New Material Monday Notes
8th Grade
1-26-15
 Watts
o Used to measure power of light bulbs and small appliances
o An electric bill is measured in kW/hrs.
o 1 kilowatt = 1000 Watts
 Horsepower (hp) = 745.5 Watts
o Traditionally associated with engines – cars, etc.
o Horsepower is used to give a unit to power – a strong horse could move a 750 N object
one meter in one second
 Machines
o A device that makes work easier
o A machine can change the size, direction, or the distance over which a force acts
 Forces in Machines
o Input force – force applied to a machine
o Output force – force applied by a machine
 Work in Machines
o Work input – work done on a machine: input force x the distance through which the
force acts (input distance)
o Work output – work done by a machine: output force x the distance through which the
resistance moves (output distance)
o Work output can never be greater than work input!
 Mechanical Advantage
o A ratio that is the number of times a machine multiplies the input force
o MA = output force/input force
 Types of Mechanical Advantage
o Ideal
 No friction
 Is different for different machines
 Input distance/output distance
o Actual
 Involves friction
 The same for all machines
o MA equal to 1 (output force = input force)
 Change the direction of the applied force
o MA is less than 1
 An increase in the distance an object moves
 Efficiency
o Efficiency can never be greater than 100%, because of friction
o Some work always needs to overcome friction
o This is a percentage comparison of work output to work input