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What is Energy?

What is Energy?. The ability to do work The ability to create a change Energy comes in many forms Mechanical Potential Kinetic Electrical Thermal Nuclear Chemical It can be transformed from one form to another Like work, it is measured in Joules. Mechanical Energy.

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What is Energy?

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  1. What is Energy? • The ability to do work • The ability to create a change • Energy comes in many forms • Mechanical • Potential • Kinetic • Electrical • Thermal • Nuclear • Chemical • It can be transformed from one form to another • Like work, it is measured in Joules

  2. Mechanical Energy • Deals with the energy an object has because of either it’s motion (kinetic energy), and/or the outside forces acting on the object trying to make it move (potential energy) • A car moving at 50 mph can do a lot of damage to a pedestrian if they hit--this is because of the car’s motion. • On the other hand, a 16 pound bowling ball being held 10 ft above a person’s head (but not moving) can be dangerous, because the force due to gravity pulls down on the ball. The ball has the potential to fall on the person’s head. The ball is not falling, BUT IT COULD because of the force of gravity.

  3. Potential Energy • Energy that is not being applied to the object’s motion, BUT IT COULD BE LATER ON. • It is the work done by something, other than you (such as gravity), naturally on its own. • Gravity causing things to fall • Attraction/repulsion of electrical charges • Expansion/contraction of a spring • Potential energy depends on the position and condition of the object (the forces that act on it). • Think of a stretched rubber band on a slingshot. It has potential energy due to its position. If the rubber band is released, it is capable of doing work. • Some call potential energy “stored energy” but that term is very misleading (the energy does not come from the object).

  4. Gravitational Potential Energy • Potential Energy due to gravity (Epg). • Gravity is always trying to do work on objects by pulling them down to the ground. • Epg of an object is equal to the work done by gravity to make an object fall. • Work = Force*distance • Workgravity = Forcegravity* distance dropped = mag(-h) • Since ag = -9.8 m/s/s we use g. (g = 9.8 m/s/s) • Epg = mgh • Gravitational potential energy = /Fg/ * height

  5. Calculating Epg • Calculate the change in potential energy of 8 million kilograms of water dropping 50 m over Niagara Falls. • Know: m= 8 million kg h = 50 m g = 9.8 m/s2 • EPg = mgh =(8,000,000 kg)(9.8 m/s2)(50 m) = 3,920,000,000 J or 3.92 x109 J

  6. Elastic Potential Energy • Since a compressed spring wants to expand, and is willing to do work on any object in the way of its expansion, we call the work done by a spring the elastic potential energy (Epel). • This is also true for a stretched spring. It wants to contract back to its original length, and will pull anything attached to it. • Some people call the elastic potential energy of a spring the potential energy of a spring (Eps). • elastic potential energy (Epel) is a more general name. • (Eps) = work of a spring = (1/2)k(Dx)2

  7. Kinetic Energy • Energy of motion • It is the energy an object has BECAUSE it is moving. It IS NOT, IN ANY WAY, BY NO MEANS the energy an object uses to keep moving. (REMEMBER INERTIA). • Depends on the mass and velocity of the object. • Kinetic energy = (½)(mass)(velocity)2 Ek = (½)mv2 • Notice an object’s velocity has a greater impact on its kinetic energy than it’s mass.

  8. Work - Energy Theorem • The work done on an object is defined as the change in that object’s energy. • Work = Change in Ep + Change in Ek • W = DEp+ DEk

  9. Work Kinetic Energy Potential Energy Work - Energy Theorem • A moving object is capable of doing work. • It can lose some of it’s kinetic energy (slow down) and create a change on another object in the process. • Car hitting an object • A bat hitting a ball • Electrical charges moving through a wire • Hot steam moving the turbines of an electrical generator • An object can also convert it’s potential energy into kinetic energy so that it can do work.

  10. Net Work • This is the overall work done on an object • The net work is done by the net force acting on an object • The net work always equals the change in kinetic energy • Fnet//d = (1/2) mvf2 -(1/2) mvi2 • Objects in equilibrium never have any net work done on them • Fnet = 0 N • Objects that move with a constant speed, never have any net work done on them.

  11. A quick comparison Objects A and B are initially sliding on smooth ice and come to a rough surface. Object A slides a distance of D as it comes to a stop. How far (in terms of D) will object B have to slide to also come to a stop? (The coefficient of kinetic friction is the same for both boxes.) VA Rest mA mA D Since object B has twice the velocity of A it has 4X the EK. This means friction must do 4X the work on B than on A. VB = 2VA mB And since the masses of A and B are the same, they both have the same frictional force. So object B must slide 4X farther than object A to do all that work. This is why you should never speed when driving.

  12. Another comparison Objects A and B are initially sliding on smooth ice and come a rough surface. Object A slides a distance of D as it comes to a stop. How far (in terms of D) will object B have to slide to also come to a stop? (The coefficient of kinetic friction is the same for both boxes.) VA Rest mA mA D mB = 2mA Since object B has 2X the mass of object A, it has 2X the EK of A. This means friction must do 2X the work on B than on A. VB = VA Because Object B has 2X the mass of object A it also has 2X the frictional force of object A. So object B slides the same distance as object A.

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