1 Simple Machines

2 Types of Simple Machines: lever gear ramp wheel pulley

3 What Do Simple Machines Do? Simple machines make it easier to do work. They allow you to use less force – TO GET THE WORK DONE! Example: Your body alone could never produce enough force to lift a car. A mechanical jack is a combination of simple machines that allows you to exert more force to lift the car...

4 Less Work? Although you use less force, you usually need to move something more distance. Actually, you may end up doing more total work, but with less force it will be easier or more manageable.

5 It’s a trade off, you gain one thing, but lose another...

6 Machine Input & Output When describing many types of systems, the terms input and output are often used. Input: refers to what you are doing. Output: refers to what the machine is doing.

7 Input/output could refer to many types of quantities. Ex: You push on a lever with 100N of force F input = 100N Ex: A pulley system lifts a boulder 1m up. d output = 1m

8 Efficiency One way to describe, rate, or quantify a machine is by finding its efficiency. Efficiency is the ratio of output work/input work. Output work Efficiency = x 100 Input work

9 Efficiency Work Output Work Input

10 Efficiency Example: A man uses a combination of pulleys to lift a 500N object. He pulls with a force of 70N, and reels in 10m of rope. However, the weight itself only moves 1m upward. What is the efficiency of the pulley system?

11 Solution: W = F x d (what you do!)W input = (70N) ( 10m) = 700J (what the machine does) W output = (500N) (1m) = 500J efficiency = W output / W input eff = 500J / 700J =.71 71%

12 Question: Q: The efficiency wasn’t 100%. Why not? Where did the “missing” work go? A: Some energy was “lost” due to friction. The energy is “unrecoverable” - we can’t use it… It takes work to spin or rotate the pulleys. Air resistance is minimal.

13 Ideal Mechanical Advantage: d input IMA = d output IMA is a ratio of input and output distances. It tells how much your effort is “multiplied” by using the machine! This is without friction – Mr. C’s perfect Physics world!

14 Actual Mechanical Advantage: F output AMA = F input AMA is a ratio of output and input forces. The AMA will never be greater than the IMA because of friction.

15 Example: Using the information from the previous pulley example, find the AMA and IMA. AMA = F output /F input = 500N / 70N = 7.14 IMA = d input /d output =10m / 1m =10

16 The Ideal Lever A car jack is used to lift a car. The force on the car is 4500 N. The force applied by the lifter is just 300 N. Each time the lifter presses the lever down, they move their end of the lever a distance of 0.8 m. If this lever is ideal, how far does the car rise for each press of the lever?

17 The Ideal Lever Since the lever is ideal, we won’t lose any of our work to friction, it will all go into lifting the car. W input =W output F input d input = F output d output (300 N) (0.8 m) = (4500 N) d output solve for d output d output = m = 5.3 cm

18 Compound Machines: A compound machine combines one or more simple machines. Ex: A mechanical watch where a lever may be attached to a gear, pulley, or spring.