Energy

Work An object that has energy has the ability to produce a change Work is how we transfer energy, it is equal to the change in energy In order to do work on an object, you must increase the energy within it Any type of energy can do work

Kinetic Energy Kinetic Energy (KE) is the energy of motion More speed means more KE

Gravitational Potential Energy Potential Energy is Stored Energy

Heat Energy Heat is energy that is created by friction This could be from sliding, rubbing or deformation

Work

Work Examples Work or Not? work not work not work Let’s look at the following scenarios: Work or Not? Lifting a box above your head Holding that box there for 2 hours Sliding a box across a frictionless surface at constant speed work not work not work

Energy Equations: Work Work is the product of the force applied in the direction of motion and the distance it is applied When the force and the movement are parallel, work is simply θ Force (F) F||

Energy Units Notice: from the work formula, energy units are a combo of Force (Newtons) and distance (m) or Newton•meters (N•m) The SI units for energy are Joules (J). So, one Joule is equal to 1 Newton•meter.

Power In physics, power just means the rate of doing work So, faster work means more power The units come out to Joules per second. We call this a Watt (W) for short

Work Example In the 1950s, an experimental train, which had a mass of 2.5 x 104 kg, was forced across a level track by a jet engine that produced a thrust of 5.0 x 105 N for a distance of 509 m. Find the work done on the train. Given: Equation: Unknown:

Work Example In the 1950s, an experimental train, which had a mass of 2.5 x 104 kg, was forced across a level track by a jet engine that produced a thrust of 5.0 x 105 N for a distance of 509 m. Find the work done on the train. Given: Equation: Unknown:

If you finish early, try #2 on pg 408 Time to practice! Turn to pg. 409 Complete #5-11 If you finish early, try #2 on pg 408

Kinetic Energy Energy is the ability to do work Kinetic Energy (KE) is the energy of motion More speed means more KE

Kinetic Energy

Energy Equations: Kinetic E Let’s throw a block Work can transfer energy into the block Work is done while the block is being accelerated by the hand a distance of d

Energy Equations: Kinetic E So, the work done is: This time the force is simply ma Remember that acceleration equation?:

Energy Equations: Kinetic E Let’s substitute: The normal equation assumes starting from rest (vi = 0):

HW Q #1 pg 408 You will need to do some estimating for parts of this problem. I am purposely leaving these a little vague. Specify where you got information that you had to look up or explain how you arrived at estimates for mass and velocity. a. Estimate the Kinetic Energy of a Chihuahua moving as fast as it can.

Potential Energy Potential Energy is stored energy Gravitational Potential Energy (GPE) is when energy is stored in an objects position (height) The higher an object goes, the more GPE (and the faster the speed it will have when it hits the ground)

Elastic Potential Energy (EPE) The other type of Potential Energy we will look at is Elastic Potential Energy (EPE) Instead of height, the energy is stored by stretching an object. More stretching means more EPE ex. rubber band, spring

Potential Energy

Energy Equations: GPE For GPE, we still have force x distance, but this time the force is the objects weight, mg This gives us the equation: We use h instead of d since it will always be height for GPE F=mg m

Energy Equations: Elastic PE EPE is trickier than GPE force changes depending on how much you stretch the object This force depends on both the distance stretched (x) and a spring constant (k) This equation is known as Hooke’s Law

Energy Equations: Elastic PE This k comes from how much force is needed to stretch a spring per a certain distance What is the k for this spring?

Energy Equations: Elastic PE Since the force at the beginning of the stretch is different than the end, we use an average to calculate the EPE: Since we usually start the stretch from rest:

Energy Equations: heat When pushing a block at constant speed across a surface, the friction force is turned into heat Since added force is only working against friction (no a), all of the work done on the block is then turned into heat f

Energy Equations: heat Remember that d is only during the friction

Proportionality Example By what factor does the Kinetic Energy of a car change if the speed doubles? Given: Unknown:

Labette pg 479-481 Everyone should calculate their own personal Power (in other words, everyone should get some exercise) There are 3 stations Free Weights (Biceps) Scales & Push ups (Triceps) Stairs (legs)

Labette pg 479-481 You will need: A group composed of 2-3 people A stopwatch (use a cellphone) A meter stick A pencil & Your lab (duhhh!) When you are done with data collection, start your calculations

Conservation of Energy Energy cannot be created nor destroyed, but only changed from one form to another What does this mean?

Conservation of Energy All of the energy that you start with… you end with! initial energy = final energy Total energy at top equals Total energy at bottom Total energy anywhere

Conservation of Energy All of the energy that you start with… you end with! initial energy = final energy Total energy at top equals Total energy at bottom Total energy anywhere

Conservation of Energy All GPE GPE and KE All KE

Conservation of Energy Problems Identify type of energy at beginning and end Full law in equation form: For most problems, many are zero

Conservation of Energy: Example Rolling down a hill from rest Top (initial): all GPE Bottom (final): all KE Left with: or:

Conservation of Energy: Example A bow is used to shoot a .050 kg arrow into the air. If the average force used to draw the bow is 110 N and the bow is drawn 0.50 m, how fast is the arrow moving when it has risen 35 meters above the bow? (Assume air resistance is negligible) Define: initial : and final : What type of energy is it? when bow is drawn (work) when arrow is at 35 m (KE & GPE)

Conservation of Energy: Example Write out CoE eqn and cross out missing E’s at rest moving start at h = 0 goes higher finding through work (no k) nothing stretched/pressed no air resistance

Conservation of Energy: Example A bow is used to shoot a .050 kg arrow into the air. If the average force used to draw the bow is 110 N and the bow is drawn 0.50 m, how fast is the arrow moving when it has risen 35 meters above the bow? (Assume air resistance is negligible) Givens: 0.050 kg 110N 0.50 m how fast 35 meters above Unknown m = 0.050 kg v = ? (speed in /s) F = 110N d = 0.50 m h = 35 m

Conservation of Energy: Example rewrite and expand solve for v

Conservation of Energy: Example plug and chug

Time to practice in pairs Turn to pg. 495 If you finish early, start pg 496

Hopper Popper Lab Strategy Write down everything you could measure with the resources you haves Write down your unknowns

Brain Break! What’s wrong with Energy in this movie?

Discussion Question A bowling ball attached to a wire is released one inch away from someone’s face. It swings across the room, and then back towards the person. It will… Gain speed on the way back and hit the person in the face. Stop one inch from the person’s face. Lose speed and not make it all the way to the person’s face.