Work By A Constant Force


Work is what is done on an object when a force is applied to move an object through a distance. It has nothing to do with the amount of sweat generated by thinking about Physics. In general, the formula for work:

Worknet (W)= Forcenet X distance

or

Wnet= Fnet x d

Work is measured in Joules.
A Joule is also called a Newton-meter (N.m) and is equivalent to a kg.m2/s2. The Worknet is the total work done on (or by) an object, and can be found by finding the work done by each individual force and adding them, or by finding the net force and applying it over a distance. Actually, in the next lessons, we'll develop a slightly different approach to finding the work. One key point to remember is that work is only done on an object when the force is applied in the direction of motion of the object. Consider:

The force required to lift a box is the weight (mg) of the box. The work done on the box lifting it is the weight of the box times the height (distance) the box is lifted, so W = Force x distance, or W =mgh. Once the box is lifted, carrying the box across the room does no additional work on the box, because the direction the force is being
applied on the box to hold it up is perpendicular to the direction of motion of the box.

Example: How much work is required to lift a 5 kg box 2 meters from the floor to a shelf?

From above, W = mgh = (5kg)(9.8 m/s2)(2m) = 98 Joules.

To be completely correct, we examine that vector of force that only applies in the direction of motion that force is being applied. For example, when pulling a wagon, a total force is being applied up the handle. But the component of force that is being applied in the direction of motion is given by Fcosq. Thus,

Wnet = Fcosq .d

Example: How much work is done in pulling a rider on a wagon (as pictured above) at a constant velocity for 50 meters if the pulling force of 200 N is applied at an angle of 40o up the handle?

Wnet = Fcosq .d= (200 N)(cos 40o)(50 meters) = 7700 Joules

Often it is helpful to see what someone else has to say on the topic. Be sure to check out:

http://www.physicsclassroom.com/Class/energy/U5L1a.html

http://www.physicsclassroom.com/Class/energy/U5L1aa.cfm



For Practice Problems, Try:

From the University of Oregon:

Benchpressing

Lifting a pipe

Dragging a suitcase - note: this one will take a little more thought and covers some previous material nicely.

Giancoli Multiple Choice Practice Questions (Select "Practice Problems) and try some. Don't worry - you won't be able to do all of them yet.