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Using Machines

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

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Using Machines

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  1. Using Machines Chapter 14.2 Pgs 432-436 SPI 0707.11.2

  2. Learning Objectives • Explain how a machine makes work easier. • Calculate mechanical advantages and efficiency of a machine. • Explain how friction reduces efficiency.

  3. New Vocabulary • Input force • Output force • Mechanical advantage • Efficiency

  4. What is a machine? • Machines make work easier for us • Machinesdo 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

  5. 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 isinput 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.

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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 • Efficiencyis the ratio of output work to input work

  12. 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

  13. 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

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