Ch. 8 Work and Machines

power

the rate at which work is done; power = work / time

Watt

the unit to express power; equivalent to joules per second; (J/s)

joule

the unit used to express work and energy; equivalent to the newton-meter; (N*m)

work output

the work done by a machine; the product of the output force and the distance through which it is exerted

work input

the work done on a machine; the product of the input force and the distance through which it is exerted

Newton

A unit of measure that equals the force required to accelerate 1 kilogram of mass at 1 meter per second per second. Kg x m/s/s

horsepower

a unit of power equal to 746 watts; amount of power needed needed to move 33,000 lbs, one foot per minute; amount of power needed to lift 550 pounds one foot per second; unit for measuring the power of motors and engines,

classes of levers

based on where the input force (E) and fulcrum (F) are placed in relation to the output force (load, L): first class - (E F L), second class - (F L E), third class - (F E L)

fixed pulley

input force (E) and output force (L) move in opposite directions

movable pulley

input force (E) and output force (L) move in same directions

ideal machine

machine in which work input equals work output; no friction; 100% efficiency

work

the action that results when a force causes an object to move in the direction of the force; work = force x distance

output force

the force exerted by the machine on the load (usually the weight of the load or resistance)

input force

the force that is applied to a machine; also called the effort force

inclined plane

a simple machine that is a straight, slanted surface; a ramp MA = length of plane /height

screw

a simple machine that is an inclined plane wrapped in a spiral

fulcrum

a bar that pivots at a fixed point

same work, different force

Machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work.

equation for power

P = W / t

mechanical efficiency

a comparison -expressed as a percentage- of a machine's work output with the work input; can be calculated by dividing work output by work input anf then mutliplying by 100 ME = (work output / work input) x 100

machine

a device that helps make work easier by changing the size or direction (or both) of a force

equation for work

W = F x d

force-distance trade off

When a machine changes the size of the force, the distance through which the force is exerted must also change. Force or distance can increase, but both cannot increase. When one increases, the other must decrease.

compound machine

a machine made up of two or more simple machines

mechanical advantage

a number that tells how many times a machine multiplies force; can be calculated by dividing the output force by the input force MA = output force / input force

lever

a simple machine consisting of a bar that pivots at a fixed point

pulley

a simple machine consisting of a grooved wheel that holds a rope or cable

wheel and axle

a simple machine consisting of two circular objects of diffrent sizes; the wheel is the larger of the two circular objects MA = radius of wheel / radius of axle

wedge

a simple machine that is a double inclinded plane plane that move; often used for cutting MA = length of wedge / greatest thickness

block and tackle

fixed and movable used together