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Aim: How can we explain work and power?

Aim: How can we explain work and power?. Do Now: A man pushes on a wall with a force of 50 N for 60 s. Has he done any work?. Work. The force acting upon an object to cause a displacement There must be a displacement Force and displacement must be in the same direction Scalar quantity.

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Aim: How can we explain work and power?

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  1. Aim: How can we explain work and power? Do Now: A man pushes on a wall with a force of 50 N for 60 s. Has he done any work?

  2. Work The force acting upon an object to cause a displacement There must be a displacement Force and displacement must be in the same direction Scalar quantity

  3. Work or No Work? A student applies a force to a wall and becomes exhausted. A calculator falls off a table and free falls to the ground. An out of shape Army soldier hangs on a pull-up bar for 10 seconds and can’t do a single pull-up. A rocket accelerates through space. NO WORK!!! WORK!!! NO WORK!!! WORK!!!

  4. W = Fd Units Joule = N ·m J = James Prescott Joule 1818-1889

  5. A 5 kg box is pushed with a force of 20 N over a distance of 4 m. How much work was done? W = Fd W = (20 N)(4 m) W = 80 J

  6. The same 5 kg box is now lifted a vertical distance of 4 m. How much work was done? W = F d W = mgd W = (5 kg)(9.8 m/s2)(4 m) W = 196 J The force required to lift an object is equal to the object’s weight g

  7. Area under a force - displacement graph is equal to the WORK done by the force 10 Calculate the work done: F (N) 5 W = bh W = (5m)(10 N) W = 50 J d (m)

  8. Power • The rate of doing work • Rate means divide by time • Scalar quantity James Watt 1736-1819 Units:

  9. A force of 50 N is applied to an object which gives it a constant velocity of 10 m/s. At what rate is work being performed?

  10. A 680 N student runs up a flight of stairs 3.5 m high in 11.4 s. On a second run, the same student completes the same stair run in 8.5 s. • What is the work done by the student? W = Fd W = (680 N)(3.5 m) W = 2380 J

  11. b) What is the power developed for the 11.4 s run? c) Compare this power to the power developed during the 8.5 s run Power and time are indirectly related Time increases, power decreases The 11.4 s run developed less power

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