The discussion revolves around the conservation of energy in the context of a frictionless roller coaster ride. Participants explore the implications of external forces, particularly the role of gravity and the forces exerted by the rails, in relation to energy conservation principles.
Participants express differing views on the nature of external forces and their impact on energy conservation. There is no consensus on the interpretation of external forces in relation to energy conservation, particularly in the context of the roller coaster and bouncing ball examples.
There are unresolved aspects regarding the definitions of external forces and how they relate to work done on a system. The discussion also touches on the importance of system boundaries in determining energy conservation.
The only force doing any work in this situation is gravity, which is a conservative force. (Perhaps you are thinking of conservation of momentum; the momentum of a system is conserved if there is no outside force.)pkc111 said:Could some one please explain to me why energy is conserved in a roller coaster car on a frictionless roller coaster car ride when the conservation law says that energy will only be conserved when their are no outside forces applied.
Sure (which is why the momentum of the car changes), but that force is always perpendicular to the car's direction of motion, thus it does no work.Isnt the force supplied by the rails to change the cars direction an outside force?
Absolutely, but strictly speaking, that force does no work on the ball. (The ground doesn't move.) During the collision, the kinetic energy of the ball will be transformed into elastic potential energy. If the collision is perfectly elastic, the ball will bounce back with the same kinetic energy.pkc111 said:What about an elastic bouncing ball isn't the ground supplying an external force in the direction of motion ?
As long as the external forces do no work, they won't change the mechanical energy.The definition of the law of conservation of mechanical energy I have is:
"Conservation of mechanical energy applies to a mechanical process in which external force and non-conservative internal forces are absent"
http://cnx.org/content/m15102/latest/
It just implies that all external forces must be absent, not just ones in any particular direction.