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Lesson 5.1 Work

Lesson 5.1 Work. Essential Question: How do you calculate work?. Which situation is an example of work? Holding a set of exercise weights up in the air Pushing your car down the street because it ran out of gas. What is work?.

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Lesson 5.1 Work

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  1. Lesson 5.1Work Essential Question: How do you calculate work?

  2. Which situation is an example of work? • Holding a set of exercise weights up in the air • Pushing your car down the street because it ran out of gas

  3. What is work? • In everyday life, work means to do something that takes physical or mental effort. • In physics, work has a different meaning.

  4. What is work in physics? When a force causes a displacement of the object Work is not done on an object unless the object is moved with the action of a force.

  5. When does a force cause work? • When the components of the force are parallel to a displacement. • If many forces are acting on an object, you can find the net work done on the object by first finding the net force on the object.

  6. Imagine pushing a crate along the ground. If the force you exert is horizontal, all of your effort moves the crate. If your force is at an angle, only the horizontal component of your applied force causes a displacement and contributes to the work.

  7. How do you calculate work? Work is the product of the component of a force along the direction of displacement and the magnitude of the displacement.

  8. A student is carrying a bucket of water along a horizontal path while walking at a constant velocity. Is this work? NO! The upward force exerted by the student to support the bucket is perpendicular to the displacement of the bucket, which results in no work done on the bucket.

  9. What is the unit for work? Work has the dimensions of force times length or newtons times meters, Joules (J). The work done from lifting an apple from your waist to the top of your head is about 1 J.

  10. How much work is done on a vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0° above the horizontal?

  11. A 20.0 kg suitcase is raised 3.0 m above a platform by a conveyor belt. How much work is done on the suitcase?

  12. A tugboat pulls a ship with a constant net horizontal force of N and causes the ship to move through a harbor. How much work is done on the ship if it moves a distance of 3.00 km?

  13. A weight lifter lifts a set of weights a vertical distance of 2.00 m. If a constant net force of 350 N is exerted on the weights, what is the net work done on the weights?

  14. A shopper in a supermarket pushes a cart with a force of 35 N directed at an angle of 25° downward from the horizontal. Find the work done by the shopper on the cart as the shopper moves along a 50.0 m length of aisle.

  15. If 2.0 J of work is done in raising a 180 g apple, how far is it lifted?

  16. The Sign of Work is Important • Work is a scalar quantity. • It is positive when the component of force is in the same direction as the displacement. • It is negative when the force is in the direction opposite the displacement.

  17. If the work done on an object results only in a change in the object’s speed, the sign of the net work on the object tells you whether the object’s speed is increasing or decreasing.

  18. Sign of Net Work • If the net work is positive then the object speeds up and the work is done ON the object. • If the net work is negative then the object slows down and work is done BY the object to something else. • Because the force of friction always acts to oppose the motion, the work done by frictional forces is always negative.

  19. A worker pushes a N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is 0.220. • How much work is done by the worker on the crate? • How much work is done by the floor on the crate? • What is the net work done on the crate?

  20. You’re pushing a huge brick of gold home. The coefficient of kinetic friction between the gold and the ground is 0.250 and the gold brick has a mass of 1000 kg. Your house is 3 km away. How much work do you have to do to get it home?

  21. You’re pulling a chest of drawers, applying a force of 600.0 N parallel to the slope. The angle of the slope is 40.0°. The chest has a mass of 40.0 kg, and there’s a coefficient of kinetic friction of 0.12. How much net work do you do in pulling the chest of drawers 10.0 m up the inclined plane?

  22. How do you calculate work?

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