Chapter 6 - Work & Machines (Science - 8th Grade)

Pulley

a grooved wheel with a rope, chain or cable in the groove; a pulley can change the direction of the input force or increase the output force depending on whether the pulley is fixed or moveable.

Forces

two forces are involved when a machine is used to do work; they are called input and output forces

efficiency (formula)

Efficiency is measured in percent Efficiency = output work/input work x 100% Efficiency of a machine is always less than 100%

Energy

Energy is the ability to cause change; if something has energy it can transfer energy to another object by doing work in that object.

Mechanical Advantage of an Inclined Slope

Ideal Mechanical Advantage = length of slope (m)/ height of slope (m)

Input Work vs. Output Work

Input: the work is done by you on the machine Output: the works that is done by the machine When you use a crowbar you do work when you apply the force of the crowbar handle and make it move. See page 162

Class of Levers

The difference among the three classes of levers depends on 1. the location of the fulcrum 2. the input force 3. the output force

First-class Lever

The fulcrum is located between the input and output force. The output force is always in the opposite direction to the input force in a first-class lever.

Third-class Lever

The input force is applied between the output force and the fulcrum. The output force is always less than the input force (like a baseball bat).

Second-class Lever

The output force is located between the input and fulcrum. For the second-class lever, the output force is always greater than the input force.

Mechanical Advantage of a Lever

The ratio of the output force to the input force. The further the input to the fulcrum (compared to output/fulcrum) the greater the mechanical advantage.

Conditions for a Force to do Work

1. Object Must Move 2. Movement must be in same direction as force

Why machines make work easier (Short Answer)

1. increase force applied to object; 2. increase distance over which work applied; 3. change direction of applied force Machines help humans increase and/or redirect the force applied to an object so the main benefit of machines is that they allow us to do the same amount of work by applying a smaller amount of force over a greater distance.

3 ways machines work

1. increase force that can be applied to an object 2. increase the distance over which is a force can be applied 3. change the direction of an applied force

Mechanical Advantage of a Lever (Formula)

Length from input to fulcrum/Length of Output to Fulcrum (Lin/Lout)

Power (Formula)

Power (watts) = work (joules)/time (seconds) P=w/t

Work and Energy (how are they related)

When work is done, a transfer of energy always occurs; energy is transferred when work is done.

Work (Formula)

Work (joules)= applied force (Newtons) x distance (meters) W=Fd

Levers

a bar that is free to pivot or turn around a fixed point

Compound Machine

a combination of two or more simple machines

machine

a device that makes doing work easier; machines can be simple like knives, scissors or door knobs

Simple Machine

a machine that does work with only one movement of the machine

efficiency

a measure of how much of the work put into the machine is changed into useful output work by the machine; in real machines, some energy put in converts to thermal energy because of friction

Moveable Pulley

a pulley in which one end of the rope is fixed and the wheel is free to move

Wheel & Axle

a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so the wheel and the axle rotate together; usually the input force is applied to wheel and the output force is exerted by the axle (pencil sharpener, rotary phone or faucet handle).

Inclined Planes

a sloping surface, such as a ramp, that reduces the amount of force required to do work.

Gears

a wheel and axle having teeth around its rim; when the teeth of the two gears interlock the turning of one gear causes the other gear to turn.

Fixed Pulley

attached to something that does not move, such as a ceiling or wall.

how can we increase efficiency

decreased friction by adding a lubricant such as oil or grease to surface that rub together; lubricant fills the gaps between the surfaces, enable the surfaces to slide past each other more easily.

conserving energy

energy is always conserved when you do work on a machine, you transfer energy to the machine when the machine does work on an object, energy is transferred from the machine to the object in a perfect machine there would be no frictions so the input would equal the output.

Output force

force exerted by a machine

Wedge

inclined plane with one or two sloping sides; moves through an object or materials;

Screw

inclined plane wrapped in a spiral around a cylindrical post; thread gets thinner farther from the post.

Two kinds of work considered when you use a machine

input work and output work (Wout is always smaller than Win)

Work

is the energy transferred when a force makes an object move; if you push a desk and it doesn't move, you haven't done any work on the desk

Types of Simple Machines

lever pulley (modified lever) wheel and axle (modified lever) inclined plane screw (modified inclined plane) wedge (modified inclined plane)

MA

output force/input force

Efficiency of a Machine (Short Answer)

page 164 in book For real machines, some of the energy put into a machine is always converted to thermal energy by frictional forces. So output work is always less than work put into the machine. Definition under other card You can make a machine more efficient by reducing friction (rubbing a oil or grease).

Power and Energy

power is the rate at which energy is transferred (when energy is transferred (work is done). Example: when a light bulb is connected to an electrical circuit, energy is transferred from the circuit to the light bulb filament.

Pulleys: Block and Tackle

system of pulleys consisting of fixed and moveable pulleys; systems of pulleys can change direction of the input and make the output force increase.

Power

the amount of work done in one second and the rate at which work is done measured in watts (w) 1 watt = 1 joule of work done in 1 second (kilowatts)

Fulcrum

the fixed point the lever pivots on

output force

the force that is applied by the machine

input force

the force that is applied to a machine