Energy Something that enables an object to work is called energy. What are some different forms of energy? –Potential –Electrical –Mechanical –Kinetic
Potential Energy Potential Energy: energy that is stored and held in readiness to do work. Any substance that can do work has potential energy. –Fossil Fuels –Electric Batteries –Food
Potential Energy Work is required to elevate objects This potential energy is called –Gravitational Potential Energy (GPE) The amount of gravitational potential energy is equal to the work done against gravity by lifting it.
Energy is the physical agent that allows work to be done. Energy has many forms: –In motion (KE) –In position (GPE) –In the physical/chemical properties of materials (EPE) Energy, like work, is measured in Joules (J). AB Energy
Which path will take the most work to get the ball to the top of the tower?
Gravitational Potential Energy Work = Force x Distance Upward Force = weight –Work = Weight x Distance –Work = mg x Distance –GPE = mg x Distance (height) GPE = mgh
Which path will take the most work to get the ball to the top of the tower?
GPE Example A cannon fires a 10kg cannon ball 150m into the air. What is the GPE at its highest point? h=150m
GPE Example A cannon fires a 10kg cannon ball 150m into the air. What is the GPE at when the cannonball lands back on the ground?
Kinetic Energy Kinetic Energy: is the energy of motion Note that KE quadruples when the velocity doubles
30km/hr 60km/hr 120km/hr Skid 10m Skid 40m Skid 160m How far would a car skid?
Sample Problem #1 (KE) A train (m = kg) travels along a stretch of track with a velocity of 16m/s. Tunnel What is the KE of the train?
A train (m = kg) travels along a stretch of track with a velocity of 16m/s. How much work is required to stop the train in 84.6s?
A train (m = kg) travels along a stretch of track with a velocity of 16m/s. How much work is required to stop the train in 84.6s? Work-Energy Theorem
Elastic Potential Energy EPE: is the energy stored in a spring or flexible object k is the spring constant (Units N/m) (material dependent) –Which spring would be harder to compress? x is the distance compressed or stretched (Units m)
The Law of Conservation of Energy The Law of Conservation of Energy: in a closed and isolated system, the total energy remains constant. –Energy can not be destroyed. –Energy transforms, but the total amount never changes.
What energy transformation take place in this example? Conservation of Energy
What energy transformation take place in this example? Conservation of Energy
What energy transformation take place in this example?
Conservation of Energy PE PE + KE KE PE + KE PE
Simple Harmonic Motion X1 X2 mgh 1 mgh 0 V=0
Hooke’s Law
Understanding the MVE – Hoops, Anyone?
Compressing
The MVE A statement of the conservation of energy that includes most of the forms of mechanical energy. KE GPE EPE RKE Energy BeforeEnergy After
External Work ( W o ) W o is any work done by the system or on the system during an energy transition. In cases where energy is added to the system, W o is positive(+). Examples include motors and muscles. In cases where energy is lost by the system, W o is negative(-). Examples include friction and air resistance.
How far does it fall? If the block slides a distance d down the plane, then how far does it fall at the same time?
Conservation of Energy Example A ball, initially traveling at a velocity of 14m/s is rolled up a frictionless hill until it stops.. How high up the hill did it go?
Understanding the MVE – Free Fall Key Factor What is the ball’s final height? What is the ball’s GPE? The height is always equal to zero, and the GPE is always equal to zero at the Lowest Point.
MVE – Free Fall WS 11a #3 Find GPE of the ball at the surface of the table Find GPE of the ball at the top of the platform A beach ball is.82m above a picnic table which is.45m tall. The table is on a 2.9m platform.
Conservation of Energy What are the kinetic and potential energies at the following points? Explain why. A B C
Conservation of Energy Example A car’s engine (m car = 1500 kg) puts 10,000 J of energy into getting the car to the top of a hill. Calculate the GPE & KE of the car at the three points below. h 3h/4 h/4 A B C
Understanding the MVE - Launch The projectile will be launched up from the ground. Key Factor What did the explosion do to the ball? Work-Kinetic Energy Principle? The net work done on a body is equal to the change in its kinetic energy.
Understanding the MVE - Archery Key Factor What was the GPE of the arrow just as it struck the target? Why? 8-25
Understanding the MVE – More Archery Let’s get a bulls eye hit this time! What was the GPE of the arrow at the beginning and the end of the arrow’s flight?
Understanding the MVE – In the Factory Key Factor What role did friction play in this problem? Friction resulted in the apparent loss of energy to the system. However, the energy is still accounted for as work other (W O ).
MVE – Archery WS Intro #3 An arrow (m=.15kg) is drawn back in a bow (k=1120N/m) a distance of.35m. What is the EPE? What is the GPE
Conservation of Energy Examples
Simple Harmonic Motion Simple Harmonic Motion: Motion caused by a linear restoring force that has a period independent of amplitude. Period: The time required to repeat one complete cycle Amplitude: Maximum displacement from equilibrium.