Work, Machines and Energy Systems Reference: Ch. 10 & 11
Mechanical systems and machines A machine is a device with moving parts that work together to accomplish a task. They do something useful. Input-what YOU do Output-what the MACHINE does. This was the bicycle manufactured by the Wright brothers in 1897.
Simple Machines A simple machine is an unpowered mechanical device. Lever, Wheel and axle, block and tackle, gear, ramp Simple machines work by manipulating forces. Most machines we use today are compound machines-they have many simple machines combined.
Work and Power Simple machine change the amount of force or the direction of the force or both. Force is measured in newtons (N) or pounds (lb) 4.48 N = 1 lb Work- force X distance W= F(d) W in joules F in Newtons d in meters Power: how fast you do work; P=W/t P in watts (or horsepower) W in joules T in seconds
Mechanical Energy Two most common forms: Energy due to MOTION = KE Energy due to POSITION = PE Potential Energy has the potential to do WORK. Ex. Fossil Fuels, Food Gravitational PE: The PE due to ELEVATED positions. Ex. Water in an elevated reservoir GPE is = to the WORK done in lifting it.
Kinetic Energy KE = ½ mv 2 relationships or Fd = ½ mv 2 a. double the speed and the KE quadruples b. it takes 4x the work to double the speed c. objects moving twice as fast takes 4x as much work to stop d. Ex: a car going 100 km/hr has 4x the KE it would have at 50 km/hr e. Ex: a car going 100 km/hr will skid 4x as far when brakes are locked-as it will at 50 km/hr because speed is squared for kinetic energy.
Mechanical Advantage and Efficiency MA=F o /F i MA > 1, output force is larger. MA < 1, output force is smaller. Output input If a machine is efficient most of the work input becomes output An ideal machine would be 100% efficient Friction always lowers efficiency Efficiency = useful work output X 100 = % Total work input If the work output<work input then the machine is not efficient!
Block and Tackle Uses ropes and pulleys to multiply forces. Input force-100 lbs Output force-200 lbs (the weight of the object being lifted) # of ropes = # of times the force is multiplied. Tension force
# Ropes = MA Two Ropes Output Force=100 lb Input Force=50 lb 100 = 2 50
Levers A lever has several different parts. Fulcrum-fixed point. Input arm-between fulcrum and force you apply. Output arm -the side where output force is applied.
Energy Energy- ability to do work; in Joules Heat Energy Radiant Energy (sun/light) Nuclear Energy Electrical Mechanical Potential (p=mgh) and Kinetic (KE= 1/2mv 2 ) Law of Conservation of Energy Energy cannot be created or destroyed; it can only be transformed to another type
Heat Energy Heat Transfer heat transfer- heat travels from a material at a higher temperature to a material at a lower temperature 3 ways heat can transfer: 1. Conduction 2. Convection 3. Radiation
Forms of heat transfer ConvectionRadiation Conduction Transfer through direct contact of particles Example: Heating food in a pot on stove Electromagnetic waves transfer heat. Does not require matter. Example: Radiation from sun feels hot on skin Motion of fluids Example: currents form when hot air rises and cool air falls (weather, oven, heater)