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Work and Power
Work
Work is the transfer of energy that occurs when a force makes an object move. 2 conditions for work to be done
An applied force must make the object moveMovement must be in the same direction as the
force.
Force
Movement
Work
How is the scientific definition of work different from the every day meaning?
For
ce
Mov
emen
t
Work
When work is done transfer of energy always occurs.
When you work on an object, you increase its energy.
Calculating Work
Work = applied force * distance Or w=Fd
Create a calculation triangle to make it easier to solve for “w”.
W
F x d
Calculating Work cont.
A force of 75N is exerted on a 45 kg couch and the couch is moved 5m. How much work is done in moving the couch?
The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50m while the brakes are being applied, what is the total force of the brakes on the car?
Power
Power is the amount of work done in one second. Rate at which work is done
Calculating Power
Power = work/ timeCreate a calculation triangle to make it
easier to solve for “p”.
W
P x t
Calculating Power cont.
To lift a baby from a crib 50 J or work are done. How much power is needed to lift the baby in 0.5s?
The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?
Machines
A machine is a device that makes doing work easier. It increases the force that you apply.
Machines cont.
Input Force: force that is applied to the machine. = Fin
Output Force: the force applied by the machine. = Fout
Mechanical advantage is the ratio of the output force to the input forceMA = Fout/ Fin
Efficiency % = Output work/ input work x 100
Machines cont.
Friction causes energy of the output force to be lost as heat.This means Wout is smaller than Win
How would lubricating a machine affect the output force exerted by the machine?
Ideal MA is one without friction
MA and Efficiency Triangles
Create a triangle for
Mechanical
Advantage
Create a triangle for
efficiency.
Fout
MA x Fin
Fout
Eff. x Fin
x 100%
Calculating MA and Efficiency
The mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N.
Find the efficiency of a machine that does 800 J of work if the input work is 2,400 J.
Simple Machines
6 basic typesLeversPulleysWheel and Axel Inclined PlanesScrewWedge
View Simple Machines
Levers
There are 3 types of Lever:First-Class: Fulcrum is between input &
output force.Second-Class: Output force is between the
fulcrum & the input forceThird-Class the input force is between the
fulcrum and the output force.
View Examples
Pulleys
Fixed: Changes only the direction of YOUR force. (4N in gets 4N out)
Movable: the attached side of the rope supports half of the weight. Ex. For a 4N object attached side supports 2N while you only do 2N of work.
Block and Tackle: Made up of 2 fixed & 2 movable pulleys & weight is divided evenly among all of them.
View Pulleys