Mechanical advantage does NOT always equal speed Ratio.
Friction can affect mechanical advantage, but not speed ratio
Speed ratio represents the ideal mechanical advantage (No friction).
FRICTION: Force that opposes motion.
caused by the roughness of surfaces.
as roughness of a surface increases so does the effect of friction.
friction creates HEAT, and must be released to protect the system.
EFFICIENCY
EFFICIENCY: Measurement of how well a machine or device uses energy.
Efficiency = (Mechanical Advantage / Speed Ratio) x 100
Efficiency of a system is negatively affected by friction.
Most energy is lost and unusable (Ex. Lost as heat)
Most complex machines are very inefficient: waste energy. ex) Car – 15% efficient.
THE SCIENCE OF WORK
WORK: Done when a force acts on an object to make the object move.
Movement is needed before one can say that work has been done. Also referred to as 'Energy'
Measure in Joules (J) or Newton-meters (N x m)
Amount of work done depends on 2 things:
amount of force exerted on the object
distance the object moved in the direction of the applied force.
Work (J) = Force (N) x Distance (m)
The joule is also used in calculating Energy!
EX 1 You apply 300 N of force in 15 m. How much work did you do? EX 2 A forklift lifts a box 2 m by applying 1000 N of force. How much work did the forklift do? EX 3 You have used 1000 J of energy to apply a force of 200 N to an object. How far did that object move?
Fun fact ! 1Kg = 9.81N , I wonder how many 10 Kgs is equal too?
POWER: How much work is achieved in a certain amount of time, measured in Watts Power (W) = Work (J) / Time (s)
EX 1 You apply 200 J to an object in 15 s. How much power did you use? EX 2 An engine provides 5000 J of work to an axle over the period of 10 seconds. How much power does the engine have? EX 3 A horse pulls a wagon by working 4500 J over a time of 6 s. How much power does the horse produce?
ENERGY AND WORK
Energy and work are closely related, can not have one without the other. Ex. Car – needs energy (gasoline) in order to work (move)
WORK AND MACHINES
Using a machine does not decrease the amount of work, it decreases the force needed.
Work output = Work input This equation is affected by friction (just like mechanical advantage)
CALCULATING EFFICIENCY Efficiency can be calculated without using Mechanical Advantage nor Speed Ratio Efficiency = (Work Output/ Work Input) x 100
EX 1 A construction worker puts 20 J of energy in to one strike of his hammer on the head of a nail. The energy transferred to driving the nail in to the wood is 8.0 J. What is the efficiency of the construction worker's hammering?
EX 2 Mr. K spent 25 J of energy to spike a volleyball over the net. A player received that volleyball and determined that the energy transferred to the ball was 20 J of energy. What is the efficiency of Mr. K's spike?
EX 3 A particular chemical process has an energy efficiency of only 3.00%. To complete this large-scale chemical process, 140,000 J of energy is input. What is the energy output of this process?
SIMPLE MACHINES CALCULATION ASSIGNMENT For this assignment you will be drawing out four different situations and calculating the following for each situation:
Draw the simple machine
Label the force & the direction of force
Calculate work input and work output
Calculate mechanical advantage
Calculate efficiency
Draw and calculate the following:
Robin is using a pulley to lift a box, she pulls with a force of 155 N for 1.5 m. The box weighs 20 Kg moves up a distance of 0.6 m.
Josh is using an axe to chop some wood, with a force of 80 N he drops the axe head 0.5 m onto a piece of wood. The wood splits 0.02 m apart with a force of 1000 N.
Helen is using a screw to fasten two pieces of wood together. She rotates the screwdriver once with 100 N of force, the screwdriver rotates 0.05 m. The screw bites downward into the wood with a force of 350 N and moves approximately 0.01 m into the wood.
Mark uses a second class lever to lift a load that weighs 10 Kg up by 0.35 m. He applied a force of 50 N to the lever which moved a total of 0.85 m.