0.19 Phy1170: energy -- work  (Page 2/7)

Conditions for constant velocity

We know that the statement means at least the following things:

• At some point in the past, there was an unbalanced force applied to the object to cause it to begin moving to the right. That force is ignored bythe statement of the problem.
• At the beginning of the time frame covered by the problem, the object was in equilibrium, moving at a constant velocity to the right.
• During the time frame of the problem, there must be a 1 newton balancing force being exerted to the left to prevent the 1 newtonforce being applied to the right from causing the object to accelerate towards the right. That force is also ignored by the statement of theproblem.

Use your graph board for sketches

As a sighted person, I often find it useful to make sketches of the physical scenarios for problems of the types that I will discuss in this module.

You may also find it useful to use your graph board to create "sketches" of the physical scenarios in order to better understand the solutions to theproblems.

Work in everyday life

You hear the word work in many situations in everyday life. For example, someone might say "My gasoline bill is high because I have to drive a long wayto work every day."

Someone else might say "She works very hard at being popular."

Or, you might hear someone say "It's going to take a lot of work to lift that heavy sofa into the back of that truck."

The latter expression comes closest to describing what is mean by work in physics.

Work in physics

According to College Physics by Mendenhall et al

If a body on which a force F is acting is displaced by a distance d, then the work done by the force is F*d*cos(theta), where theta is the angle betweenthe path d and the line of action of F.

Thus, the term work has a very precise meaning in physics. In equation form, we can represent work as

W = F * d * cos(theta)

where

• W represents work
• F represents a force that causes the displacement of an object
• d represents the magnitude of the displacement
• theta is the angle between the line of action of the force and the direction of the displacement

Three key ingredients

According to the Physics Classroom (see (External Link) ), there are three key ingredients to work: force , displacement , and cause .

In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.

Commonplace examples of work

Examples of work are commonplace in everyday life:

• A bulldozer pushing dirt. The bulldozer exerts a horizontal force to move the dirt.
• An elevator lifting people from the first to the tenth floor. The cable on the elevator exerts an upward force to move the people upward byapproximately 110 feet.
• You lifting your backpack full of books onto your back. Your muscles exert a force to raise several heavy books, possibly displacing them bothvertically and horizontally at the same time.

However, some things that we might initially think of as work don't really qualify as work according to the physics definition of work.