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Transcript of Physical Science Chapter 5 Work and Machines 1 Note to self: Find videos.
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Physical Science Chapter 5
Work and Machines
1
Note to self: Find videos
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5.1 Work
A simple machine is a system that has work done on it. It, in turn, does work on an object or another system.
Work—transfer of energy that occurs when a force makes an object move
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5.1 Work
For work to occur, an object must move.
The motion of the object must be in the same direction as the applied force on the object.
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5.1 Work
Work and energy are related, since energy is always
transferred from the object doing the work to the object on which the work is done.
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5.1 Work
Work is done on an object only when a force is being applied to the object and the object moves.
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5.1 Work
Calculating work—work equals force in newtons times distance. Work measured in Joules.
W = f x d6
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5.1 Work
Power—amount of work done in a certain amount of time; rate at which work is done1. Calculating power—power
equals work divided by time. p = w/t
2. Power is measured in watts (W)7
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5.1 Work
3. Since work and energy are related, power can also be calculated—power equals energy divided by time. P = E/t
8Practice Problems—Work, Power. Pg 128, 130.
End Sect. 1
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5.2 Using Machines
Device that makes doing work easier is a machine.
Machines increase applied force and/or change direction of applied force to make work easier.
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5.2 Using Machines
Same amount of work can be done by applying a small force over a long distance as can be done applying a large force over a short distance, since work equals force times distance.
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5.2 Using Machines
Increasing distance reduces the amount of force needed to do the work.
Some machines change the direction of the applied force to do work.
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5.2 Using Machines
Machines help move things that resist being moved.Force applied to machine is effort
force.
Resistance force is force applied by machine to overcome resistance.
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5.2 Using Machines
Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine.
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5.2 Using Machines
Some energy transferred is changed to heat due to friction.
An ideal machine with no friction would have the same
input work and output work.14
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5.2 Using Machines
Mechanical advantage (MA) is the number of times a machine multiplies the effort force. It is calculated by MA equals resistance force divided by effort force.
Fe =effort force. Fr =resistance force.
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Calculate:
Calculate the mechanical advantage of a crowbar to which you apply 100 N of force to lift a 250-N rock.
MA = resistance force / effort force = 250 N / 100 N = 2.5
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5.2 Using Machines
Efficiency—measure of how much of the work put into a machine is changed into useful output work by the machine.
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5.2 Using Machines
Calculating efficiency—efficiency equals (output
work divided by input work) times 100%.
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5.2 Using Machines
Efficiency of a machine is always less than 100%.
Lubricants can make machines more efficient by reducing friction.
18End 2
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5.3 Simple Machines.
A machine that does work with only one movement is a simple machine.
Lever—bar that is free to pivot about a fixed point called a fulcrum
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5.3 Simple Machines.
Effort arm is part of the lever on which effort force is applied.
Resistance arm is part of the lever that exerts the resistance force.
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5.3 Simple Machines.
Three classes of levers based on positions of effort force, resistance force, and fulcrumFirst-class lever—fulcrum is located between the effort and resistance forces; multiples and changes direction of force.
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5.3 Simple Machines.
Second-class lever—resistance force is located between the effort force and fulcrum; always multiplies force.
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5.3 Simple Machines.
Third-class lever—effort force is between the resistance force and fulcrum; doesn’t multiply force but does increase distance over which force is applied.
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5.3 Simple Machines.
Calculating ideal mechanical advantage (IMA) of a lever—IMA equals length of effort arm divided by length of resistance arm.
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5.3 Simple Machines.
Grooved wheel with a rope, simple chain, or cable running along a groove is a pulley, which is a modified first-class lever. 25
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5.3 Simple Machines.
A fixed pulley is attached to something that doesn’t move; force is not multiplied but direction is changed; IMA=1. 26
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5.3 Simple Machines.
A movable pulley has one end of the rope fixed and the wheel free to move; multiples force; IMA = 2.
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5.3 Simple Machines.
Block and tackle—system of pulleys consisting of fixed and movable pulleys’ IMA = number of ropes supporting resistance weight. 28
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5.3 Simple Machines.
Wheel and axle—machine with two wheels of different sizes rotating together; modified lever form
1. IMA= radius of wheel divided by the radius of axle.
2. Gears are a modified form of a wheel and axle.
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5.3 Simple Machines.
Incline plane—sloping surface that reduces the amount of force required to do work.
1. IMA = length of slope (effort distance) divided by height of slope (resistance distance.)
2. Less force is required if a ramp is longer and less steep.
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5.3 Simple Machines.
Screw—inclined plane wrapped in a spiral around a cylindrical post.
Inclined plan with one or two sloping sides in a wedge.
Compound machine—uses a combination of two or more simple machines.
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