Bell Work Turn in lab Solve the following: A girl pushes a cart 20 m. The force she applies in the direction of motion is 20N. How much work did she do? Show your work- Given, Formula, Plug it in with units, answer with units

Work, Power and Energy

WORK RECALL: Force = push or pull - In order for WORK to be done, a FORCE must make something MOVE. -So: If you push your hand straight down on the table right now (apply a force) and the table doesn’t move, have you really done any work??

Doing Work Two Things are Required for Work to be done: - The force must make object move - Movement must be in the same direction as the force Force is Up, Movement is Up

Work and Energy Another way to think about energy is: -ENERGY is the ability to do WORK -If something has energy, it can transfer it to another object by doing work on that object. -When you do WORK on something, you INCREASE its ENERGY -Energy is always transferred from the object doing work to the object on which work is done.

CALCULATING WORK Amount of Work done depends on: 1. Amount of Force used 2. Distance over which force was applied (distance the object moves) Work (in joules) = applied force (newtons) X distance (meters) W = Fd

Example Problem You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m, how much work do you do? F= 100 N d = 5 m W = ?? W= Fd W = (100N)(5m) W = 500Nm W = 500 J

= The amount of work done in one second/the RATE at which WORK is done POWER = The amount of work done in one second/the RATE at which WORK is done For example: If you and a friend push two boxes (identical in mass) for the same distance = YOUR WORK DONE IS EQUAL BUT: If your friend pushes the box faster than you, your friend is MORE POWERFUL!

Power (in watts) = Work (in Joules) time (in seconds) P = W t CALCULATING POWER Power (in watts) = Work (in Joules) time (in seconds) P = W t SI Unit for Power = Watt (W) 1 Watt = 1 J of work done in 1 sec (small unit) Use kilowatt (kW) to often express power 1 kW = 1000 W

Example Problem You do 900 J of work in pushing a sofa. If it took 5 sec to move the sofa, what was your power? W = 900 J t = 5 sec P = ??Watt P = W = 900 = 180 J/s = 180 Watts t 5

Power is also the RATE at which ENERGY is TRANSFERRED Power and Energy Power is also the RATE at which ENERGY is TRANSFERRED Power (in Watts) = energy transferred (in joules) time (in seconds) P = E t Ex: A light-bulb transfers electrical energy into light and heat.

ENERGY The ability to cause CHANGE Anything that causes change must have ENERGY!! You use energy to function Different Forms of Energy Electrical Chemical Radiant Thermal These are like different forms of money

KINETIC ENERGY (KE) -The energy of MOTION -All moving objects have Kinetic Energy! -Depends on: MASS and SPEED of the object -Equation: Kinetic Energy (KE) Joules = 1/2mass(kg) X speed2 KE = 1/2mv2 -Joule (J) = SI unit for Energy

Example Problem with Kinetic Energy -A jogger whose mass is 60kg is moving at a speed of 3 m/s. What is the jogger’s Kinetic Energy? Mass = 60kg Velocity = 3 m/s KE = ??J KE = 1/2mv2 KE = 1/2(60kg)(3m/s)2 KE = 270 J

POTENTIAL ENERGY (PE) -The energy of REST -Objects at REST have POTENTIAL ENERGY -Potential Energy is CHANGED into KINETIC ENERGY when MOTION occurs

TYPES OF POTENTIAL ENERGY Elastic Potential Energy = Energy stored by something that can be STRETCHED or COMPRESSED Ex: Rubber band 2. Chemical Potential Energy = Energy stored in CHEMICAL BONDS Ex: Food, Natural gas 3. Electrical Potential Energy = Energy stored due to ELECTRICAL CHARGES 4. Nuclear Potential Energy = Energy stored in the NUCLEI OF ATOMS

TYPES OF POTENTIAL ENERGY 5. Gravitational Potential Energy: Energy stored by objects due to their POSTION ABOVE EARTH -Ex: Anything with the potential to FALL -Depends on: MASS and HEIGHT above ground -Equation: GPE (J) = Mass (kg) X gravity (m/s2) X height (m) GPE = mgh Remember: Gravity on Earth = 9.8 m/s2

GPE Example Problem -What is the GPE of a ceiling fan that has a mass of 7kg and is 4m above the ground? GPE = mgh Gravity = 9.8 m/s2 Mass = 7 kg Height = 4 m GPE = (7kg)(9.8m/s2)(4m) = 274 kgm2/s2 = 274 J

To INCREASE GPE 1: INCREASE object’s HEIGHT 2. INCREASE object’s MASS 2KG 2KG 7KG 2KG

The Change of GPE to KE -As objects fall, GPE is changed into KE -KE is LARGEST right before the object hits the ground, thus GPE is the SMALLEST right before hitting the ground -Objects with MORE GPE move FASTER because they have more KE

Converting between KE and PE -MECHANICAL ENERGY = Total amount of POTENTIAL and KINETIC energy in a system Mechanical energy = PE + KE What happens to the mechanical energy as PE and KE are converted into each other? ME stays the same!! As PE and KE are converted, the FORM of energy changes, but the TOTAL AMOUNT STAYS THE SAME

The Law of Conservation of Energy -States that: ENERGY CAN’T BE CREATED OR DESTROYED!! -So does this mean the total amount of energy in the Universe is the same at all times??? Friction and Air Resistance These forces can cause some mechanical energy to change into THERMAL ENERGY!!