What is Work?

1. What is Work?

2. What is work in science? • Work is exertion of a force on an object causing it to move a measurable distance.

3. Work Requirements • Force exerted on object • Object moves in direction of the force

4. Is this work?

5. Is this work?

6. Calculating Work • The amount of work done depends on: • Force exerted • Distance the object moves

7. Another magic triangle W = F x D F = W/D D = W/F W F D

8. What is the SI unit for work? • The unit for force is … newton. • The unit for distance is … meter. • Therefore, unit for work is N∙m.

9. Who was James Prescott Joule? • Mr. Joule, a physicist, studied “work” in the mid-1800s. • Unit for work is joule (J). • One joule is the amount of work when one newton of force is exerted to move an object one meter. • J = N∙m

10. Let’s try some problem solving. • You exert a force of 20 N to push a desk 10 m. How much work did you do? 20 N 10 m W = F x D = 20 N x 10 m = 200 N∙m = 200 J

11. A hydraulic lift raises a 12,000 N car a mere 2 meters. How much work is done on the car? • W = F x D • = 12,000 N x 2 m • = 24,000 N∙m • = 24,000 J

12. Now do the rest of the problems. • Pencil—0.2 N to lift 150 cm • Soup can—600 g lifted 500 cm • Total work to lift and plant 2 trees—100 N 183 cm & 250 N. • 3rd tree weighing 375 N • All 3 trees lifted 283 cm

13. pencil • F = 0.2 N • D = 150 cm ÷ 100 cm/m = 1.5 m • W = F x D • = 0.2 N x 1.5 m • = 0.3 J

14. Soup Can • M = 600 g ÷ 1000 g/kg = 0.6 kg • F = 0.6 kg x 9.8 m/s/s • = 5.88 N • D = 500 cm ÷ 100 cm/m = 5 m • W = F x D • = 5.88 N x 5 m • = 29.4 J

15. 2 Trees • D = 183 cm ÷ 100 cm/m = 1.83 m • F total = 100 N + 250 N = 350 N • W = F x D • = 350 N x 1.83 m (to lift) • = 640.5 J • To lift and plant W = 640.5 J x 2 (twice the distance to move up and then down) • = 1281 J

16. Trees continue • Third tree • W = 375 N x 1.83 m • = 686.25 J to lift • To lift and plant (x2) = 1,372.5 J • To lift all three 283 cm • W = total F x D • = 725 N x 2.83 m = 2,051.75 J

17. Now that you know about work… • What is power? • Power is the rate at which work is done. • In other words, power is work per unit time

18. Magic triangle again • Power = Work/Time • = Force x Distance • Time • W = P x T • T = W/P W P T

19. SI unit for power is …. • Joule per second: J/s • Also know as the watt (W), after James Watt • 1 J/s = 1 W • Usually talk about power in kilowatts (kW) • Power also discussed in horsepower unit for engines • One horsepower = 746 W