Using Simple Machines INB Page 145

REVIEW! What is Work? Work occurs when a force causes an object to move in the direction of the force. The unit for work is the joule (J). Work is done on an object only when a force makes an object move and only while that force is applied. For work to be done on an object, the direction of the object’s motion must be in the same direction as the force applied. Work can be calculated by multiplying force by distance. W = Fd

Machines Do Not Save Work Machines make work easier… because they change the size or direction of the input force. Using a machine does not mean that you do less work. Using a screwdriver to open a paint can changes both the size and direction of the input force

INPUT FORCE – OUTPUT FORCE The work you do on a machine is called work input. You apply a force, called the input force, to the machine and move it through a distance. The work done by the machine is called work output. The machine applies a force, called the output force, through a distance.

You might think that machines help you because they increase the amount of work done. NO!!! If you multiplying the forces by the distances through which they are applied (remember, W = Fd), shows the screwdriver does not do more work on the lid than you do on the screwdriver. Work output can never be greater than work input.

MACHINES MAKE WORK EASIER A machine can change the size or direction (or both) of a force.

MACHINES MAKE WORK EASIER A machine can change the size or direction (or both) of a force.

MACHINES MAKE WORK EASIER A machine can change the size or direction (or both) of a force.

MACHINES MAKE WORK EASIER A machine can change the size or direction (or both) of a force.

Simple Machines and Work Eureka! The Incline Plane Eureka! The Lever

Mechanical Advantage Do some machines make work easier than others? Yes, because some machines can increase force more than others. A ma- chine’s mechanical advantage tells you how many times the machine multiplies force. In other words, it compares the input force with the output force. You can find mechanical advan- tage by using the following equation: Mechanical advantage (MA) = output force input force Eureka! Mechanical Advantage

Mechanical Advantage - Example In this example, the output force is greater than the input force. Using the equation for Mechanical Advantage: (MA) = output force input force , you can find the mechanical advantage of the handcart:

MATHBREAK Finding the Advantage You apply 200 N to a machine, and the machine applies 2,000 N to an object. What is the mechanical advantage? MA = 2000 N / 200 N MA = 10

Which of the following makes work easier to do? a.) a machine with a mechanical advantage of 15 b.) a machine to which you apply 15 N and that exerts 255 N a and b have a mechanical advantage of 15!

Mechanical Efficiency The work out- put of a machine can never be greater than the work- input. Some of the work done by the machine is used to over- come the friction created by the use of the machine. No work is lost. The work out- put plus the work done to overcome friction equals the work input. The less work a machine has to do to overcome friction, the more efficient it is. The following equation is used to calculate Mechanical Efficiency: Mechanical Efficiency = output force input force X 100

Review Questions: Write the question and the answer on page 146 in your INB 1. Explain how using a ramp makes work easier. 2. Why can’t a machine be 100 percent efficient? 3. Suppose you exert 15 N on a machine, and the machine exerts 300 N on another object. What is the machine’s mechanical advantage? 4. Comparing Concepts For the machine described in question 3, how does the distance through which the output force is exerted differ from the distance through which the input force is exerted?