IB Physics 11 Mr. Jean November 3 rd, 2014

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

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. 1)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.

Gravitational Potential:

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/s 2 ) x 1.5m Work = J

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

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

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

Kinetic Energy: E k = 0.5(m)(v 2 ) v 2 = v o 2 + 2ad v 2 = 0 + 2(-9.81m/s 2 )(1.5m) v 2 = m 2 / s 2 E k = 0.5(2.5kg)(29.43 m 2 / s 2 ) E k = J

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

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.

Who has the most E k and by how much more?

Who has the most E p and by how much more?

How much kinetic energy does each racer have?