Chapter 6 Work and Energy Essential Questions: What is work? What is energy?

Where must we start? For something to happen (pendulum to swing, coaster to roll, etc.), we must start with….

Work is…

Work “When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. There are three key ingredients to work - force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.” PCR http://www.physicsclassroom.com/Class/energy/u5l1a.cfm

Lab: How do you compute Work? The amount of work can be determined by what the worker (you in this case) gets done, or by mathematically combining force and displacement. You will “get done” the same thing in each trial. However, you will do it with varying forces each trial, over varying displacements. Your goal is to determine how to mathematically combine force and displacement to compute work. Also determine possible units for work.

Which is more work?

Inclined Plane Activity Tilt the ramp against a support. Determine the displacement needed (length of ramp) to raise the lab cart exactly 1.00m vertically. Measure the force needed to drag the cart up the incline at a constant speed. Be sure to pull scale parallel to the track! Repeat at three other angles for the track. Try to use as large a range of angles as possible. Perform one more “control” trial. Compute the product of the force and displacement for each of the trials. Record results neatly in a table on WB.

Work W = (F)(Dr) cos q Units: joule (J) one J = 1.0N x 1.0m 3.6 million joules equals 1 kWhr 4.19 J = 1 calorie Work is a scalar quantity, but can be + or - Work is defined as: The process where a force acts through a displacement to transfer energy from one object (or one form) to another. The “use” of energy to accomplish something.

Examples PCR W&E Lesson 1b (Calculating the amount of work done) CYU #1, 2, 5

Start QUEST Complete to #14 on the Work, Energy, Power Quest by tomorrow!

Work Review: What is the SI unit for work? What formula is used to compute work? A parent pushes horizontally with a force of 30.0N onto a 5.00kg stroller carrying a 10.0kg baby, keeping the stroller moving 20.0m up a 5.00° incline at a constant speed of 1.5m/s. How much work does the parent do?

What is needed to do work?

What is needed to do work? ENERGY

Energy is … The ability to do WORK

Energy comes in many forms

Mechanical Energy is either: Potential (gravitational or elastic) or Kinetic

On what does (gravitational) Potential Energy depend?

James Prescott Joule’s Experiment

Gravitational Potential Energy (PEg) PEg = mgh OR mgy any height may be set as the vertical origin, but this should not change throughout a problem. If the origin is moved, the problem is re-started Units: joule (J) one J = 1.0N x 1.0m PEg is a scalar quantity, but can be + or - Gravitational Potential Energy is defined as: The ability an object has to do work due to its weight and vertical position. The gravitational potential energy an object has at one height (relative to another height) is equal to the work done to move the object to that height.

Elastic Potential Energy PEs = ½ k(Dl)2 (we are NOT doing this)

Example Pat (60kg) must walk from the 1st floor of South to the 3rd floor. Each floor is separated by 4.73m. How much more gravitational potential energy does Pat have: on the second floor than on the first floor? on the third floor than on the second floor? on the third floor than on the first floor?

On what does kinetic energy depend?

Net Work = DKE What is the mathematical relationship between the final speed (m/s) of a system and the net work (J) done on it (if it started from rest)?

Kinetic Energy KE = ½ mv2 Units: joule (J) one J = 1.0N x 1.0m KE is a scalar quantity and can only ever be positive Kinetic Energy is defined as: The energy a mass has due to its motion. The kinetic energy an object has is equal to the NET work done to put it in motion, from a state of rest.

Kinetic Energy Example A 1200kg car accelerates from rest to 10.0m/s in 2.0 seconds. How much kinetic energy does it gain? If the same car now accelerates from 10.0m/s to 20.0m/s in 6.0 seconds, how much kinetic energy does it gain?

Continue QUEST assignment … up to #31

Power is… …the rate at which…

Power is… …the rate at which work is done or the rate at which energy is transferred P= W/Dt or P = DE/Dt

Power is… But, Since: P=W/Dt P= (F . Dx)/Dt Then P= F . vavg

Assignment: CYU Lesson 1 d & f