Monday, February 22 nd Agenda  Collect Worksheet: “Muscles and Work”  Finish Section 8.1: Work, Power, and Machines  Work input, work output, mechanical advantage  In-Class Assignments:  Pg. 256: Practice #1-3  Section 8.1 review: Pg. 256 #1-2, 5-7  Study Guide Section 8.1: Work, Power, and Machines  Homework  None!

“Muscles and Work” Worksheet  Hand In

Machines and Mechanical Advantage  Which is easier, lifting a car yourself or using a jack?  Which requires more work?  Using a jack is obviously easier, but you may be surprised to learn that using a jack doesn’t require less work.  You do the same amount of work either way, but the jack makes the work easier by allowing you to apply less force at any given moment.

Machines Multiply and Redirect Forces  Machines help us do work by redistributing the work that we put into them.  Machines can change the direction of an input force or they can increase an output force by changing the distance over which the force is applied.  This process is called multiplying the force.

Work Input = Work Output  When lifting a box straight up, a mover applies a large force over a short distance.  Using a ramp to lift the box, the mover applies a smaller force over a longer distance.  In both cases, the work done is the same, 225 J.

Work Input = Work Output  Both a car jack and a loading ramp make doing work easier by increasing the distance over which the force is applied.  As a result, the force required at any point is reduced.  But the amount of work you put into the machine, the work input, is equal to the amount you get out, the work output. =

Mechanical Advantage  A ramp makes doing work easier by increasing the distance over which force is applied.  But how long should the ramp be?  A very long ramp would allow the mover to use very little force, but he would have to push the box a long distance.  A very short ramp would be too steep and would not help him very much.

Mechanical Advantage  To solve problems like this, scientists and engineers use a number that describes how much the force or distance is multiplied by a machine.  This number is called the mechanical advantage.  Mechanical advantage: a quantity that measures how much a machine multiplies force or distance.

Mechanical Advantage Equation Mechanical Advantage = output force input force  Output force = amount of work you get out  Input force = amount of work you put into the machine

Mechanical Advantage Equation Mechanical Advantage = input distance output distance  Input distance = length of ramp, lever, pulley, etc.  Output distance = how far the object moved (height)

Mechanical Advantage Equations  A machine with a mechanical advantage greater than 1 multiplies the input force.  This kind of machine can help you move or lift heavy objects, such as a car.  A machine with a mechanical advantage less than 1 does not multiply the force, but increases the distance and speed.  When you swing a baseball bat, your arms and the bat together form a machine that increases speed without multiplying force.

Practice Problem Pg. 255 Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. Equation: mechanical advantage = input dist output dist Input distance = 5.0 m Output distance = 1.5 m Solve: 5.0 m = m Since the units cancel, there are NO units for mechanical advantage.

Additional Practice A person pushes a 950 N box up a ramp. If the person exerts a force of 350 N along the ramp, what is the mechanical advantage of the ramp? Equation: mechanical advantage = output force input force Output force: 950 N Input force: 350 N Solve: mechanical advantage = 950 N = N

In-Class Assignments  Pg. 256: Practice #1-3  Section 8.1 review: Pg. 256 #1-2, 5-7  Study Guide Section 8.1: Work, Power, and Machines