Work and machines

1. Work and machines Making jobs easier

2. Work • Work is done on an object when a force is exerted on an object that causes the object to move some distance. • No work without motion • No work without force in the same direction

3. Person does not work on weights – weights do not move Person does work on weights – weights moved Force on weights Force on weights Motion and force Summer Olympics 2000 Sydney Australia By Lou Jones http://www.pricespower.com

4. Horse does not work on the rider – force not in same direction Horse does work on the cart - force and motion in the same direction Direction and force Force on person Force on cart motion motion

5. Calculating work • Work = Force X Distance • Force units = Newton, N • Distance units = meter, m • Work units = Joule, J • J = N*m

6. Making work easier • Machines • Change the amount of force you exert • Change the distance over which you exert force • Change the direction in which you exert force • Do change the amount of work done

7. Total work does not change • Work = Force X Distance • If a machine allows you to apply a smaller force the distance will increase • If a machine reduces the distance the force will increase • If a machine allows you to apply a force in a different direction it doesn’t change how much force you have to apply or how far you move it

8. Mechanical advantage • The number of times a force exerted on a machine is multiplied by the machine • Mechanical ad. = Output force Input force • If M.A. > 1 machine increases force needed but distance decreases • If M.A. < 1 machine reduces force needed but distance increases

9. Mechanical advantage output force (in newtons) Mechanical advantage ------------------------------ input force(in newtons)

10. Mechanical advantage output force (in newtons) Mechanical advantage ------------------------------ input force(in newtons) • an input force of 202 N is applied to the handles of the wheelbarrow in the sample problem, how large is the output force that just lifts the load? • Suppose you need to remove a nail from a board by using a claw hammer. What is the input distance for a claw hammer if the output distance is 2.0 cm If and the mechanical advantage is 5.5?

11. Efficiency • Efficiency compares the output work to the input work (%) • Cannot be greater than 100% • Efficiency usually < 100% • Friction

12. Efficiency output work (in newtons) Efficiency advantage ------------------------------ input work(in newtons) • Wile E. Coyote sets up a pulley to lift a gigantic boulder over a valley highway in an attempt to squish the Roadrunner. Wile E. Coyote puts 3402 J of energy into the pulley while the pulley does 2938 J of work. Calculate the efficiency of the pulley. • 2. Wile E. Coyote’s previous attempt of killing the Roadrunner failed so he decided to use an inclined plane to push a boulder down onto the Roadrunner. The output work done by the inclined plane is 875 J and the input work is 1285 J. Calculate the efficiency of the inclined plane.

13. Types of simple machines • Inclined plane • Wedge • Screw • Lever • Wheel and axle • pulley Image taken from: http://www.daniel-wright.district103.k12.il.us/ccheifetz/physics.html

14. Inclined plane teacher.scholastic.com/dirtrep/ simple/img/plane.gif

15. Wedge teacher.scholastic.com/dirtrep/ simple/img/wedge.gif

16. Screw teacher.scholastic.com/dirtrep/ simple/img/screw.gif

17. Lever teacher.scholastic.com/dirtrep/ simple/img/lever.gif

18. Wheel and axle teacher.scholastic.com/dirtrep/ simple/img/wheel.gif

19. Pulley teacher.scholastic.com/dirtrep/ simple/img/pulley.gif