chapter 5 work and machines. work transfer of energy that occurs when a force makes an object move
TRANSCRIPT
Chapter 5
Work and Machines
Work
Transfer of energy that occurs when a force makes an object move.
Work – Two Conditions
1. The object must move.2. The motion must be in the
same direction as the applied force.
Is Work Done?
Picking up a stack of books.Holding a stack of books.Walking while holding a stack
of books.
Work and Energy
When work is done, a transfer of energy occurs (energy = ability to do work)
Ex. When you pick up a box, you transfer energy from your muscles to the box, increasing its PE
Calculating Work
Work = Force X distanceW=FdF = Nd = mW = N-m = J
Power
Is the rate at which work is done.
Amount of work done in a certain amount of time.
Calculating Power
Power = Work/timeP = W/tW = Jt = secP = J/sec = Watts (W)
Power and Energy
Energy can be transferred without involving work.
Ex. Light bulb transfers energy into heat and light.
P = E/t
Machines
A device that makes doing work easier.
Work by:
–Increasing F (car jack)
–Increasing d (ramp)
–Changing direction (ax)
Work done by Machines
Two forces involved
–Effort Force (Fe) – the force applied to a machine.
–Resistance Force (Fr) – the force applied by the machine to overcome resistance.
Work done by Machines
Two types of work involved:
–Input work (Win) – work done by you on a machine.
–Output work (Wout) – work done by the machine.
Conserving Energy
What does the Law of Conservation of Energy state?
You transfer energy to the machine, and the machine transfers energy to an object.
Conserving Energy
A machine cannot create energy, so Wout is never greater that Win.
Can Wout = Win ? Why?
Ideal Machines
What is an ideal Machine?If a machine allows you to
apply less force, then how do you get the same amount of work out of the machine?
Mechanical Advantage (MA)
It is the number of times a machine multiplies the effort force.
IMA vs AMA
Calculating MA
MA = resistance force/effort force
MA = Fr/Fe
MA has no units. Why?
Efficiency
A measure of how much of the work put into a machine is converted into useful output work.
Why is work output always less than work input?
How can you efficiency?
Calculating Efficiency
Efficiency = (Wout/Win) X 100%
Simple Machines
A machine that does work with only one movement.
Six Types: 1)lever, 2)pulley, 3)wheel and axle, 4) inclined plane, 5) screw, and 6) wedge.
Lever
A bar that is free to pivot about a fixed point or fulcrum.
Effort arm = distance from fulcrum where effort force is applied.
Resistance arm = distance from fulcrum where resistance force is applied
Types of Levers
Based on positions of effort force, resistance force, and fulcrum.
Types of Levers
1st class – fulcrum is in the middle (crowbar)
2nd class – resistance is in the middle (wheelbarrow)
3rd class – effort is in the middle (baseball bat)
IMA of Levers
IMA = length of effort arm/length of resistance arm
IMA = Le/Lr
Pulleys
A grooved wheel with a rope, chain or cable running along the groove.
How is a pulley a modified 1st class lever?
Fixed Pulleys
Attached to something that does not move
Only change the direction of the force
IMA = 1
Moveable Pulleys
One end of the rope is fixed and the wheel is free to move.
Multiplies effort forceIMA = 2
Block and Tackle
A system of pulleys made of fixed and moveable pulleys
IMA = # of supporting rope segments. Only count effort segment if effort force is in the direction of the object’s movement.
Wheel and Axle
Machine consisting of two wheels of different sizes that rotate together.
Modified form of a lever.IMA = rw/ra
Gears
Modified Wheel and Axle with two wheels of different sizes with interlocking teeth.
Large wheel = effort gear, small wheel = resistance gear.
Larger effort gear = more turns of resistance gear = effort force
Inclined plane
Sloping surface that reduces the amount of force required to do work.
Increases distanceIMA = length/height
Screw
Inclined plane wrapped, in a spiral, around a cylinder.
Wedge
Moving inclined plane with one or two sloping sides.
Changes the direction of effort force
Compound Machines
When two or more simple machines are used together.
Examples?