Chapter 14 Work, Power, and Machines
14.1 Work and Power
A.  What is Work?

• Work is the product of force and distance
• work is done when a force acts on an object in the direction the object moves

1.  Work Requires Motion

• for a force to do work on an object, some of the force must act in the same direction as the object moves
• if there is no movement, no work is done.

2.  Work Depends on Direction

• the amount of work done on an object depends on the direction of force and the direction of movement.
• Any part of a force that does not act in a direction of motion does no work on an object.

B.  Calculation Work

• Work done is calculated by multiplying the constant force acting in the direction of motion by the distance that the object moves.
• Work = Force X Distance              W= F D

1.  Units of Work

• Force is measured in Newton, and distance is measured in meters and the result is work is measured in Newton-meters
• Joule (J) is the SI unit of work.  1J = 1Nm

2.  Using the work formula

• The weight lifter who lifts a 1600 N barbell 2.0 m over his head.
• Work = 1600 N (2.0 m)
• Work = 3200 N∙m = 3200 J

C.  What is Power?

• Power is the rate of doing work
• Doing work at a faster rate requires more power
• To increase power you can increase the amount of work done in a given time
• Or can do a given amount of work in less time to increase power

D.  Calculating Power

• Calculate power by dividing the amount of work done by the time needed to do the work
•               
• Work is measured in joules (J) and time is in seconds (s).
• The SI unit of power is the watt (W), which is equal J/s.

E.  James Watt and Horsepower

• Horsepower (hp) is another common unit of power, 1 hp = 746 watts
• Horsepower was based on the power output of  a very strong horse

Practice Problems

• What is the SI unit for work? For power?
• What conditions must exist in order for a force to do work on an object.
• When does a force do work?
• How are work and power related?
• How do you calculate work?
• How does doing work at a faster rate affect the power required?
• How much work is done when you lift a 950 N barbell up over a distance of 1.75m?
• While lifting a box 1.25m straight up with a force of 225N for 2.5s.  How much power is required to do this?
• How long does it take to do 750 J of work with a 60 watt light bulb?
• If 500w of power are used in 25s how much work is done?

14.2 Work and Machines
A.  Machines Do Work

• Machine is a device that changes a force
• Machines make work easier to do. 
• They change the size of a force needed, the direction of  a force, or the distance over which a force acts.

1.  Increasing Force

• a small force exerted over a large distance becomes a large force exerted over a short distance
• example:  a jack

2.  Increasing Distance

• some machines decrease the applied force, but increase the distance over which the force is exerted
• example oar, increasing the distance the oar travels through the water helps you go fast.

3.  Change Direction

• some machines change the direction of the applied force
• example:  row boat

B.  Work Input and work output

• because of friction, the work done by a machine is always less the the work done on the machine

1.  Work Input to a Machine

• input force is the force you exert on a machine
• input distance is the distance the input force acts through
• work input is the work done by the input force acting through the input distance

2.  Work output of a Machine

• output force is the force that is exerted by a machine
• output distance is the distance of the output force is exerted through
• work output of a machine is the output force multiplied by the output distance.

Practice Problems

• How do machines make work easier?
• How does the work done on a machine compare to the work done by a machine?
• What changes can a machine make to a force?
• Give an example of a machine that changes the direction of an applied force.
• How are work input and work out put related for a machine?

14.3 Mechanical Advantage and Efficiency
A.  Mechanical Advantage

• mechanical advantage of a machine is the number of times that the machine increases an input force.

1.  Actual mechanical Advantage

• mechanical advantage determined by measure in the actual forces acting on a machine is the actual mechanical advantage
• Actual mechanical advantage (AMA) equals the ratio of the output force  to the input force
• 

2.  Ideal mechanical Advantage

• Ideal mechanical advantage(IMA) of a machine is the mechanical advantage in the absence of friction
• Because friction is always present, the actual mechanical advantage of a machine is always less than the ideal mechanical advantage.
• Engineers often design machines that use low-fiction materials and lubricants

B.  Calculating Mechanical Advantage

• Ideal mechanical advantage is easier to measure than actual mechanical advantage
• The effects of friction are neglected when calculating ideal mechanical advantage
• 

C.  Efficiency

• Because some of the work input to a machine is always used to overcome friction the work output of a machine is always less than the work input.
• Efficiency is the percentage of the work input that becomes work output
• Because there is always friction the efficiency of any machine is always less than 100 percent
• 
• Efficiency is usually expressed as a percentage
• Reducing friction increases the efficiency of  a machine

Practice Problems

• How does the actual mechanical advantage of a machine compare to its ideal mechanical advantage?
• How does increased friction affect the actual mechanical advantage of a machine?
• What two quantities must you know in order to calculate ideal mechanical advantage?
• Why is the efficiency of a machine always less than 100%
• You apply a force on a crowbar to open a door.  The effort lengh of the crowbar is 28 cm long and the resistance length is 6 cm.  Find the mechanical advantage of the crowbar.
• If the AMA of a machine is 3.75 and the input force id 7.50N what is the output force?
• What is the efficiency of a machine with a work output of 90J and a work input of 180J?
• What is the efficiency of a lever if you push 75N over 2.5m to move 225N over 1.25m?

14.4 Simple Machines

• The six types of simple machines are the lever, the wheel and axle, the inclined plane, the wedge, the screw, and the pulley.

A.  Levers

• Lever is a rigid bar that is free to move around a fixed point
• Fulcrum is the fixed point that the bar rotates around
• Input arm of a lever is the distance between the input force ant the fulcrum
• Output arm is the distance between the output force and the fulcrum
• To calculate the ideal mechanical advantage of any lever, divide the input arm by the output arm

1.  First-class lever

• Example:  a screwdriver being used as a first-class lever to open a paint can
• Depending on the location of the fulcrum the mechanical advantage of a first-class lever can be greater than 1, equal to 1, or less than 1.
• Seesaw, scissors, and tongs

2.  Second-class lever

• The output force is located between the input force and the fulcrum
• Example: wheelbarrow
• The mechanical advantage of a second-class lever is always great than 1.

3.  Third-class lever

• The input force of a third-class lever is located between the fulcrum and output fore
• The mechanical advantage of a third-class lever is always less than 1
• Examples: baseball bats, golf clubs, and hockey sticks.

B.  Wheel and Axle.

• Wheel and axle is a simple machine that consists of two disks or cylinders, each one with a different radius.
• The wheel and axle rotate together as a unit.
• To calculate the ideal mechanical advantage of the wheel and axle
• Divide the radius (or diameter) where the input fore is exerted by the radius (or diameter) where the output force is exerted
• Can have a mechanical advantage greater than 1 or less than 1

C.  Inclined Planes

• Inclined plane is a slanted surface along which a force moves an object to a different elevation.
• The ideal mechanical advantage of an inclined plane is the distance along the inclined plane divided by its change in height.

D.  Wedges and Screws
1.  Wedges

• Wedges is a v-shaped object whose sides are two inclined planes sloped toward each other
• A thin wedge of a given length has a greater ideal mechanical advantage than a thick wedge of the same length.
• Examples: knife blade, a zipper

2.  Screws

• Screws is an inclined plane wrapped around a cylinder
• Screws with threads that are closer together have a greater ideal mechanical advantage

E.  Pulleys

• Pulley is a simple machine that consists of a rope that fits into a groove in a wheel
• The ideal mechanical advantage of a  pulley or pulley system is equal to the number of rope sections supporting the load being lifted.

1.  Fixed Pulleys

• A fixed pulley is a wheel attached in a fixed location
• They are only able to rotate in place
• The ideal mechanical of a fixed pulley is always 1

2.  Movable Pulley

• A moveable pulley is attached to the object being moved rather than to affixed location
• Movable pulleys are used to reduce the input force needed to lift a heavy object.

3.  Pulley system

• Pulley system is a combination of fixed and movable  pulleys
• Using pulley systems in combination with other simple machines, large cranes are able to lift railroad locomotives.

F.  Compound Machines

• Compound machine is a combination of two or more simple machines that operate together.
• Example:  car, washing machine, clock

Practice Problems

• What are the six typed of simple machines?
• What determines the mechanical advantage of the six types of simple machines?
• Which classes of lever can have a mechanical advantage of less than 1?
• What class or classes of lever always have a mechanical advantage greater than 1
• How is a screw similar to an inclined plane?
• How is it possible to achieve a large mechanical advantage using pulleys?
• What is the ideal mechanical advantage of a ramp with a length of 5.0m and its higher end is 0.75 m above its lower end?
• What is the IMA of a bicycle when the pedals move through a distance of 0.50m and the rear wheel of the bicycle moves 1.75m?