Ws09002Name ______

Date ______

Machines - Percent Efficiency

Info: Work output = F x d (what happened, what moved, what resulted)

Work input = F x d (what was done to make it happen, what was the effort)

Work used to overcome friction = Work input – Work output

Efficiency = (Work output / Work input) x 100

In an IDEAL machine: Work input = Work output

In a REAL machine: Work input =Work output + work used to overcome friction

  1. By applying a force of 75N over a distance of 8m to a set of pulleys, a student raised a 300N object to a height of 1.5m above the floor. What is (a) the work input (b) the work output (c) the efficiency? (600J, 450J, 75%)
  1. An effort of 120N is applied to a machine to move 720N resistance 4m. Assuming this to be an ideal machine, over what distance must to effort be applied? (24m)
  1. Using a set of pulleys, a mover raises a 1000N crate a distance of 5m. This is accomplished by pulling in 20m of cord. (a) If this were an ideal machine, how much effort (N) would have to be applied? (b) If the effort actually applied is 300N, what force is used to overcome friction? (c) What is the work output? (d) What is the work input? (250N, 50N (1000J), 5000J, 6000J)
  1. A set of pulleys is used to raise a 150N weight 4m. To do this, the operator applies a force of 40N and pulls in 20m of rope. Find the (a) work output (b) work input (c) work used to overcome friction (d) efficiency. (600J, 800J, 200J, 75%)
  1. A machine having an efficiency of 70% lifts a 5 kg object 10m. (a) What is the weight of the object? (b) What is the work output (c) What is the work input (d) How much work was expended in overcoming friction? (49J, 490J,700J, 210J)
  2. A resistance of 475N is displaced 0.0623m by an effort of 178N. The efficiency of the machine is 85%. What is the displacement of the effort? (.196m)
  1. The efficiency of a lift is 72.5%. What is the work needed to raise a 200kg mass 5.65m? (15,274 J)
  1. A force of 225N is displaced 7.35m while moving a weight of 1340N to a height of 0.975m (a) What is the work input? (b) What is the work output? (c) What is the efficiency of the machine? (1653.8J, 1306.5J, 79%)
  1. A dockhand rolls a barrel up a plank 6.4m long. The barrel weighs 425N and is displaced a height of 1.28m. The dockhand exerts a constant force of 95N parallel to the plank. (a) What work is needed to overcome friction? (b) What is the efficiency of the machine? (64J, 89.5%)
  1. A 20N force is needed to pull a box 40m up an incline. The box weighing 400N is now 1.6m above its starting level. Find (a) the work output (b) the work input (c) the work expended on friction and (d) the efficiency. (640J, 800J, 160J, 80%)
  1. A certain machine does 5000J of useful work and expends 1000J in overcoming friction. Find (a) the work input (b) the efficiency of this machine. (600J, 83%)
  1. A 100kg box is raised 10m by a machine that is 35% efficient. What is (a) the work output (b) the work input (c) the work used in overcoming friction? (600J, 800J, 200J, 75%)