Work and Energy Work by a Constant Force Work by a Varying Force Kinetic Energy and Work

Chapter 6 Work and Energy 6.1 Work Done by a Constant Force When something has energy, it has the ability to do work. We start our discussion of energy with the concept of work. I am going to ask for several daring volunteers to do some work for me (hold, push, and lift). While these volunteers are busy working away, here’s a question for the rest of the class: Define “work.” Do your best to come up with a “scientific” definition. Only one sentence, please. No peeking at later slides!

You working volunteers, also think of a definition. I might let you tell it to us after I … finish … with you.

Work is done on an object by a force when the force moves the object through some distance. Suppose I lift a 100 pound chunk of lead, put it on your shoulder, and make you stand there for 10 minutes. Do you do any work in the process? You don’t! Not “physics” work. What a stupid way to define work! No, the physics definition of work is consistent with the known laws of physics.

You'll probably object that your muscles say they did work, and you are correct.muscles In order to maintain a steady position, your muscles must continually contract and expand, and in the process they certainly do work on the tissues of your body. Go to howstuffworks to see this in action.howstuffworks

Strictly speaking, I should say you did no work on the chunk of lead (in this simple example where you did not move the chunk of lead). Did anybody do work in this example? When I lifted the chunk of lead from the floor to your shoulder, I had to exert a force. The force moved the mass of lead some distance; therefore I did work. So I lift a 100 pound chunk of lead, put it on your shoulder, and make you stand there for 10 minutes. I claim you do no work.

If you tire and let the chunk of lead fall, do you do any work? No. As the lead falls, does it do any work? However, the force of gravity causes the lead to fall, so gravity does work on the lead as it falls. No (if you ignore the air it pushes aside, and assuming the earth does not move “up” to meet the lead). Work is done when a force moves a mass through some distance. Work is proportional to FD, where F is the force that moves the mass and D the distance. In the SI system of units that scientists prefer, the units of work are joules, where 1 joule equals 1 newton-meter.

Holding a book: no work done. Lifting a book: work done. Here’s an equation: W F = F  D = FD cos  D  mg  mg  W grav = mg cos  OSE: [W F ] i  f = FD cos . Use the correct angle, not just any old  !

If you promise to read this section, I won’t test you on it. What is kinetic energy? Your text shows that the work to change the speed of a mass m from v 1 to v 2 is W = ½mv 2 2 – ½mv 1 2 so we define kinetic energy as OSE: K = ½mv Work Done by a Varying Force 6.3 Kinetic Energy and the Work-Energy Principle Sorry, have to skip this section.

With the above definition of K, we have OSE: [W net ] i  f =  K. This is often called the work-energy theorem. It is one of the BIG DEALS of physics.* Note that  (stuff) always means (stuff) final - (stuff) initial. *Calling this a BIG DEAL will cause you to want to apply it to every problem in this chapter. There are even BIGGER DEALS later that are usually better starting points for problems. So  K means K final - K initial.

Giancoli does the second part the “physicist way.” Let’s do it the “official way” here. Example 6-4. What is the KE of a baseball of mass m thrown with a velocity of magnitude V? If the ball started from rest, how much energy did it take to make it reach this speed?