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Aim: How can we apply work-energy to motion problems?

Aim: How can we apply work-energy to motion problems?. Do Now: In your own words, what does energy mean to you? List and describe different forms of energy. How can we define energy?. Energy is the ability to produce change in an object or its environment.

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Aim: How can we apply work-energy to motion problems?

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  1. Aim: How can we apply work-energy to motion problems? Do Now: In your own words, what does energy mean to you? List and describe different forms of energy.

  2. How can we define energy? • Energy is the ability to produce change in an object or its environment. • Examples of forms of energy: solar, thermal, mechanical (PE and KE), chemical • Physics definition of Energy: the ability to do work

  3. What does work mean to you? • Lifting a weight? • Holding a weight? • Pushing a cart? • Pushing a wall?

  4. What does work mean to scientists? • Definition: Work is done when a force transfers energy to an object and moves it. • Lifting a weight? • Holding a weight stationary? • Pushing a cart? • Pushing a wall?

  5. Symbol: W • Equation: W = Fd (force times displacement) • Unit: Joule (J) • One Joule equals one Newton•meter • Scalar quantity • Horizontal Work involves applying a force over a distance • Vertical Work involves lifting your weight (Fg=mg) a certain height.

  6. Rule for calculating work • W = Fd=ΔET • ΔET Change in energy • The force must be parallel (horizontal component) to the displacement.

  7. List two examples of zero work. • Applying a force and the object is not moving. • Applying a force that is perpendicular to the displacement of the object. F d

  8. Example (horizontal motion) • 1. How much work is done in pushing a crate 3 meters with a force of 10 Newtons?

  9. Example: • A student is pulling a cart with a 25 newton force at a 60° angle for 20 meters. Calculate the work done.

  10. Examples: vertical motion against gravity • How much work does it take to lift a 10 kg object 4 meters? • It takes 250 Joules to lift an object 10 meters. Calculate the force required.

  11. A constant horizontal force of 30.0-newtons applied to a box causes it to move at a constant speed of 5.0 m/s. Determine how much work is done against friction on the box in 10 seconds.

  12. Summary • How can we describe work? • Give one example of applying a force to an object but no work is done. • Identify two units for work.

  13. Aim: How can we calculate power? • 1) It takes 500 Joules to lift a 20-kilogram object. Calculate the height of the object. • 2) A constant force of 10.-newtons is used to push a 3.0 kilogram block 4 meters across a counter top. How much work is done on the block. • 3) Identify the two units used for work.

  14. How can we calculate work from a graph? Force vs Displacement On a force vs displ. Graph Area under line equals work. 10 8 6 4 2 Force (N) 5 10 15 20 Displacement (m)

  15. Calculate work Force vs Displacement 10 8 6 4 2 Force (N) 5 10 15 20 Displacement (m)

  16. How can we describe power? • Definition: amount of work done per unit time, or rate at which work is done. • Unit: 1 Watt (W) = 1 Joule/second • Scalar quantity • Note: Don’t confuse W(ork) with W(att)!

  17. Example 1 • You lift a 10 kg box a vertical distance of 2 meters in 3 second. How much power did you generate? • You lift the same box 2 meters in 1 second. Calculate the power.

  18. Example 2 • Your friend lifts a 50 kg object the same vertical distance of 2 meters, but in only 2min. Calculate the rate at which work is done.

  19. Example 3 • An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 meters in 0.5 minutes. How much force does the elevator exert?

  20. Example 4 • A 50 Newton force exerted on an object is powered by a 250-Watt motor. Calculate the velocity.

  21. Summary • 1) How can we calculate power? • 2) As the time increases the power _____________. • 3) Describe the units for power.

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