Kinetic Energy (Monday's Class)

On Monday, we started off reviewing four questions:

(1) Power = (Work)/(Time)

(2)Total Mechanical Energy of an object = KE (Kinetic Energy) + PE (Potential Energy)

(or more simply - mechanical energy is the ability to do work)

(3) The unit for work is the Joule (J).

(4) As a mass increases in velocity, the kinetic energy increases, as well.

THEN we learned about our quiz...YIKES!  It looks like as a whole, the classes combined did not understand the information, so here are some ideas of what went wrong, and how we can fix it for next time!

What went wrong:

• Units were a big problem.  People either forget them, got them wrong, or they were inconsistent throughout the problem.  Just make sure you have the correct units in the final answer.
• Simple 5th grade math errors occurred - check your work!
• Make sure your read the question correctly and underline what it is Mr. Wirth is asking for...and finally...
• Use the correct equations - that's what the reference tables are for!

How we can fix it!:

• Do your homework and ask questions - remember, the midterm is only in 16 days!
• Go in for some extra help with Mr. Wirth
• Or just ask a friend on a topic that you don't understand

24 people getting an F on the test?  Come on, people, we can do better than that!

Onto Kinetic Energy:

Kinetic Energy is associated with MOTION of an object.

An object that has motion (whether horizontal or vertical) has kinetic energy.

The equation: KE = (1/2)mv^2

and...

KE = Kinetic Energy

m = mass of the object in kilograms

v = velocity in meters per second

Remember: Kinetic energy is a scalar quantity, meaning that it does not have a direction.  It is described by magnitude alone.  Like work, the standard unit for kinetic energy is the Joule (J).

Here is an example for an equation on how to solve for kinetic energy:

Determine the kinetic energy of a 625-kg roller coaster that is moving with a speed of 18.3 m/s.

E: KE = (1/2)mv^2

S: KE = (1/2)(625 kg)(18.3m/s)

A: 1.05 x 10^5 Joules

Scribe Post 1/8/07

Potential Energy

Last week we called Potential Energy "stored energy". Today, we defined PE as "energy associated with position". There are several types of PE associated with particular forces as follows:

• Gravitational PE-gravity
• Elastic PE-elastic forces (spring)
• Chemical PE-coloumb forces (electric charges)
• Nuclear PE-strong or weak nuclear foreces

We also said that Potential Energy has the "possibility to do work". However, even if an object has PE, work doesn't need to happen and may never happen. When work is done on an object, the object's Potential Energy changes.

The change in Potential Energy (deltaPE)=the work required to go from an initial state to a final state. This was demonstrated in the staircase climbing lab we did last week (work was done by climbing the stairs, causing the gravitational PE to increase).

When working with PE energy and work, assume no energy is lost due to friction.

Gravitational Potential Energy is energy of height. It increases in response to work being done against a gravitational field (again, like the staircase lab). Some effects of Gravitational PE are seen in roller coasters, sky diving, and hydro-electric power generators.

The unit of PE is the Joule (J)

Key Equation Learned Today

• Change in PE=mg(change in h)

PE: Potential Energy
m: mass
g :gravitational field strength
h: height

work= force times distance