Chapter 7

# Chapter 7 - Work & Energy Chapter 7: Work & Energy The law...

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1 Chapter 7: Work & Energy The law of conservation of energy is very powerful and can make solving complicated problems simple. We need to understand the energy of a system and how one may introduce energy into a system by doing work . A very important and general law of physics is that the energy in a closed system is conserved . That is while it can be transformed into different forms, it cannot be created or destroyed. what types of energy do we need to know about kinetic energy - the energy a body has due to its motion potential energy - energy stored in a system that has the potential to be released example would be gravitational potential energy being changed into kinetic energy Conservation of Energy vertical motion can be analyzed in terms of the conserved energy of the particle kinetic potential kinetic Energy Dissipation kinetic and potential energies are mechanical there is also non-mechanical energy e.g. there is some air resistance on the rock, slightly slowing it down and hence reducing its energy. This energy is not lost but is transferred when it slightly heats the air - becomes internal energy of the air As another example think of friction, where the friction force slows objects down, reducing their kinetic energy.

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2 Work There’s a very specific definition of what physicists mean by ‘work’. The simplest case is when a constant force is used to move an object with this displacement parallel to the force then the work done by the force, W = F s more generally, if the force vector makes an angle with the displacement vector, then the work done by the force is W = Work SI units of work are clearly N m (Newton - meters). 1 N m is also known as 1 J, or one Joule (pron. jewel ) Work is a scalar quantity - it does not possess a direction. We’ll see soon that work is the transfer of energy. Example 7.2 - Sliding down a ramp A package with mass, m, is unloaded from a truck with an inclined ramp, as shown. The ramp has rollers that eliminate friction, and the truck unloads from a height, h . The ramp is inclined at an angle . Find an algebraic expression in terms of these quantities for the work done on the package during its trip down the ramp. normal force perpendicular to
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## This note was uploaded on 04/09/2008 for the course PHYS 111n taught by Professor Sukenik during the Spring '08 term at Old Dominion.

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Chapter 7 - Work & Energy Chapter 7: Work & Energy The law...

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