the boat, how much work is done? • It takes 1.20x104J of work to pull a loaded sled weighing 8.00x102N a distance of 2.00x102m. To do this, a force of 1.20x102N is exerted on a rope, which makes an angle with the horizontal. At what angle is the rope held?

  13. Check Your Understanding • A force of 6.00x102 N is applied to a metal box to pull it 15.0m across the floor. The rope used to pull the box is held at an angle of 46.0o with the floor. How much work is done? W = Fd cos θ W = (6.00x102N)(15.0m)(cos 46.0o) = W = 6251.9 N∙m = W = 6250 J

  14. Check Your Understanding • A person uses a rope to pull a 1.00x103kg boat 50.0m along a wharf. The rope makes an angle of 45o with the horizontal. If a force of 40.0N is used to move the boat, how much work is done? W = Fd cos θ W = (40.0N)(50.0m)(cos 45o) = W = 1414.2 N∙m = W = 1400 J

  15. Check Your Understanding • It takes 1.20x104J of work to pull a loaded sled weighing 8.00x102N a distance of 2.00x102m. To do this, a force of 1.20x102N is exerted on a rope, which makes an angle with the horizontal. At what angle is the rope held? W = Fd cos θ 1.20x104J = (1.20x102N)(2.00x102m)(cos θ) cos θ = 0.500 θ = 60.0o

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