Chapter 14 Work, Power, and Machines

14.1 Work and Power

SPS8: Students will determine relationships among force, mass, and motion.

SPS8.e: Calculate amounts of work and mechanical advantage using simple machines.

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 DistanceW= 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

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.

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

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