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Understanding Work and Power in Physics

Learn about work and power in physics through examples and calculations. Understand the concept of force, displacement, and the relationship between them to determine if work is done. Explore power as the rate of doing work and its calculation.

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Understanding Work and Power in Physics

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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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