Class_Notes

Work and Energy

Forces on a body do work IF

• The forces are along the path
• The object moves

Work done by force F defined as

FPath Dx

Units of work = Newton*meter = Joules

Example:

 Let F = 5 N, FPath = 4 N, FPerp = 3 N, N = 7 N, W = 10 N, k = 2 N, Dx = 6 m F Dx = FPath Dx = 24 J N Dx = 0 Dx = 0 J W Dx = 0 Dx = 0 J k Dx = -2 Dx =-12 J Total work = 12 J

But why talk about doing work? What is the result of doing work?

First let's look at the Net work done on a body by the Net force.

From Newton's Law, the net force equals mass times acceleration.

Newton's Law of Non-Inertia

Multiply both sides by Dx to give total work.

To evaluate this we draw a graph of mv vs v (recall v vs t) since then the area under graph will give total work.

Conclusion:

Total Work = Change in Kinetic Energy

Now let's investigate a very important work done by gravity. Let's do that by raising a mass up a height h. We must exert an upward force = mg, and we do not do any work when we move the mass sideways since we won't be exerting any of our force "along the path" so work done by gravity depends only on the vertical distance we move. So work done by gravity equals -mg h.

Wg = - mgh

Thus

W'Total + Wg = DKE where prime means we removed one work term.

W'Total -mgh = DKE

W'Total = DKE + mgh

We then call mgh a change in Potential Energy due to gravity. i.e. we say that an object raised above the earth a distance h has Changed its Potential Energy. DPEg = mgh

Now let's apply our ideas

Interactive Physics

Pendulums

Conservation concepts in energy but doesn't some energy get "lost"?