# JWhite – CH 5

What is Work? The transfer of energy that occurs when a force makes an object move.
Properties of work: Force is a push or pull. For work to be done, a force must make something move, For example, If you push against a desk and it does not move, you have not done any work.
How are work and energy related? When work is done, a transfer of energy always occurs. For example, if you are carrying books you are increasing the potential energy if you raise the books higher.
What is energy? The ability to cause change. If something has energy, it can transfer energy to another object by doing work on that object.
What is the formula for Work? Work = force X distance
What are the units for work? Joules ( J)
What is Power? The amount of work done in a certain amount of time.
How is Power related to Work? Power is the amount of work done in a certain amount of time.
What is the formula used to calculate Power? Power = Work/Time
What are the units for Power? J/s ( Joules per second)
Work can also be done using electrical energy to produce heat and light. IN this case, energy is still transfered from one object to another. What is the equation for Power? Power = Energy/time
How are Power, work and time related? Power equals work divided by time.
A person pushed a bowling ball 20 m. The amount of work done was 1470J. How much force did the person exert? F = w / d 1470N/ 20m = 73.5 N
What is a machine? A machine is a device that makes work done easier.
How can machines make work easier? by increasing the force that can be applied to an object. For example, no one can lift a car, but a car jack can multiply your force to help you lift the car.
How does a ramp make lifting an object easier? Less force is needed to lift the object.
What is effort force? The force applied to a machine. (the force YOU apply to the machine)
What is the resistance force? The force applied by the machine to overcome resistance ( the force THE MACHINE exerts on the object you are trying to move)
When you pull a nail out with a hammer, what is the resistance force and what is the effort force? Effort Force – The force you apply to the handle Resistance Force – the force the claw on the hammer end applied to the nail.
What is Win (work input)? The work done by you on the machine
What is Wout (work output)? The work done by the machine
Conservation of Energy and Work Because energy can not be created or destroyed, the amount of energy the machine transfers can not be greater than the amount of energy you transfer to the machine. A machine can not create energy, so Wout is always smaller than Win.
Ideal Machine A perfect machine where there is no heat or friction. Win = Wout
Work Input = Effort force X Effort distance
Work out = Resistance force X resistance distance
Mechanical Advantage The number of times a machine multiplies the effort force
AMA (actual mechanical advantage) AMA = Resistance force/ effort force
AMA = 1 When the effort force is equal to the resistance force
IMA ( ideal mechanical advantage) Calculated using distances ( but AMA is calculated using forces since you have already done work)
IMA of a lever lever effort arm/lever resistance arm
IMA of a pulley IMA = number of supporting strands If the string is being pulled down, do not count it as a support strand.
IMA of an inclined plane = length of ramp/height of ramp
When is the test? Monday!!!
Efficiency a measure of how much of the work put into a machine is changed into useful output work by the machine. A machine with high efficiency produces less heat from friction so more of the input work is changed to useful output
Calculating efficiency efficiency = wout/ win x100%
A claw hammer is used to pull a nail from a board. If the claw exerts a resistance of 2500N to the applied force of 125N, what is the AMA of the hammer? AMA = 2500N / 125N = 20
What is a Wheel and Axle? machine with 2 wheels of different sizes rotating together. The IMA is the radius of the wheel divided by the radius of the axle
Inclined plane sloping surface that reduces the amount of force required to do work. Less force is required if a ramp is longer and less steep
Screw inclined plane wrapped around a spiral around a cylindrical post
wedge an inclined plane with one or more sloping sides
compound machine uses a combination of two or more simple machines
First class lever fulcrum is located between the effort force and the resistance force.
second class lever resistance force is located between the effort force and the fulcrum
third class lever effort force is located between the resistance force and the fulcrum
block and tackle system of pulleys consisting of fixed and moveable pulleys
When a fixed pulley lifts an object (performs work), what does the PULLEY do? The pulley decrease the force required to move the object AND changes the direction of the force required
How do you find the ideal mechanical advantage of a lever? the effort distance ( distance of the effort arm – effort force to the fulcrum) divided by the resistance force ( distance from the resistance force to the fulcrum) IF YOU DON'T GET THIS PLEASE LET ME KNOW!!!
How would you find the IMA of a lever that is 4 m long and has an effort arm of 3m? FIrst you have to find what the resistance arm is. If the effort arm is 3m then the resistance arm must be 1m, since the fulcrm would be between the resistance and the effort. To calculate IMA it is effort/resistance so, IMA = 3
What does IMA mean? AMA? IMA – ideal mechanical advantage ( used when you have distances in meters) AMA – Actual Mechanical Advantage – used when the work has alread been done, units are in N ( force)