Monday, February 22 nd Agenda Collect Worksheet: “Muscles and Work” Finish Section 8.1: Work, Power, and Machines Work input, work output, mechanical.

Monday, February 22nd

AgendaCollect Worksheet: “Muscles and Work”Finish Section 8.1: Work, Power, and

MachinesWork input, work output, mechanical advantage

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

MachinesHomework

None!

“Muscles and Work” Worksheet

Hand In

Machines and Mechanical AdvantageWhich 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 ForcesMachines 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 OutputWhen 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 OutputBoth 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 AdvantageA 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 AdvantageTo 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 outInput 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 EquationsA 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 ProblemPg. 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 mOutput distance = 1.5 mSolve: 5.0 m = 3.3

1.5 mSince the units cancel, there are NO units for

mechanical advantage.

Additional PracticeA 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 NInput force: 350 NSolve: mechanical advantage = 950 N = 2.7

350 N

In-Class Assignments

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

Machines