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Chapter 14 Work, Power and Machines
The Physics Classroom – Work, Energy & Power
14.1 Work and Power
14.1.1 Describe the conditions that must exist for a force to do work on an object.
14.1.2 Calculate the work done on an object.
14.1.3 Describe & calculate power.
14.1.4 Compare the units of watts and horsepower as they relate to power.
- Work = Force x Distance. Work is done on an object when a force acts in the same direction as the object moves.
- Power = Work / Time. Power is the rate of doing work. Doing work faster requires more power.
work – the product of the distance and the force in the direction an object moves
joule – (J) the SI unit for work, equal to 1 newton-meter
power – the rate of doing work
watt – (W) the SI unit of power, equal to one joule per second
horsepower – (hp) a common unit of power, equal to about 746 watts
Practice Packet:
Work Worksheet (jeff)
What it Means to do Work (theresa)
14.2 Work and Machines
14.2.1 Describe what a machine is and how it makes work easier to do.
14.2.2 Relate the work input to a machine to the work output of the machine.
- Machines make work easier by changing the force needed, the direction of the force, or the distance over which the force acts.
- Work output (done by a machine) is always less than the work input (done by the machine).
machine – a device that changes force
input force – the force exerted on a machine
input distance – the distance through which the input force acts in a machine
work input – the work done on a machine as the input force acts through the input distance
output force – the force exerted by a machine
output distance – the distance an output force acts through in a machine
work output – the work done by a machine as the output force acts through the output distance
14.3 Mechanical Advantage and Efficiency
14.3.1 Compare a machine’s actual mechanical advantage to its ideal mechanical advantage.
14.3.2 Calculate the ideal and actual mechanical advantages of various machines.
14.3.3 Explain why the efficiency of a machine is always less that 100%.
14.3.4 Calculate a machine’s efficiency.
- Friction is present in all machines. Because of friction, the actual mechanical advantage is alwaus less than the ideal mechanical advantage.
- Actual mechanical advantage = Output force / Input force.
- Ideal mechanical advantage = Input distance / Output distance.
- Efficiency = Work output / Work input x 100%.
- Because of friction, the efficiency of any machine is always less than 100%.
mechanical advantage – the number of times that a machine increases an output force
actual mechanical advantage – the ratio of the output force to the input force in a machine
ideal mechanical advantage – the mechanical advantage of a machine n the absence of friction
efficiency – the percentage of the work input that becomes work output in a machine
14.4 Simple Machines
14.4.1 Name, describe, and give an example of each of the six types of simple machines.
14.4.2 Describe how to determine the ideal mechanical advantage of each type of simple machine.
14.4.3 Define and identify compound machines.
- The six types of simple machines are the lever, the wheel and axle, the inclined plane, the wedge, the screw, and the pulley
lever – a rigid bar that is free to move around a fixed point
fulcrum – the fixed point around which a lever rotates
input arm – the distance between the fulcrum in a lever and the output force
output arm – the distance between the fulcrum in a lever and the output force
wheel and axle – a simple machine that consists of two rigidly attached disks or cylinders, each one with a different radius
inclined plane – a slanted surface along which a force moves an object to a different elevation
wedge – a V-shaped object whose sides are two inclined planes sloped toward each other
screw – an inclined plane wrapped around a cylinder
pulley – a simple machine that consists of a rope that fits into a groove in a wheel
compound machine – a combination of two or more simple machines that operate together
