Physical Science Unit 3 Chapter 13 - Work and Machines

Work (W)

the energy transferred to a system by an external force when it acts on the system to move it

Formula for Work

W = Fd

Power (p)

The rate of doing work

Formula for Power

P = W/t

Simple Machine

a basic mechanical device that changes the magnitude, direction, or distance traveled of the force used when doing work

Mechanical Advantage (MA)

the measure of the change in input needed to do a certain amount of work

When the machine multiplies the effect of the input force, the MA is…

greater than 1

When the machine multiplies the input distance, the mechanical advantage is…

greater than 0 and less than 1

When the machine changes the direction of the force only if the mechanical advantage is…

equal to 1

MA is a…

dimensionless quantity

Ideal Mechanical Advantage (IMA)

the mechanical advantage for an ideal machine with no friction

IMA Formula

IMA = D(in)/D(out)

Actual Mechanical Advantage (AMA)

the mechanical advantage that accounts for all mechanical losses sustained through using a machine

AMA Formula

AMA = F(out)/F(in)

Efficiency

a comparison of the amount of usable energy remaining after a process with the original amount of energy that went into the process

Efficiency Formula

Efficiency = (AMA/IMA) * 100%

Lever

a simple machine that consists of a rigid bar that turns about a pivot point

Fulcrum

a point about which a lever rotates

Torque

a force that tends to cause a rotation about a pivot point

First-Class Lever

the fulcrum is between the input force and output force and the mechanical advantage is greater than 0

Examples of First-Class Levers

scissors, pliers, see-saws, can openers, hammers, water pumps, nail clippers

Second-Class Levers

the output force (load) is between the fulcrum and the input force and the mechanical advantage is greater than 1

Examples of Second-Class Levers

wheelbarrows, wrenches, nutcrackers

Third-Class Levers

the input force is between the fulcrum and the output force and the mechanical advantage is always between 0 and 1

Examples of Third-Class Levers

tweezers, tongs, shovels, brooms, fishing rods, baseball bats, (hammers)

A first class lever changes the…of the force

direction

A second class lever multiplies the…

input force

A third class lever changes the…of the force

distance

Wheel and Axle

a simple machine consisting of a wheel with a rod running through its axis that acts as a pivot point

What is the mechanical advantage of a wheel and axle?

Greater than 0

Gear

a simple machine that consists of a wheel with teeth around its perimeter

Ideal Mechanical Advantage of Gears

IMA = teeth(in)/teeth(out)

Formula for the MA of a Lever

MA = Effort Arm/Resistance Arm

Pulley

a simple machine that consists of a wheel and axle system with a groove around the perimeter of the wheel in which a rope, cable, or belt moves with the wheel as it rotates

Fixed Pulley

the simplest pulley system that has a pulley attached to a fixed structure, work is done by applying the input force to the free end of the rope

Block and Tackle

two fixed pulleys and two moveable pulleys that multiply the mechanical advantage

Inclined Plane

a simple machine consisting of a flat surface whose opposite ends are at different heights

Wedge

consists of two planes connected with an acute angle that is thick at one end and thinner at the other

What is the mechanical advantage of a wedge?

If the wedge is sharp, the mechanical advantage is high; if the wedge is blunt, the mechanical advantage is low

Screws

an inclined plane that is wrapped around a cone or a cylinder in a spiral pattern

Mechanical Advantage of a Screw

if the thread is steep, the mechanical advantage is lower; if the thread is shallow, the mechanical advantage is higher

Compound Machine

two or more simple machines that are combined

Moveable Pulley

the rope of the pulley is attached to a fixed structure, and the pulley is attached to the load