Transcript PHY 102 S08
No Lab Activity this week Chapter 3 Mallard HW quiz due by midnight Wednesday 02/20 Lunar eclipse Wednesday 02/20 – total eclipse 9 – 10 pm Chapter 3 quiz in class Thursday 02/21 Tuesday, February 19 Spring 2008 Chapter 3 Energy ...continuation... Great Idea: The many different forms of energy are interchangeable, and the total amount of energy in an isolated system is conserved. Interchangeability of Energy Fundamental property of the universe: every form of energy can be converted to every other form of energy. Interchangeability of Energy Figure 3-4 Energy changes during a bungee jump G = EP K = EK E = elastic energy T = ET thermal energy Energy in the Earth-Sun System 100% Earth’s atmosphere thermal energy reradiated solar energy absorbed incoming solar energy incident solar energy 35% ~10% solar energy reflected solar energy reflected solar energy consumed Energy Consumption on Earth Conversion of solar energy into chemical potential energy through the trophic levels Release of Energy Back to Space Use of stored energy to: generate electricity, obtain fuel, accomplish work Eventually, stored energy is released as heat, or thermal energy, back into space Law of Conservation of Energy First law of thermodynamics While the energy in an isolated system may change from one form to another, the total amount of energy does not change – it is conserved Any change in the total energy of a system arises from transfer of energy between the system and its surroundings. Our Energy Future Current world energy consumption: • ~ 4.7 × 1020 J/yr • largely based on fossil fuels (oil, coal, and gas). Advantages: cheap, high-grade Disadvantages: nonrenewable, human and environmental health impacts http://en.wikipedia.org/wiki/World_energy_resources_and_consumption Alternate Energy Sources Current world energy consumption: • ~ 4.7 × 1020 J/yr Solar energy reaching Earth: Advantages: renewable, (relatively) clean Disadvantages: expensive, lowefficiency, storage, lack of infrastructure • ~ 2 × 1024 J/yr solar panels – Berwyn, IL Ford Lock and Dam, MN Twin Groves Wind Farm, central IL In-class exercises Concept Question 1 Equal forces are used to move blocks A and B across the floor. Block A has twice the mass of block B, but block B moves twice the distance moved by block A. Which block, if either, has the greater amount of work done on it? Concept Question 2 A string is used to pull a wooden block across the floor. The string makes an angle to the horizontal as shown in the diagram. a. Does the force applied via the string do work on the block? b. Is the total force involved in doing work or just a portion of the total force? Concept Question 3 In the situation pictured below, if there is a frictional force opposing the motion of the block, does this frictional force do work on the block? Concept Question 4 If there is just one force acting on an object, does its work necessarily result in an increase in the object’s kinetic energy? Concept Question 5 Two balls of the same mass are accelerated by different net forces such that one ball gains a velocity twice that of the other ball in the process. Is the work done by the net force acting on the faster-moving ball twice that done on the slower-moving ball? Sample Problem 1 A woman does 160 J of work to move a table 4 m across the floor. What is the magnitude of the force that the woman applied to the table if this force is applied in the horizontal direction? Sample Problem 2 A rope applies a horizontal force of 180 N to pull a crate a distance of 2 m across the floor. A frictional force of 120 N opposes the motion. a. What is the work done by the force applied by the rope? b. What is the work done by the frictional force? c. What is the total work done on the crate? Sample Problem 3 A net force of 60 N accelerates a 4-kg mass from rest over a distance of 10 m. a. What is the work done by this net force? b. What is the increase in kinetic energy of the mass? c. What is the final velocity of the mass? Sample Problem 4 At the low point in its swing, a pendulum bob with a mass of 0.2 kg has a velocity of 5 m/s. a. What is its kinetic energy at the low point? b. Ignoring air resistance, how high will the bob swing above the low point before reversing direction?