1 / 30

Energy

Energy. Chapter 8. Energy. We discuss energy frequently, but what is energy? Energy is the ability to do work but energy Types of energy: Kinetic Potential Gravitational Elastic. Work. The Equation: W = Fd The units: W = Work = Joules (J) F = Force = Newtons (N)

lilike
Download Presentation

Energy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Energy Chapter 8

  2. Energy • We discuss energy frequently, but what is energy? • Energy is the ability to do work but energy • Types of energy: • Kinetic • Potential • Gravitational • Elastic

  3. Work • The Equation: W = Fd • The units: • W = Work = Joules (J) • F = Force = Newtons (N) • d = Distance = meters (m) • Work is defined as: • applied energy • It takes work to move something • Can make energy

  4. As the equations states a force must be applied over a distance in order for there to be work. • If it does not move, then no work has been done • Often times, forces occur at an angle, in this case we find the x component of the distance and use it in the equation: W = Fd(cosΘ)

  5. How do you know WORK is being done? • There is a force or a distance • There is friction • The objects slows down or speeds up

  6. Sample Problem #1 • You push a 1000 newton car 5 meters at 30° to the ground. How much work did you do? F = 1000 N W = Fd(cosΘ) d = 5 m W = (1000 N)(5m)(cos 30° ) W = ? W = 5000J (.866) Θ = 30° W = 4330 J

  7. Potential Energy • The equation: Ep = mgh • The units: • Ep = potential energy = Joules (J) • m = mass = kilogram (kg) • g = acceleration due to gravity = meters/squared seconds (m/s2) • h = height distance = meters (m)

  8. Potential Energy is defined as energy due to position. • It comes from the pull or gravity • How do you know there is potential energy? • An object is above the ground • An object is dropped or tossed up • An object goes up or down

  9. Sample Problem #2 • How much potential energy does a 4 kg object have that is 5 meters off the ground? m = 4 kg Ep = mgh h = 5 m Ep = (4kg)(9.8 m/s2)(5m) g = 9.8 m/s2Ep = 196.2 J Ep = ?

  10. Kinetic Energy • The equation: Ek=½ mv2 • The units: • Ek=kinetic energy = Joules (J) • m = mass = kilogram (kg) • v = velocity = meters/second (m/s)

  11. Kinetic Energy is defined as the energy of motion. • An object has kinetic energy when it is moving. • An object gets kinetic energy from falling (Ep) or by being pushed (from Work). • An object loses Ek by going up (due to gravity), friction, or negative force.

  12. How do you know there is Kinetic Energy? • An object is moving (velocity). • An object is thrown or is falling. • An objet changes speed.

  13. Sample Problem #3 • How much kinetic energy does a 10 kg object traveling 3 m/s have? m = 10 kg Ek=½ mv2 v = 3 m/s Ek=½ (10 kg)(3 m/s)2 Ek= ? Ek= 45 J

  14. Elastic Potential Energy • The equation: Eep=½ kx2 • The units: • Eep=elastic potential energy = Joules (J) • k = spring constant = Newtons/meters (N/m) • x = distance = meters (m)

  15. Elastic Potential Energy is defined as: • The potential stored (compress) or released (stretched) in a spring or elastic object. • The spring constant or force constant will be given. • The distance in the equation is the distance that the spring is stretched or compressed from its resting position.

  16. How do you know there is Elastic Potential Energy? • There is a spring involved (or elastic object). • An object is compressed. • An object is stretched.

  17. Sample Problem #4 • A spring has a 4 N/m spring constant and a resting position of 0.5 m. If it is stretched to 2.5 m, find its elastic potential energy. k = 4 N/m Eep=½ kx2 xi = 2.5 m Eep=½ (4 N/m)(2m)2 xf = .5 m Eep= 8 J x = 2 m

  18. Power • The equation: P = W/t • The units: • P = power = Watts (W) • W = work = Joules (J) • t = time = seconds (s)

  19. Power is defined as how fast work is done • The more powerful something is, the faster work is done • A less powerful device an do the same amount of work, but it will take a longer time

  20. Sample Problem #5 • You do 120 J of work in 2 seconds. Find your power. W = 120 J P = W/t t = 2 s P = 120J/2s P = ? P = 60 W

  21. The Law of Conservation of Energy • “Energy is never created no destroyed, just transformed into other forms of energy.” • We can answer a lot of questions using this law, but it may be trick remember which part of energy to use. • Remember: • EK if the object is moving • EP if the object is off the ground • Eep if the object is elastic or attached to a spring • W if there is a force, horizontal distance, or friction.

  22. Sample Problem #6 • A 2 kg ball going 6 m/s stops in 4 m. Find the force of the friction that stopped it. m = 2 kg EK = W v = 6 m/s ½ mv2 = Fd d = 4 m ½ (2kg)(6m/s)2 = F (4m) F = ? F = 9N * We used EK because the mass is moving.

  23. Remember that the height of EP must be straight up! • If you are given an angle, you may need to use sin, cos, or tan to find the vertical height

  24. Practice Problem #1 Potential Energy at the top d is the length of the ramp not the height! m d = 6m h = 3m m Equals kinetic energy at the bottom Θ = 30°

  25. Potential energy at the top should be equal to kinetic energy at the bottom- IF THERE IS NO FRICTION. • If there is friction slowing the object down: the potential energy at the tope equals the work done by the friction plus the kinetic energy of the object • We will only be dealing with frictionless examples…for now. 

  26. Dropped and Thrown Objects • If an object is dropped, its potential energy is transformed into kinetic energy. In the absence of friction these two amounts are equal: • Part way down the object has both potential and kinetic energy. • A the bottom the object has no potential energy and all kinetic energy.

  27. Look at the picture at the bottom left and answer these questions. • Calculate the potential energy the object has at the top of the inclined plane. • Calculate the potential energy the object has at the bottom of the inclined plane. • Calculate the kinetic energy the object has at the top of the inclined plane. • Calculate the kinetic energy the object has at the bottom of the inclined plane.

  28. Sample Problem #7 • A 4 kg ball is thrown into the air. It reaches a height of 1.8 m. How fast was it going when thrown into the air? h = 1.8 m EP = EK m = 4 kg mgh = ½ mv2 g = 9.81 m/s2gh = ½ v2 v = ? (9.81 m/s2)(1.8m) = ½ v2 36 m2/s2 = v2 v = 6 m/s

  29. Sample Problem #8 • A 4 kg ball is dropped from 12m. How fast is it going halfway down? htop = 12 m Etop = Ehalfway Hbottom = 6 m Ep = Ep + EK m = 4kg mgh = mgh + ½ mv2 g = 9.81 m/s2gh = gh + ½ v2 v = ? (9.81 m/s2)(12 m) = (9.81 m/s2)(6 m) + ½ v2 120 m2/s2 = v2 v = 11 m/s

  30. That’s All Folks

More Related