4.2 and 4.3: Simple Machines pp. 91 - 103 Mr. Richter

Agenda Warm – Up Welcome Back/Plan for the rest of the year Intro to Simple Machines Notes Today: Machines Mechanical Advantage Efficiency Tomorrow: Types of simple machines

Objectives: We Will Be Able To… Describe how a machine works in terms of input and output. Define some simple machines and name some examples. Calculate the mechanical advantage of a simple machine given the input and output force. Calculate the efficiency of a simple machine.

Warm-Up: What is a machine? Why do we use them?

Machines

Machines Humans are amazing creatures, and we are able to do many things. But there are some things we cannot accomplish on our own. A machine is a device with moving parts that work together to accomplish a task.

Machines A simple machine is a device that accomplishes a task with only one movement.

Machines Every machine, simple or complicated, has an input or output. The input is how much force, work, power or energy is used to make the machine work. What do you do for it? The output is how much force, work, power or energy the machine produces. What does it do for you?

Machines

Mechanical Advantage and Efficiency

Mechanical Advantage Machines are measured by two standards: mechanical advantage and efficiency. Mechanical advantage tells you how much the machine helps you. Mechanical advantage is the ratio of the output force to the input force.

Mechanical Advantage A large mechanical advantage means you need to put in a very small amount of force to get a large force out. A small mechanical advantage means the opposite. MA is unit-less.

Mechanical Advantage What is the mechanical advantage of a lever that allows Jorge to lift a 24 N box with a force of 4 N? 6

Efficiency Efficiency is a measure of how close to perfect a machine is. Efficiency measures how much energy is conserved, and how much is lost to friction. The efficiency of a machine is the ratio of the output work to the input work. Expressed as a percent. (%)

Efficiency An excellent machine is about 95% efficient. This means only 5% of the energy is wasted due to friction. No machine is 100% efficient. Yet?

Efficiency A person does 8000 J of work to pedal a bicycle. The bicycle outputs 6000J of energy. How efficient is the bicycle? 75%

Homework pp. 107 – 108 Solving Problems #8-10, 18-20

Warm Up If the weight at the bottom of the pulley system weighs 100 N, how much force does the man have to use to lift the weight?

Pulleys

Pulleys In an ideal (perfect) machine, the input work is equal to the output work. When we analyze simple machines, we will assume they are ideal unless otherwise noted.

Pulleys In a pulley system, the mechanical advantage is determined by the number of ropes pulling up on the load at the same time.

Pulleys The MA of each pulley system is equal to the number of ropes pulling upward on the load.

Pulleys Homework Worksheet Problems 1 and 3 as a class. Then the rest of the worksheet on your own.

Warm-Up Where do you see levers used in real life?

Levers

Levers “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” - Archimedes

Levers A lever is a rigid structure that rotates around a fixed point. The fixed point is called a fulcrum. The side of the lever where the input force is applied is called the input arm. The output arm applies the output force.

Levers The mechanical advantage of a lever is the ratio of the length of its input arm to the length of its output arm.

Three Classes of Levers Levers can be sorted into three separate classes. First class levers’ input and out put arms are on opposite sides of the fulcrum. Examples: see-saw crowbar

Three Classes of Levers Second class levers have the input and output force on the same side of the fulcrum, but the input force is farther from the fulcrum. Example: Wheelbarrow

Three Classes of Levers Third class levers also have the input and output forces on the same side of the fulcrum, but the input force is closer to the fulcrum. This means the input force is greater(!) than the output force. Used for moving objects large distances. Example: biceps

Levers A lever has a mechanical advantage of 4. Its input arm is 60 cm long. How long is its output arm? If the lever is used to lift an object that weighs 20 N, how much force is required?

Wrap-Up: Did we meet our objectives? Describe how a machine works in terms of input and output. Define some simple machines and name some examples. Calculate the mechanical advantage of a simple machine given the input and output force. Calculate the efficiency of a simple machine.

Homework Finish Lever Worksheet