Chapter 6 Work & Energy

6.1 Work Done By A Constant Force When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. 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.

6.1 Work Done By A Constant Force Examples of WORK: a horse pulling a plow through the field, a person pushing a grocery cart a student lifting a backpack, etc. In each case described here there is a force exerted upon an object to cause that object to be displaced.

6.1 Work Done By A Constant Force Work is not Effort: Work is done upon an object whenever a force acts upon it to cause it to be displaced

6.1 Work Done By A Constant Force Is the following an example of work? A student applies a force to a wall and becomes exhausted. This is not an example of work. The wall is not displaced. A force must cause a displacement in order for work to be done.

6.1 Work Done By A Constant Force Is the following an example of work? A book falls off the table and free falls to the ground. This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i.e., "fall").

6.1 Work Done By A Constant Force Is the following an example of work? A rocket accelerates through space. This is an example of work. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space.

6.1 Work Done By A Constant Force Is the following an example of work? A student carries a heavy pile of books straight across the room at constant speed. This is not an example of work. The student is exerting a normal force on the books, but the normal force is not causing the books to be displaced. More on this in a few minutes…

6.1 Work Done By A Constant Force The mathematics of work W = force x displacement Units of work = Nm 1 Nm = 1 J (joule) Work is a scalar quantity (magnitude only, no direction)

6.1 Work Done By A Constant Force Some facts about work: If the displacement is 0, no work was done Often force and displacement do not point in the same direction Only the component of force that is parallel to the displacement is used in defining work.

6.1 Work Done By A Constant Force Magnitude of the force Magnitude of the displacement Angle between the force and the displacement Positive Work Negative Work NO Work

Example 1 Find the work done by a 45N force in pulling the suitcase at an angle of 50° for a distance of 75m.

Example 2 A weightlifter is bench-pressing a barbell whose weight is 710N. He raises the barbell a distance of 0.65m above his chest, and then lowers it the same distance at a constant velocity. Determine the work done on the barbell by the weightlifter during (a) the lifting phase and (b) the lowering phase.

Example 3 A 120kg crate lies on the flatbed of a truck that is moving with an acceleration of +1.5m/s 2. The crate does not slip as the truck undergoes a displacement of 65m. What is the total work down on the crate by all of the forces acting on it?

Example 4 (#6 pg. 188) A person pulls a toboggan for a distance of 35m along the snow with a rope directed 25° above the snow. Then tension in the rope is 94N. How much work is done on the toboggan by the tension force? How much work is done if the same tension is directed parallel to the snow?

Assignment p. 187 Focus on Concepts #1,2 p. 188 #1-11 odds