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Physics 2

Physics 2. Lesson #2. Work. Work is related to “force”. Calculating Work. Work = force x distance Or W = Fd. Work Units. Force = kg m/s 2 or N Distance = m So if W=Fd, work units are kgm 2 /s 2 or Nm. kg m 2 /s 2 …..What is that!?. Let’s invent another simpler unit.

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Physics 2

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  1. Physics 2 Lesson #2

  2. Work Work is related to “force”

  3. Calculating Work • Work = force x distance • Or • W = Fd

  4. Work Units • Force = kgm/s2 or N • Distance = m • So if W=Fd, work units are kgm2/s2 or Nm

  5. kg m2/s2 …..What is that!? • Let’s invent another simpler unit. • James Joule is an important scientist here. • 1 kgm2/s2 = 1 J (Joule) • Also, 1 Nm = 1 J

  6. Work Calculations • Some work formulas include: • W=Fd • W=mad • BUT….we sometimes have to calculate acceleration first (a=∆v/ t, or a=(v2-v1)/ ∆t or a=(v2-v1)/(t2-t1)

  7. Eg. 1 • Ian applies a force of 25 N when sliding a heavy packing crate along the floor for 20 m. Calculate his work. • W=Fd • W=25N x 20m = 500 Nm or 500 J

  8. Eg. 2 • How far can a dog pull a sled if it can exert a force of 240 N doing 8000 J of work? • W=Fd • d=W/F • d= 8000 Nm / 240 N = 33.3 m

  9. Eg. 3 • How much force is needed to walk up a flight of stairs if the distance up is 120m and the work done is 24.0 kJ (24,000J). • W=Fd • F=W/d • F= 24000Nm / 120m = 200N

  10. Eg. 4 • Josh accelerates a 150 kg bobsled over a distance of 80.0 m at a rate of 1.20 m/s2. How much work was done? • W=Fd • W=mad • W= 150kg x 1.20 m/s2 x 80.0m • W= 14,400 kgm2/s2 or 14.4 kJ

  11. Eg. 5 • It takes 4.70s and 180m for a 1700 kg car to go from 22.0 m/s to 28.0 m/s. How much work does the engine do to accomplish this? • W=Fd • W=mad • W=m x (v2-v1)/t x d • W= 180kg x 6.0m/s / 4.70s x 180m = 41.4kJ

  12. Work Graphs • Force goes on the y-axis • Distance goes on the x-axis

  13. F (N) D (m) Eg. 1

  14. F (N) D (m) Eg. 2

  15. F (N) D (m) Eg. 3

  16. Calculating Work From a Graph • Work is equal to the area under a force-distance graph. • This can be done with geometry or by counting squares.

  17. F (N) D (m) Eg. 4 5 W=5x6=30J 6

  18. F (N) D (m) Eg. 5 8.0 W =1/2 x 8 x 20 = 80J 20

  19. Counting Squares • The trick here, is to count full and “1/2 or more” squares, then multiply by the work represented by each square. • ie. 120 squares x 2 J per square = 240 J • See text for example.

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