Chapter 5: Work and Machines 

Section 1: Work

• What is work?
  • In order for work to be done, a force must make something move.
  • Work: the transfer of energy that occurs when a force makes an object move.
  • There are two conditions that have to be satisfied for work to be done on an object.
  • One is that the applied force must make the object move, and the other is that the movement must be in the same direction as the applied force.
  • When you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books.
• Work and Energy
  • When work is done, a transfer of energy always occurs.
  • If something has energy, it can transfer energy to another object by doing work on that object.
  • When you do work on an object, you increase its energy.
  • By carrying a box up the stairs, you are doing work. You transfer energy to the box.
  • The amount of work done depends on the amount of force exerted and the distance over which the force is applied.
  • The distance you use to calculate the work you did is how far the object moves while the force is being applied
• Power: the amount of work done in one second.
  • Power is the rate at which work is done.
  • The SI unit for power is the watt (W).
  • Doing work is a way of transferring energy from object to another.
  • Just as power is the rate at which work is done, power is also the rate at which energy is transferred.
  • When energy is transferred, the power involved can be calculated by dividing the energy transferred by the time needed for the transfer to occur.

Section 2: Using Machines

• What is a machine?
  • Machine: a device that makes doing work easier.
  • Machines can be simple.
• Making Work Easier
  • Machines can make work easier by increasing the force that can be applied to an object.
  • A second way that machines can make work easier is by increasing the distance over which a force can be applied.
  • Machines also can make work easier by changing the direction of an applied force.
  • A car jack is an example of a machine that increases an applied force.
  • The work done in lifting an object depends on the change in height of the object.
  • Some machines change the direction of the force you apply.
  • An ax blade changes the direction of the force from vertical to horizontal.
• The Work Done by Machines
  • A crowbar increases the force you apply and changes its direction.
  • When you use a machine such as a crowbar, you are trying to move something that resists being moved.
  • Two forces are involved when a machine is used to do work.
  • Input Force: The force that is applied
  • Output Force: The force applied by the machine
  • Two kinds of work need to be considered when you use a machine—the work done by you on the machine and the work done by the machine.
  • When the machine does work on an object, energy is transferred from the machine to the object.
  • Because energy cannot be created or destroyed, the amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine.
  • The input work is the product of the input force and the distance over which the input force is exerted.
  • The output work is the product of the output force and the distance over which that force is exerted.
• Mechanical Advantage: The ratio of the output force to the input force
  • Machines like the car jack, the ramp, the crow bar, and the claw hammer make work easier by making the output force greater than the input force.
  • The mechanical advantage of an a machine without friction is called the ideal mechanical advantage, or IMA.
  • The IMA can be calculated by dividing the input distance by the output distance.
• Efficiency: a measure of how much of the work put into a machine is changed into useful output work by the machine.
  • For real machines, some of the energy put into a machine is always converted into heat by frictional forces.
  • A machine with high efficiency produces less heat from friction so more of the input work is changed to useful output work.
  • To calculate the efficiency of a machine, the output work is divided by the input work.
  • In an ideal machine there is no friction and the output work equals the input work.
  • In a real machine, friction causes the output work to always be less than the input work.
  • Machines can be made more efficient by reducing friction.
  • Oil reduces the friction between two surfaces. Oil fills the space between the surfaces so high spots don’t rub against each other.

Section 3: Simple Machines

• Types of Simple Machines

  • Simple Machine: a machine that does work with only one movement of the machine.
  • There are six types of simple machines
  • Lever, pulley, wheel and axle, inclined plane, screw, and wedge.
  • The pulley and the wheel and axle are modified levers, and the screw and the wedge are modified inclined planes.

• Lever: a bar that is free to pivot or turn around a fixed point.

  • The fixed point the lever pivots on is called the fulcrum.
  • The input arm of the lever is the distance from the fulcrum to the point where the input force is applied.
  • The output arm is the distance from the fulcrum to the point where the output force is exerted by the lever.
  • The output force produced by a lever depends on the lengths of the input arm and the output arm.
  • There are three classes of levers.
  • The differences among the three classes of levers depend on the locations of the fulcrum, the input force, and the output force.
  • For a first-class lever, the fulcrum is located between the input and output forces. The output force is always in the opposite direction to the input force in a first-class lever.
  • For a second-class lever, the output force is located between the input force and the fulcrum.
  • The output force is exerted between the input force and fulcrum. For a second-class lever, the output force is always greater than the input force.
  • For a third-class lever, the input force is applied between the output force and the fulcrum.
  • The output force is always less than the input force in a third-class lever. Instead, the distance over which the output force is applied is increased.
  • For a lever, the input distance is the length of the input arm and the output distance is the length of the output arm.

• Pulley: a grooved wheel with a rope, chain, or cable running along the groove.

  • A fixed pulley is a modified first-class lever.
  • The axle of the pulley acts as the fulcrum.
  • The two sides of the pulley are the input arm and out- put arm.
  • A pulley can change the direction of the input force or increase input force, depending on whether the pulley is fixed or movable.
  • A system of pulleys can change the direction of the input force and make it larger.
  • A fixed pulley changes only the direction of your force. You need to apply an input force of 4 N to lift the 4-N weight.
  • A pulley in which one end of the rope is fixed and the wheel is free to move is called a movable pulley.
  • For a fixed pulley, the distance you pull the rope downward equals the distance the weight moves upward.
  • For a movable pulley, the distance you pull the rope upward is twice the distance the weight moves upward.
  • A system of pulleys consisting of fixed and movable pulleys is called a block and tackle.
  • The IMA of a pulley system is equal to the number of rope segments that support the weight.
  • The IMA of a block and tackle can be increased by increasing the number of pulleys in the pulley system.

• Wheel and Axle: a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate together.

  • Usually the input force is applied to the wheel, and the output force is exerted by the axle.
  • A wheel and axle is another modified lever.
  • The center of the axle is the fulcrum.
  • The input force is applied at the rim of the wheel.
  • The output force is exerted at the rim of the axle.
  • The ideal mechanical advantage of a lever is the length of the input arm divided by the length of the output arm.
  • A gear is a wheel and axle with the wheel having teeth around its rim.
  • When the teeth of two gears interlock, turning one gear causes the other gear to turn.
  • When two gears of different sizes are interlocked, they rotate at different rates.
  • If the input force is applied to the larger gear, the output force exerted by smaller gear is less than the input force.
  • If an input force is applied to the larger gear, and it rotates clockwise, the smaller gear rotates counterclockwise. The output force exerted by the smaller gear is less than the input force applied to the larger gear.

• Inclined Plane: A sloping surface, such as a ramp that reduces the amount of force required to do work,

  • The IMA of an inclined plane for a given height is increased by making the plane longer.

• The Screw: an inclined plane wrapped in a spiral around a cylindrical post.

  • A screw has an inclined plane that wraps around the post of the screw.
  • You apply the input force by turning the screw.
  • The output force is exerted along the threads of the screw.
  • The IMA of a screw is related to the spacing of the threads

• The Wedge: an inclined plane with one or two sloping sides

  • Like the screw, the wedge is also a simple machine where the inclined plane moves through an object or material.

• Compound Machine: Two or more simple machines that operate together

  • Some of the machines you use every day are made up of several simple machines.

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