Work and Machines

What is Work? Work is force times distance. To be exact, work is force times the distance moved in the direction of the force. The unit of measurement for work is the joule. One joule is equal to one newton of force times one meter of distance.

Work 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 (2) When you carry books while walking, you might think that your arms are doing work. However, in this case, the force exerted by your arms does no work on the books. The force exerted by your arms on the books is upward, but the books are moving horizontally.

Doing Work Energy is always transferred from the object that is doing the work to the object on which the work is done. He is transferring energy from his moving muscles to the box and is increasing its potential energy by increasing its height.

Power The rate at which work is done is called power. The power of a machine is calculated by dividing the amount of work done by the time it takes in seconds.

Power (2) The unit of power is called the watt, named after James Watts, the inventor of the steam engine. One watt is equal to one joule of work done in one second. Another unit commonly used is horsepower. One horsepower is equal to 746 watts.

Calculating Power Suppose you drag a box with a force of 100 newtons for 10 meters, and it takes you 10 seconds. You have done 1,000 joules of work (100 N x 10 m). Suppose your friend drags a similar box but takes 60 seconds. You and your friend did the same amount of work but you used six times more power because you did the work six times faster.

Machines A machine is a device with moving parts that work together to accomplish a task. Machines can be simple. Some, like knives, scissors, and doorknobs, are used everyday to make doing work easier.

Machines and Forces Machines are best understood through the concepts of input and output forces. The input force is the force applied to the machine. The output force is the force the machine applies to accomplish a task. Rule for all machines: You can never get more work out of a machine than you put into it.

Mechanical Advantage Mechanical advantage tells how much a machine multiplies force or increases distance. It is defined as the ratio between the output force and the input force.

Mechanical Advantage (2) A machine with a MA of greater than 1 multiplies the input force. Such a machine can help you move or lift heavy objects. A machine with a MA of less than 1 increases distance and speed.

Efficiency Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine.

Efficiency (2) In an ideal machine there is no friction and the output work equals the input work. So the efficiency of an ideal machine is 100 percent. The efficiency of a real machine is always less than 100 percent (oil or grease can be added to make it more efficient).

Simple Machines A simple machine is 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.

Lever A lever is 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.

Lever (2) First Class Lever (Screwdriver opening a paint can) Second Class Lever (A wheelbarrow where the fulcrum is the wheel) Third Class Lever (a bat where the fulcrum is the batter’s wrist)

MA of a Lever

Pulley A pulley is 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.

Pulley (2) A pulley in which one end of the rope is fixed and the wheel is free to move is called a movable pulley. A system of pulleys consisting of fixed and movable pulleys is called a block and tackle.

Wheel and Axle A wheel and axle is 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.

MA of the Wheel and Axle

Inclined Plane A sloping surface, such as a ramp that reduces the amount of force required to do work, is an inclined plane.

MA of an Inclined Plane

Screw A screw is an inclined plane wrapped in a spiral around a cylindrical post. You apply the input force by turning the screw. The output force is exerted along the threads of the screw.

Wedge A wedge is an inclined plane with one or two sloping sides. It changes the direction of the input force.

Compound Machines Two or more simple machines that operate together form a compound machine.