What is Work?
In classical physics terms, you do work on an object when you exert a force on the object causing it to move some distance. The amount of work you actually do may have little relationship to the amount of effort you apply.
For example, if you push on a car stuck in a snow drift, you may exert a lot of force (and effort) but if the car does not budge, you have not done any work! In order for work to be done on an object, the object must move some distance as a result of the force you apply. There are also constraints on the force you apply. Only force exerted in the same direction as the movement of the object result in work. You may think that you do a lot of work if you carry an arm full of books from home to school. In reality you do no work at all! In carrying the stack of books, you exert an upward force to hold the books so they don”t fall to the ground. There is no movement associated with this force. As you walk, the motion of the books is horizontal not vertical. Since the force applied to the books is vertical, and the motion is horizontal, you do not do any work on the books.
Work is a transfer of energy so work is done on an object when you transfer energy to that object. The amount of work done on an object depends on the amount of force exerted on the object and the amount of distance the object moves.
You are watching: A person does 100 joules of work
Work = Force x Distance
According to Newton”s Second Law of Motion, the net force on an object is dependent on the mass of the object, and its acceleration during the movement.
Force = Mass x Acceleration
The common unit of force is the Newton (N). One Newton is the force required to accelerate one kilogram of mass at 1 meter per second per second.
1 N = 1kg m/s2
The amount of work done to push a 10,000 N car a distance of 10 meters positiveeast.orgld be
10,000 N x 10 m = 100,000 N m or 100,000 J
The Newton-meters are termed joules (J). The joule is named after James Prescott Joule (1818-1889) who first calculated the amount of electrical work needed to produce a unit of heat. In his experiments, Joule discovered that the same amount of heat was produced by the same amount of either electrical or mechanical work (“the mechanical equivalent of heat”).
Learn more about work here.
What is Energy?
On March 10, 2005 sounding like an approaching locomotive, a tornado dropped out of billowing black clouds on the New Zealand town of Greymouth. The winds hurled a truck into a lagoon, snapped power poles in half, roofs sailed through the air and buildings were destroyed (go here to see a video of this disaster).
Although wind is just moving air, it possesses energy. When the wind moves the leaf on a tree or picks up and hurls a truck, it has caused a change in the position of the object. Therefore it has done work. A measure of the ability to do work or cause change is called energy. Any time an object does work on another object, some of the energy of the working object is transferred to that object raising its energy state. Like work, the units of energy are joules.
Energy is the amount of work a physical system is capable of performing so energy can be defined as that which changes the position, physical composition or temperature of an object. There are two categories of energy, kinetic energy and potential energy. The difference between them is whether the energy is being transferred (kinetic) or stored (potential). They are interconvertible.
Kinetic energy is the energy of motion (the motion of waves, electrons, atoms, molecules) while potential energy is stored energy or energy of position that has the potential to do work (follow the kinetic and potential energy links for a more in depth discussion). The kinetic energy of an object is dependent both the mass of the object and its velocity.
K.E. = 1/2 m v2
Thus a 3,000 lb car moving at 50 mph will transfer more kinetic energy than a 2,000 lb car moving at the same rate. Energy can be stored in an object by lifting it up. The amount of potential energy positiveeast.orgld be
P.E. = mgh
where m is the mass of the object; h is the height of the object, and g is the force of gravity acting on the object.
This animation shows how energy converts between kinetic and potential energy.
Animation courtesy of: http://www.bsharp.org/physics/stuff/swings.html
View how kinetic and potential energy are interrelated using a roller coaster demonstration.
A waterfall possesses both potential and kinetic energy. The water at the top of the falls possesses potential energy. As the water flows over the edge of the falls, its energy is changed into kinetic energy.