Work, Energy and Power Chapter 5 Section 4

Wagon Example Push a wagon on a sidewalk and it starts to roll down the sidewalk. Push a wagon on a sidewalk and it starts to roll down the sidewalk. The wagon eventually comes to a stop shortly after the push. The wagon eventually comes to a stop shortly after the push. Friction slows the wagon down. Friction slows the wagon down. Mechanical Energy is not conserved in the wagon since there is a change in kinetic energy. Mechanical Energy is not conserved in the wagon since there is a change in kinetic energy.

Work-Kinetic Energy Theorem Work-Kinetic Energy Theorem – The net work done on an object is equal to the change in the kinetic energy of the object. Work-Kinetic Energy Theorem – The net work done on an object is equal to the change in the kinetic energy of the object.

Work-Kinetic Energy Theorem Equation W net = ΔKE W net = Net Work W net = Net Work ΔKE = Change in kinetic Energy ΔKE = Change in kinetic Energy Force is not required and applies to all objects universally. Force is not required and applies to all objects universally.

Friction When dealing with the work done by friction, the Work-Kinetic Energy Theorem can be put into an alternative form. When dealing with the work done by friction, the Work-Kinetic Energy Theorem can be put into an alternative form. W friction = ΔME

Frictionless When a problem deals with frictionless objects or where friction is neglected. When a problem deals with frictionless objects or where friction is neglected. W friction = 0 W friction = 0 ΔME = 0 ΔME = 0 ME i = ME f ME i = ME f This is the Conservation of Mechanical Energy This is the Conservation of Mechanical Energy

Work-Kinetic Energy Theorem & Work It doesn’t matter if friction is present or its frictionless, the Theorem demonstrates that work is a method of transferring energy. It doesn’t matter if friction is present or its frictionless, the Theorem demonstrates that work is a method of transferring energy. Perpendicular forces to the displacement cause no work, cause the energy is not transferred. Perpendicular forces to the displacement cause no work, cause the energy is not transferred.

Distinction Between W and W net Its important to make the distinction between the two expressions: Its important to make the distinction between the two expressions: W = Fd(cosθ) W = Fd(cosθ) This expression applies to the work done on an object due to another object This expression applies to the work done on an object due to another object Definition of work Definition of work W net = ΔKE W net = ΔKE Shows only the NET FORCE on an object Shows only the NET FORCE on an object Relates to the net work done on an object to change the kinetic energy of an object Relates to the net work done on an object to change the kinetic energy of an object

Example Problem A 10.0 kg shopping cart is pushed from rest by a N force against a 50.0 N friction force over a 10.0 meter distance. A 10.0 kg shopping cart is pushed from rest by a N force against a 50.0 N friction force over a 10.0 meter distance. 1. How much work is done by each force on the cart? 2. How much kinetic energy has the cart gained? 3. What is the cart’s final speed?

Example Problem Answers J J m/s

Everyday Power What is power? What is power? A few everyday uses of power. A few everyday uses of power. Electricity Electricity Engines Engines Etc… Etc… Basically any time work is done, power is generated. Basically any time work is done, power is generated.

Power Power – The rate at which energy is transferred. Power – The rate at which energy is transferred. In other words, power is the rate at which energy is transferred. In other words, power is the rate at which energy is transferred.

What is Power? Power is the amount of work done over a certain time interval. Power is the amount of work done over a certain time interval. P = W/t P = Power (watt) W = Work (J) t = Time (s)

Alternative Power Form Power can also be described through forces and the speed of the object. Power can also be described through forces and the speed of the object. P = Fv Power = Force Speed

SI Units of Power The SI units for Power is the “watt” The SI units for Power is the “watt” A watt is equal to one joule or energy per second A watt is equal to one joule or energy per second Horsepower is often used with power when dealing with mechanical devices such as engines. Horsepower is often used with power when dealing with mechanical devices such as engines. 1 horsepower = 746 watts 1 horsepower = 746 watts

Road Design Why are many mountain roads built so that they zigzag up the mountain rather than straight up? Why are many mountain roads built so that they zigzag up the mountain rather than straight up?

The Physics Behind Road Design The same energy is needed to reach the top of the mountain regardless of the path. The same energy is needed to reach the top of the mountain regardless of the path. Therefore the work is the same. Therefore the work is the same. The zigzag path has a longer distance and takes more time to reach the top The zigzag path has a longer distance and takes more time to reach the top Therefore less power is needed on the zigzag path vs. straight up. Therefore less power is needed on the zigzag path vs. straight up.

Machine Power Machines with different power ratings do the same work, but do so over different time intervals. Machines with different power ratings do the same work, but do so over different time intervals. The only main difference between different power motors is that more powerful motors can do the work in a shorter time interval. The only main difference between different power motors is that more powerful motors can do the work in a shorter time interval.

Example Problem Two horses pull a cart. Each horse exerts a N force at a 2 m/s speed for 10.0 minutes. Two horses pull a cart. Each horse exerts a N force at a 2 m/s speed for 10.0 minutes. 1. Calculate the power delivered by the horses. 2. How much work is done by the two horses?

Example Problem Answers W or 1kW ,000 Joules or 0.60MJ

Light Bulbs A common everyday thing that you take for granite is artificial light. A common everyday thing that you take for granite is artificial light. A light bulb usually has marked on it the wattage it uses. A light bulb usually has marked on it the wattage it uses. Example: 60 watt light bulb (most common) Example: 60 watt light bulb (most common) A 60 watt light bulb will use 60 joules of energy over the course of 1 second. A 60 watt light bulb will use 60 joules of energy over the course of 1 second. Where does the energy come from? Where does the energy come from?

From Sunlight to Artificial Light Sunlight  Plants  Fossil Fuel (coal)  Steam  Turbine  Electricity  Light Sunlight  Plants  Fossil Fuel (coal)  Steam  Turbine  Electricity  Light Whenever energy is transferred, heat is produced. Whenever energy is transferred, heat is produced. 2 nd Law of Thermodynamics 2 nd Law of Thermodynamics So it takes light to produce light and its very inefficient. So it takes light to produce light and its very inefficient.