Simple Machines Chapter 14

What do All These Things Have in Common?

Questions What is the function of the machines? What is the function of the machines? How many moving parts do they have? How many moving parts do they have? How do the parts work together? How do the parts work together? How does this machine make work easier? How does this machine make work easier?

Machines change the Way Force is Applied A machine is any device that helps people do work. A machine is any device that helps people do work. They change the way in which work is done. They change the way in which work is done.

How Could we Move this Couch Easier?

Machines If machines do not reduce the amount of work required, how do they help people do work? Machines make work easier by changing If machines do not reduce the amount of work required, how do they help people do work? Machines make work easier by changing –The size of the force needed to do the work and the distance over which the force is applied. –The direction in which the force is exerted.

Have you ever tried to open a door by turning the doorknob’s shaft instead of the handle? The doorknob allows you to use less force to turn the handle instead of the shaft.

Machines: Changing Size & Direction The doorknob is a machine. It allows you to apply a smaller force over a greater distance. The doorknob is a machine. It allows you to apply a smaller force over a greater distance. The boy using the rake The boy using the rake is a machine. He moves his hands a short distance to move the end of the rake a large distance, Allowing him to rake more Leaves.

Machines Input Force is the force exerted on a machine. Input Force is the force exerted on a machine. Output Force is the force that a machine exerts on an object. Output Force is the force that a machine exerts on an object. Picture on page 450 Picture on page 450

Changing Direction Machines also help you work by changing the direction of force. Machines also help you work by changing the direction of force. You pull down on a rope You pull down on a rope and the flag moves up.

Mechanical Advantage When machines help you work, there is an advantage—or benefit– to using them? When machines help you work, there is an advantage—or benefit– to using them? Mechanical =Output Force Advantage Input Force

Mechanical Advantage For machines that allow you to apply less force over a greater distance– such as a doorknob– the output force is greater than the input force. For machines that allow you to apply less force over a greater distance– such as a doorknob– the output force is greater than the input force. Therefore, the mechanical advantage of this type of machine is greater than 1. Therefore, the mechanical advantage of this type of machine is greater than 1. For example, if the input force is 10 newtons and the output force is 40 newtons, the mechanical advantage is: For example, if the input force is 10 newtons and the output force is 40 newtons, the mechanical advantage is: 40 N / 10 N 40 N / 10 N MA= 4 MA= 4

Mechanical Advantage Sometimes changing the direction of the force is more useful than decreasing the force or distance. Sometimes changing the direction of the force is more useful than decreasing the force or distance. Therefore, the mechanical advantage of the machine is 1. Therefore, the mechanical advantage of the machine is 1.

Practice Problem The output force of a machine is The output force of a machine is 600 N and the input force is 200 N. What is the mechanical advantage of the machine? 600 N and the input force is 200 N. What is the mechanical advantage of the machine? MA= Output force = 600 N = 3 Input Force 200 N Input Force 200 N

Work Transfers Energy Machines transfer energy to objects on which they do work! Machines transfer energy to objects on which they do work!

Energy When you lift an object, you transfer energy to it in the form of gravitational potential energy– that is, potential energy caused by gravity When you lift an object, you transfer energy to it in the form of gravitational potential energy– that is, potential energy caused by gravity

Work When you use a machine to do work, there is always an exchange, tradeoff, between the force you use to do the work and the distance over which you apply that force. When you use a machine to do work, there is always an exchange, tradeoff, between the force you use to do the work and the distance over which you apply that force.

Mountain Climbing Page 453 Short Route: Climbing straight up covers a shorter distance but more force must be applied against gravity Short Route: Climbing straight up covers a shorter distance but more force must be applied against gravity Long Route: Climbing a gentle slope, the climber covers more distance but uses less force. Long Route: Climbing a gentle slope, the climber covers more distance but uses less force.

Efficiency The work you put into a machine will always be greater than the work done by the machine. Some input work is always lost in overcoming friction. The work you put into a machine will always be greater than the work done by the machine. Some input work is always lost in overcoming friction. Efficiency (%) = Output work x 100 Input work Input work

Efficiency and Energy Work transfers energy and that machines make work easier. Work transfers energy and that machines make work easier.

Energy Efficiency Many appliances come with energy guides that can help a buyer compare the energy efficiency of different models. Many appliances come with energy guides that can help a buyer compare the energy efficiency of different models. A washing machine with the highest energy rating may not always save the most energy, because users may have to run those machines more often. A washing machine with the highest energy rating may not always save the most energy, because users may have to run those machines more often.

Review Questions In what ways can a machine change a force? In what ways can a machine change a force? –By changing the size of a force, changing its direction, or both How is a machine’s efficiency calculated? How is a machine’s efficiency calculated? Efficiency (%) = Output work x 100 Input work Input work