When is mechanical energy conserved

How do you know if total mechanical energy is conserved? Is mechanical energy conserved between A and B? In which of the following situations is mechanical energy conserved? Which of the following is a type of mechanical energy? What happens to mechanical energy as an object falls? Can any object have momentum if its mechanical energy is zero?

Can any object have momentum even if its mechanical energy is zero class 9? Can a body have momentum without kinetic energy? Can a body have momentum when its mechanical energy is negative?

Any KE due to increases in delivery speed will be lost when motion stops. Explain that the word potential means that the energy is available but it does not mean that it has to be used or will be used. This simulation shows how kinetic and potential energy are related, in a scenario similar to the roller coaster. Observe the changes in KE and PE by clicking on the bar graph boxes.

Also try the three differently shaped skate parks. Drag the skater to the track to start the animation. This animation shows the transformations between KE and PE and how speed varies in the process. Later we can refer back to the animation to see how friction converts some of the mechanical energy into heat and how total energy is conserved.

On an actual roller coaster, there are many ups and downs, and each of these is accompanied by transitions between kinetic and potential energy. Assume that no energy is lost to friction. At any point in the ride, the total mechanical energy is the same, and it is equal to the energy the car had at the top of the first rise.

This is a result of the law of conservation of energy , which says that, in a closed system, total energy is conserved—that is, it is constant. Using subscripts 1 and 2 to represent initial and final energy, this law is expressed as. Either side equals the total mechanical energy. The phrase in a closed system means we are assuming no energy is lost to the surroundings due to friction and air resistance. If we are making calculations on dense falling objects, this is a good assumption.

For the roller coaster, this assumption introduces some inaccuracy to the calculation. When calculating work or energy, use units of meters for distance, newtons for force, kilograms for mass, and seconds for time. This will assure that the result is expressed in joules.

Compare it to the amount of work it would take to walk to the top of the roller coaster. Ask students why they may feel tired if they had to walk or climb to the top of the roller coaster they have to use energy to exert the force required to move their bodies upwards against the force of gravity. This video discusses conversion of PE to KE and conservation of energy. The scenario is very similar to the roller coaster and the skate park. It is also a good explanation of the energy changes studied in the snap lab.

Before showing the video, review all the equations involving kinetic and potential energy and conservation of energy. Also be sure the students have a qualitative understanding of the energy transformation taking place. Refer back to the snap lab and the simulation lab. A 10 kg rock falls from a 20 m cliff. What is the kinetic and potential energy when the rock has fallen 10 m? Substitute the known values into the equation and solve for the unknown variables.

Alternatively, conservation of energy equation could be solved for v 2 and KE 2 could be calculated. Note that m could also be eliminated. If there was no frictional force between the object and the floor and air resistance wasn't a factor, the object would never decelerate as it moved across the floor, and would travel on forever at a constant velocity in that direction until stopped by an outside force Newton's First Law.

When you push the object, the object gains kinetic energy and begins to move across the floor, but eventually it stops. Where did the kinetic energy go? Energy is "lost" to friction in the sense that it is not converted between potential and kinetic energy but rather into heat energy, which we cannot put back into the object.

If we expand our system to include the entire universe, total energy is conserved, but mechanical energy is not. Note that the above example assumes the floor is level and horizontal so that there is no change in gravitational potential energy.

As the car descends hills and loops, its potential energy is transformed into kinetic energy as the car speeds up. As the car climbs up hills and loops, its kinetic energy is transformed into potential energy as the car slows down. Yet in the absence of external forces doing work, the total mechanical energy of the car is conserved.

Conservation of energy on a roller coaster ride means that the total amount of mechanical energy is the same at every location along the track. The amount of kinetic energy and the amount of potential energy is constantly changing. Yet the sum of the kinetic and potential energies is everywhere the same. This is illustrated in the diagram below. The total mechanical energy of the roller coaster car is a constant value of 40 Joules.

The motion of a ski jumper is also governed by the transformation of energy. As a ski jumper glides down the hill towards the jump ramp and off the jump ramp towards the ground, potential energy is transformed into kinetic energy.

If it can be assumed that no external forces are doing work upon the ski jumper as it travels from the top of the hill to the completion of the jump, then the total mechanical energy of the ski jumper is conserved. Consider Lee Ben Fardest esteemed American ski jumper. He starts at rest on top of a meter hill, skis down the degree incline and makes a world record setting jump. Assuming that friction and air resistance have a negligible effect upon Lee's motion and assuming that Lee never uses his poles for propulsion, his total mechanical energy would never change.

Of course it should be noted that the original assumption that was made for both the roller coaster car and the ski jumper is that there were no external forces doing work. In actuality, there are external forces doing work. Both the roller coaster car and the ski jumper experience the force of friction and the force of air resistance during the course of their motion. Friction and air resistance are both external forces and would do work upon the moving object.