Chapter 12 Work and Energy

Defn: Work is done when a force is exerted on an object causing it to move a distance in the direction of the applied force.

Example: In physics this man is doing work not because he is at his place of employment but because he applies forces to the boxes causing them to move.

Force in direction of motion... The easiest scenario is when the force is applied in the direction of motion. Then, W = F • d

Work Work Who is doing work? What must you ask to determine if work is being done? Work By carrying the box up the stairs, force and distance is in the same direction

What are the units of WORK? Work is measured in Newton-meters (N•m) or foot-pounds (ft•lb) A Newton-meter is called a “JOULE” (sounds like ‘jewel’) Named after James Prescott Joule (1818-1889) British physicist who developed the first law of thermodynamics.

Ex: How much work is done when a man pushes a car with an 800 N constant force over a distance of 20 m? Work = Force X Distance Work = (800 N X 20 m) = 16,000 Nm or 16,000 J

Example: No work! Although you may be doing a lot of thinking, from a physics point of view you may not be doing any work. Why? Is ‘home work’ really work at all?

Try this example in your notebook A student lifts a 12 N textbook 1.5 m in 1.5s. How much work does the student do on the textbook? Work = Force x Distance W = 12N x 1.5m W = 18 J

Power... power is the rate at which work is done. The faster you do work the more power you need. The SI units of power are the J/s which is called the Watt (W). in the English system the Horse Power (hp) is used. 1hp = 746 W

What is the unit of power? Watt is the unit of power.

Try This Example A student lifts a 12 N textbook 1.5m in 1.5 s. What is the power output of the student? Power = Work/time Power = 18J/1.5s Remember that the Work came from the previous answer you worked out in your notebook! P = 12W

Ex: How much power is expended when you walk up ten stairs in 10 sec? It takes more power to do the SAME work in less time! Ex: How much power is expended when you run up 10 stairs in 1.2 s?

How machines are rated... The mechanical advantage (MA) of a machine is the ratio of the force out (Fout) to force put in (Fin). MA = Fout/Fin Which would be easier, lifting a box straight up onto a truck with your hands or using a ramp to lift the box? Which would require more work?

Actually... The basic principle is: Workin > Workout Because some work (energy), is lost to friction! So, SIMPLE MACHINES allow us to do the same job with less FORCE, but using MORE ENERGY!

Ideal Mechanical Advantage Ideal Mechanical Advantage (IMA) assumes no frictional losses deffort is the distance the person applies upon the machine. dresistance is the distance the machine moves an object. Let’s look at some examples IMA = deffort/dresistance

IMA of a ramp = deffort/dresistance IMA of a lever equals the effort arm divided by the resistance arm Effort Resistance

Ex: What is the MA of a 10 ft ramp used on a moving van if a mover applies a 70 lb force over the length of the ramp to lift a 400 lb refrigerator a height of 1.5 ft?

MA = Fout/Fin Force output is the force applied by the machine (400 lbs). Force input is the force applied by the person (70 lbs). MA = 400 lbs / 70 lbs = 5.7

Steepness of a ramp Is the force different with different angles? Is the distance different with different angles? Is the work different with different angles? Why do we use a incline plane (ramp)?

Pulleys - IMA To find the IMA count the number of supporting ropes

Simple Machines A simple machine is one of the six basic types of machines, which are the basis for all other forms of machines. The most basic machines

There are six types of Simple Machines... levers pulleys wheel and axles screws wedges inclined planes

How Simple Machines Work... The basic principle is: Workin = Workout f • D = F • d person machine a small force applied over a large distance yields a large force over a small distance.

Pulleys A pulley system has two main functions to multiply force to change direction A series of pulleys hooked together is called a block and tackle

First class lever = fulcrum in the middle Third class lever = effort is in the middle. Levers... Second class lever = resistance in the middle.

The Inclined Plane Family Inclined planes multiply and redirect force. An inclined plane turns a small input force into a large output force by spreading the work out over a large distance. A wedge is a modified inclined plane. A screw is an inclined plane wrapped around a cylinder.

Compound Machines A machine made of more than one simple machine is called a compound machine. Examples of compound machines are: scissors, which use two first class levers joined at a common fulcrum a car jack, which uses a lever in combination with a large screw

Energy

Electrical Energy Solar Energy The Nature of Energy What is energy? Energy is the ability to cause a change or the ability to do work What are the different forms of energy? Electrical Energy Mechanical Energy Solar Energy Heat Energy Nuclear Energy

The energy of different objects may fall into two main categories: Kinetic Energy and Potential Energy Kinetic Energy is the energy of a moving object Potential Energy is stored energy. This can be thought of in two ways. Stored energy in our bodies or in a battery is called chemical potential energy. Stored energy due to the position of an object above the surface of the earth is gravitational potential energy.

Calculating Kinetic Energy Kinetic Energy depends upon two factors: the mass of the object and its velocity. Mass is measured in kg and velocity is measured in m/s When the units are multiplied, the result is kg m2/s2 Since a Newton (N) is a kg m/s2, therefore kg m2/s2 is the same as a N-m, which is the same as a joule (J). Hence, the unit of kinetic energy is the joule.

Practice Problems KE = 1.1 X 103 Joules (J) How much energy does a soccer player with a mass of 75 kg and a velocity of 5.5 m/s possess? KE = ½mv2 KE = ½(75 kg)(5.5 m/s)2 KE = 1.1 X 103 kgm2/s2 1 kgm/s2 = 1N KE = 1.1 X 103 Nm 1 Nm = 1 Joule (J) KE = 1.1 X 103 Joules (J)

Solve the following: 2( ) 2( ) What is the velocity of a car with a mass of 675 kg and a kinetic energy of 1690 joules? 2KE = v2 2( ) 2( ) KE = ½mv2 2KE = mv2 m 2KE = v 2(1690J) v = m 675 kg v = 2.24 m/s

Calculating Gravitational Potential Energy Gravitational Potential Energy (GPE) depends upon three factors: the mass of the object, the height of the object from the surface of the earth, and the gravitational acceleration constant (on earth = 9.8 m/s2) Notice how the units multiply : mass (kg) x m/s2 x height (m). Hence the unit is kg m2/s2 which is the joule (J). Let’s do a practice problem…

Potential Energy PE = 1.0 X 105 Joules Calculate the energy possessed by a rock on the side of a cliff. The rock has a mass of 237 kg at an elevation of 45 meters. PE = mgh PE = (237 kg)(9.8 m/s2)(45m) PE = 1.0 X 105 kgm2/s2 PE = 1.0 X 105 Nm PE = 1.0 X 105 Joules

What is the Law of Conservation of Energy? Energy cannot be created nor destroyed….but…. Energy can be transformed from one type to another. In this diagram, electrical energy is converted into thermal (heat) energy and light energy.

There are many energy transformations in an automobile. The explosion of the gasoline involves the release of chemical energy. The chemical energy causes the pistons in the engine to move (mechanical energy) The moving pistons affect many moving parts including a generator which produces electrical energy which keeps the battery charged. The moving pistons and other moving parts also produce heat energy.

Objects can convert potential energy to kinetic energy or kinetic energy to potential energy. The diagram below illustrates this relationship.

In nuclear reactions some mass is converted into energy.

The major source of energy is the SUN. In photosynthesis plants convert sunlight into stored energy in the plants. Animals and humans eat the plants to get energy. The human body “burns” this stored energy in units called Calories. One Calorie is equal to 4.18 J. The human body stores energy in Calories and will utilize “burn” this energy as needed.

What is Thermal Energy?

Particles of matter are in constant motion Particles of matter are in constant motion. This motion relates directly to the state of matter of the object (solids, liquids, or gases). Temperature affects how fast these particles move. The higher the temperature the faster the particles move. Moving particles possess kinetic energy. Temperature is defined at the average kinetic energy of the particles of an object.

Thermal Energy is the sum or total of all of the energy of the particles of an object. Thermal energy and temperature are related though DIFFERENT. Temperature is the average kinetic energy of the particles of an object. Thermal energy is the total amount of energy of the particles of an object. A bathtub full of water at 100oF has more thermal energy than a thimble of water at 100oF. The temperature is the same but the total amount of energy is different. The bathtub has more energy.

What is heat? Heat is thermal energy that flows from something at a higher temperature to something at a lower temperature. What is specific heat? Specific heat is a property of matter which determines how readily a material is to absorb heat and change temperature. Specific Heat is defined as the amount of heat energy needed to raise the temperature of 1 kg of a substance by 1 oC or 1 K.

Did you notice that water has a very high specific heat, whereas iron has a low specific heat? Water requires a lot of heat energy to raise its temperature.

Metals require little energy to raise its temperature. Thermal energy can be calculated using the following formula:

Try this one!! Answer = 376 J/kg K Did you do this problem?

Transferring Thermal Energy How is Thermal Energy Transferred? YOU MUST KNOW THIS!!!!! Conduction – direct contact Convection – through a fluid Radiation – by electromagnetic waves

What is heat transfer by conduction? Heat transfers as particles of an object increase their collisions as heated. These collisions transfer the heat energy through the object by colliding with adjacent particles.

What is heat transfer by convection? Heat is transferred through a substance through currents. This occurs in fluids (liquids AND gases) Convection currents are caused by heating of a liquid or gas, the liquid or gas rises, then cools and falls. This occurs in the mantle of the earth And in the atmosphere. Most of our weather patterns are the result of convection currents in the atmosphere.

What is heat transfer by radiation? Radiation is heat transfer by electromagnetic waves. These wave may pass through all states of matter and also through NO matter – such as the vacuum of space. This energy is often called radiant energy. Radiant energy from the sun travels through the vacuum of until it reaches the earth.

How is heat flow controlled? Insulators – a material which does not allow heat to pass through it easily. Some animals have good insulation to survive severe winters.

Buildings and houses are insulated so that heat does not pass out of (winter time) and into (summer time).

What are some other uses of insulation?

Using heat – How do we use heat in our lives? Forced-Air Systems – a fuel is burned in a furnace and a fan circulates the heat in the house

Radiator Systems – closed metal containers that contain hot water or steam. The thermal heat is transferred to the air and circulated by convection currents. Electric radiators – heat metal coils which transfer the thermal heat to the surrounding air.

Using heat to do work – the internal combustion engine

Automobile engines are usually four-stroke engines since each four-stroke cycle converts chemical/thermal energy to mechanical energy.

Refrigerators, air conditioners, and heat pumps – how do they work? A coolant is circulated through pipes by a compressor. When the pipes get small and narrow the coolant compresses and changes to a liquid giving off heat energy. When the pipes get larger, the coolant changes from a liquid to a gas (evaporates) causing the area to absorb heat and get cooler

The human coolant - In-built cooling system The human body has a cooling system which function automatically – perspiration – sweat. Like a refrigeration system, the evaporation of the sweat from the surface of the body causes energy to be absorbed from the skin, causing the skin to feel cooler. No sweat!!!