Using Machines Chapter 14.2 Pgs 432-436 SPI 0707.11.2 Learning Objectives • Explain how a machine makes work easier. • Calculate mechanical advantages and efficiency of a machine. • Explain how friction reduces efficiency. New Vocabulary • • • • Input force Output force Mechanical advantage Efficiency What is a machine? • Machines make work easier for us • Machines do not decrease the amount of work we need to do • We still exert a force over some distance to use machines – Raking leaves: you exert a force on a rake to gather leaves – Using a wheelbarrow: you exert a force to lift the handles of the wheelbarrow Input & Output Forces • The force that you apply to a machine is the input force. • The work you do on the machine is equal to the input force times the distance over which your force moves the machine • The work you do on the machine is input work • The machine also does work by using force to move an object • The force the machine applies is the output force. • The work the machine does is output work. Using a Machine • When you use a machine, the output work can never be greater than the input work • The machine makes the work easier by: – Changing the amount of force you need to exert – Changing the distance over which force is exerted, or – Changing the direction in which you exert your force Changing Force • Some machines make work easier by reducing the force you apply to complete a task • These machines increase input force so output force is greater • The number of times a machine increases the input force is the mechanical advantage of the machine Calculating Mechanical Advantage • Mechanical advantage is the ratio of output force to input force • Can be calculated using this equation: • Force is in newtons • Applying math pg 433 (1 &2) • Mechanical advantage does not have any units because it is a ratio with two numbers of the same units Changing Distance • Changing Distance (figure 5 pg 434) – Some machines allow you to exert a force over a shorter distance – The output force is less than the input force – Example: using a rake – The mechanical advantage of this type of machine is less than one because output is less than the input Changing Direction • Changing Direction (pg 434, fig.5) – Sometimes its easier to apply a force in a certain direction – Example: raising a flag – Neither force nor distance is changed – The mechanical advantage of this type of machine is 1 because both forces are equal Efficiency • Machines can make output force greater than input force but they cannot make OUTPUT WORK greater than INPUT WORK • When you use a machine there is friction between moving parts • Friction converts some input work to thermal energy & the rest is converted to output work • Output work is always less than the input work • Efficiency is the ratio of output work to input work Friction • Imagine pushing a heavy box up a ramp • The surfaces of the two objects are in contact • At points along each surface, atoms and molecules stick and bond together • These bonds remain until a force breaks them • To keep the box moving, force must be applied to break the bonds as they continue to form Assess What You’ve Learned • Identify three specific situations in which machines make work easier. • Why does the output force exerted on a rake have to be less than the input force? • How does the efficiency of an ideal machine compare with the efficiency of a real machine? • How does friction reduce the efficiency of machines? • Applying math pg 436, 6-7