Physics 11 Mr. Jean April 19th, 2012

The plan: Video clips of the day Work Potential energy kinetic energy Gravitational potential kinetic energy

Physics wins! http://latimesblogs.latimes.com/lanow/2012/04/ucsd-scientist-evades-400-traffic-ticket-with-research-paper-.html

Quick Field Trip: To the LAB! ….. Add in physics theme song here….

Work: In physics work is the product of the force and the total change of distance. W = F∆d W = Energy required to move an object in joules (J) F = Newton’s of force required to move an object (N) d = Total distance moved (m)

Example: A force of 800N is required to push an object. The object is pushed at a constant rate of speed for 3 meters. How much work was needed to move this object? 800N x 3m = 2400J 2400J of energy were needed.

Kinetic Energy:

Units:

Kinetic Energy Example: A 6kg curling stone is at rest. It is accelerated to 20m/s. Calculate the kinetic energy at rest. 2) Calculate the kinetic energy in motion.

Frame the problem: Key points: The stone started out at rest A moving object has kinetic energy. An object at rest has no kinetic energy. The amount of kinetic energy is directly related to the stones mass and velocity.

Potential Energy: This is the energy which is stored in an object. Example: A rock sitting on a desk has potential energy (The rock could fall). The height difference from the table to the ground creates a potential energy.

Potential Energy Units:

Example: What is the potential energy stored if a 3kg rock is 0.68 meters above the ground?

ENERGY = JOULES What I would like to accomplish with this example is to show you (and prove to you) that the unit joules are the same for Work, Potential Energy, and Kinetic Energy

Situation: A 2.5 kg brick is pick up off the ground. It is raised 1.5 meter, it stays there for 5 seconds, then dropped back to the ground.

Work: Calculate the work involved with lifting the brick. Work = Force x Distance

Work: Calculate the work involved with lifting the brick. Work = Force x Distance Work = (2.5kg)(-9.81m/s2) x 1.5m Work = 36.79 J

Potential Energy: Calculate the potential Energy of a brick which is resting 1.5 meters in the air. Eg= mgh

Potential Energy: Calculate the potential Energy of a brick which is resting 1.5 meters in the air. Eg = mgh Eg = (2.5kg) (-9.81)(1.5m) Eg = 36.79 N m

Kinetic Energy: After the brick is released calculate the kinetic energy as the brick hits the ground. Ek = 0.5(m)(v2) v 2 = vo2 + 2ad

Kinetic Energy: Ek = 0.5(m)(v2) v 2 = vo2 + 2ad v2 = 0 + 2(-9.81m/s2)(1.5m) v2 = 29.43 m2 / s2 Ek = 0.5(2.5kg)(29.43 m2 / s2) Ek = 36.79 J

Summary: Work = 36.79 J Potential Energy = 36.79 J Kinetic Energy = 36.79 J The joules are the same when looking at these three formulas.

Try This: Prove that work, Potential Energy, and Kinetic energy are the same for the following: A block sitting at 0meters is slide up a frictionless inclined plane to a height of 80 meters. The block is then held in place for 5 minutes. The block is then released.