Work Part I: Energy notes

Definitions Everyday/Common Work is done in order to accomplish some task or job To get the work done energy is expended Scientifically, work does not depend on how long it takes to do a task, how tired you get completing the task or with the task itself. Scientific Work involves force and motion Work = force x distance W = f  d – F = m  a W = (m  a)  d

Is work being done?

Is any work being done on the barbell? He is applying a large force to hold the barbell up but b/c the barbell is motionless, no work is being done on the barbell

Work Work is equal to the change in energy Work is the transfer of energy through motion – In order for work to take place, a force must be exerted through a distance – The object must move – The motion must be in the direction of the force You do work when you lift a weight against Earth’s gravity. The heavier the weight or the higher you lift it, the more work you do

Units for Work Work = Force  Distance Force is measured in Newtons. Distance is measured in meters. Work is measured in Newton-meters (Nm) or Joules (J).

Requirements for Work to be done….  A force must be applied  The object must move in the direction of the applied force. When you lift a book up onto a shelf, you are doing work. You apply an upward force to the book so that it moves upward.

Is work being done now? To lift the barbell work is done – It moves by the force applied to it – A force is applied to it

Is work being done…. When you do a pushup what are you working against?

When you stand, holding the book, you are NOT doing any work. The force you apply is used to overcome gravity but the book is not moving.

If you carry the book across the room, you apply an upward force to overcome gravity but the direction of motion is not in that direction! Therefore, you do no work to the book.

Example How much work is needed to lift a 5 kg backpack to a shelf 1.5 meters above the floor? Work =? force = m  g = 5 kg x 9.8 m/s 2 = 49 N distance = 1.5 m Work = 49 N x 1.5 m = 73.5 J When work is done there is a change in energy. The amount of work done is equal to the change in energy.

Is work being done here? If they move everything in 3 hours then they will not have worked as rapidly as if they had done it in 2 hours. The rate at which the work was done will be different.

Power ✿ How quickly you do this work is power. ✿ Power is the rate at which work is done. ✿ Power = work/time ✿ P = w t ✿ P = (F  d) t

Units for Power ✿ Work is measured in Joules. ✿ Time is measured in seconds. ✿ Power is measured in watts.

Horsepower 1 horsepower = 746 watts

Power Example #1 A constant force of 150 N is used to push Brian’s stalled motorcycle 10 m, along the flat school driveway, in 20 seconds. How much power did Brian have to supply to move his bike? Force = 150 N Distance = 10 m P = W/t = F  d / t= (150 N  10 m) / 20 s P = 75 watts

Power Example #2 A crane can lift a 500 kg mass a distance of 10 m in 5 seconds. What is the power of the crane? Power = work/time Work = force  distance Force (weight)= mass  gravity M= 500 kgG= 9.8 m/s 2 Distance = 10 mTime = 5 s Power = m  g  d / time P=(500 kg  9.8 m/s 2  10 m) / 5 s P= 49,000 / 5 P= 9,800 watts 9,800 watts = 9.8 kw

What is energy? Energy is defined as the ability of an object to cause a change in its environment. Energy is also the ability to do work. So an object that possesses energy has the ability or capability to do work. A moving baseball has energy because you can see and feel the change in motion of your hand as you catch it.

Energy and Work Energy is sometimes called stored work. When work is done by a system then the amount of energy of the system decreases. When work is done on a system then the amount of energy of the system increases. If something has energy, it can do work. Therefore, work is really a form of energy.

Is work being done on the barbell? Does this increase or decrease the amount of energy of the barbell?

Is it Energy? Is sunlight considered energy? Does it have the ability to cause a change in its environment?

Types of Energy There are several kinds of energy, but all can be grouped into two large categories: Potential and Kinetic Energy

Gravitational Potential Energy Potential energy is energy due to position or location. Its potential actually comes from gravity! An apple at the top of a tree has more gPE with respect to the Earth than a similar apple on a lower branch

gPE Equation Potential energy = mass  gravity  height PE = mgh But, mg = weight. So, PE = weight  height PE is measured in Joules!

PE Example #1… A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff? PE = m  g  h m = 65 kg G = 9.8 m/s 2 H = 35 m gPE = 65 kg  9.8 m/s 2  35 m gPE= 22,295 J

PE Example #2 What is the PE of a 25 N book that is 4 m above the floor? PE= m  g  h PE = w  h W = 25 N h = 4 m PE = 25 N x 4 m= 100 J

Elastic Potential Energy Energy is stored while the elastic is stretched. Energy is stored when the spring is compressed.

Chemical Potential Energy Whenever we eat food, we are supplying our bodies with energy. The body uses that energy to pump blood, and breath, among many other things. Form of energy involved in chemical reactions.

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Kinetic Energy Moving objects have the ability to do work to other objects as well. The energy of motion is called kinetic energy. A baseball moving at 90 mph can do more work than a baseball moving at 10 mph. Which do you think can do more work, a baseball or a soccer ball moving at the same speed?

KE equation KE depends on mass, as well as velocity. KE= ½ mass  velocity 2 KE = ½ m  v 2 Make sure that you square ONLY the velocity!

Factors for KE Kinetic Energy depends on speed more than mass. If you double the mass what happens to the KE? If you double the velocity what happens to the KE? This is why car crashes at high speeds are much more dangerous.

KE Example A 7 kg bowling ball is moving in a bowling lane with a velocity of 5 m/s. What is the KE of the ball? KE = ?m = 7 kgv = 5 m/s KE = ½ 7 kg  (5 m/s) 2 = ½  7 kg  25 m/s 2 = ½  175 J = 87.5 J

Wait til you see this one! The Story of Kinetic Energy and Potential Energy The Story of Kinetic Energy and Potential Energy kinetic-and-potential-energy kinetic-and-potential-energy

Other Forms of Energy Mechanical Energy – The sum of the KE and PE in a system

Other Forms of Energy Radiant Energy – Travels through space – Electromagnetic Spectrum – Ex. Visible light, sunlight, Infrared Electrical energy – Energy from moving particles

Other Forms of Energy Nuclear Energy – Energy stored in the nucleus of an atom – Nuclear Fission When the atoms are split, this energy is released Produces energy for nuclear power and nuclear weapons (Atomic Bomb) – Nuclear Fusion The joining of nuclei Happens in the core of stars The hydrogen bomb

Conservation of Energy In the course of a roller coaster ride, energy changes form many times. A conveyor belt pulls you up the first hill. At the top you are not moving very fast then…

All of the energy required for the entire ride must come from the work done by that conveyor belt to get to the top of the first hill. The energy is then stored as gPE. This is why the first hills of roller coasters are always the tallest. From this point energy can change forms but it will not have more energy than it does at the top of the hill. Some energy is transferred as heat to the wheels due to friction, vibrations, and sound.

Energy Transformation Almost all PE PE decreasing, KE increasing Almost all KE

Law of Conservation of Energy Energy cannot be created or destroyed, it is transferred from one object to another. Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source.

Law of Conservation of Energy Energy does not disappear, – It can change form…heat, sound, vibrations – It can be transferred from PE to KE and vise versa. – These changes can be difficult to measure, but energy does NOT disappear.