Work December 2, 2014

Many of the terms encountered in physics have meanings similar to their meanings in everyday life. In its everyday sense, the term work means to do something that takes physical or mental effort. But in physics, work has a distinctly different meaning. Consider the following situations: ▫A student holds a heavy chair at arm’s length for several minutes. ▫A student carries a bucket of water along a horizontal path while walking at a constant velocity. It may surprise you to know that as the term work is used in physics, there is no work done on the chair or the bucket, even though effort is required in both cases (more to be explained later).

Quiz

Definition of Work Work is done on an object when a force causes a displacement of the object. Work is done only when components of a force are parallel to a displacement. ▫A student holds a heavy chair at arm’s length for several minutes. ▫A student carries a bucket of water along a horizontal path while walking at a constant velocity.

In physics, work is the magnitude of the force (F) times the magnitude of the displacement (d) in the same direction as the force. What are the SI units for work? ▫Force units (N)  distance units (m) ▫Nm are also called joules (J).

How much work is 1 joule? ▫Lift an apple weighing about 1 N from the floor to the desk, a distance of about 1 m.

Work Pushing this car is work because F and d are in the same direction. Why aren’t the following tasks considered work? ▫A student holds a heavy chair at arm’s length for several minutes. ▫A student carries a bucket of water along a horizontal path while walking at a constant velocity.

Practice A A 20.0 kg suitcase is raised 3.0 m above a platform. How much work is done on the suitcase?

Practice B If 2.0 J of work is done in raising a.18 kg apple, how far is it lifted?

An ant sits on the back of a mouse. The mouse carries the ant across the floor for a distance of 10 meters. How much work was done by the mouse? Practice C

Practice D You lift a 45-newton bag of mulch 1.2 meters and carry it a distance of 10 meters to the garden. How much work was done?

Which person did the most work? A)John walks 1,000 meters to the store. He buys newtons of candy and then carries it to his friend’s house which is 500 meters away. B)Sally lifts her 22-newton cat a distance of 0.5 meter. C)Henry carries groceries from a car to his house. Each bag of groceries weighs 40 newtons. He has ten bags. He lifts each bag up one meter to carry it and then walks 10 meters from his car to his house.

A people-moving conveyor-belt moves a 600 N person a distance of 100 meters through the airport. A)How much work was done? B)The same 600 N person lifts his 100 N carry-on bag upward a distance of 1 meter. They travel another 10 meters by riding the “people mover.” How much work was done in this situation?