Wednesday, May 7, 2008

Review of energy
1. Kinetic energy - energy of motion = (1/2) * m * v^2
Did examples calculating KE

KE cannot be negative since m and v^2 are always positive.

If you double the mass, the KE doubles. Mass is proportional to KE. For the same velocity, a graph of KE vs m is a straight line through the origin with slope (1/2)v^2.

If you double the velocity, the KE increases by a factor of 4 (=2^2). Because velocity is squared, it has an even greater effect on the KE. If you make the velocity 3 times greater, the KE increases by a factor of 9. Did several examples including giving a KE of 3x10^6 J at 20 mph and finding KE at 40 mph, 120 mph.

A graph of KE vs v gives a parabola. A graph of KE vs v^2 gives a straight line through the origin with a slope of m/2

Slamming on brakes example. Energy balance equation is KE = work done by friction

(1/2) m v^2 = Ff * d
Since the friction force does not depend on the speed, if you double the speed, you increase the KE by a factor of 4 and increase the stopping distance by a factor of 4. Did several examples.

2. Gravitational potential energy = GPE = m*g*h
Did examples calculating GPE.
Graphs of GPE vs m and GPE vs h are straight lines through the origin.

Examples of choosing different locations for h = 0 (and thus GPE = 0).
GPE can be negative depending on the choice of h = 0. What is important is the difference in GPE between locations.

GPE does not depend on the path you took to get there.

3. Energy balance equations (equations for conservation of energy)
Energy you start with + energy you add = energy you end up with + places energy went

Energy balance equations:
Roller coaster lab
Introduced energy balance equation for Looney Tunes Lab
Showed energy balance equation for Looney Tunes Lab half-way down.
Energy balance equation for dropping an object with no air resistance.

Calculated speed of dropped object using energy. If you know the speed you can easily find the time.

Energy balance equation for an object that is thrown down.

4. Machines
Principle of machines is that work in = work out
Machines can change the force at the expense of distance. Machines do not "create" energy.

Showed examples using one and two strings to support and then lift a weight.

Introduced pulley lab ideas.

5. Handed out worksheet on energy problems. Gave students 15-20 minutes to work on it in class.

Roller Coaster Lab due tomorrow.