Physics 201 Lecture 6 Work, Power, and Energy

Machines and the definition of work The purpose of a mechanical machine is to multiply an input force to create a much larger force for useful work This mechanical advantage is gained by increasing the displacement over which the force is applied This motivates defining work as the product of displace- ment and component of force driving the displacement Though both force and displacement are vectors, work is not – in order to account for direction we have Is this work or weight? If they both appear in a problem, substitute mg for weight and use W for work

Real machines and real work In any real machine there are frictional effects that limit its efficiency: The rate at which work is done is called power – a more powerful machine will perform work faster Work does not always mean what it does in English Work is not synonymous with effort Work does not involve time Work can be negative

The energy level of a system represents its ability to do work Work done on an internal conservative force is released when the system returns to the original configuration This is why potential energy depends on the configuration of the system rather than the means whereby it is rearranged But friction opposes motion – it always does negative work

Work can create energy in three different ways Kinetic Energy Creates motion (linear or rotation) Work Potential Energy Displacement from equilibrium Heat Energy Temperature or phase change

Work done on a single rigid object creates kinetic energy

When driving from equilibrium, work creates potential energy Potential energy is work against the internal forces of the system Potential energy ultimately depends on the internal configuration of the system Different forces will involve different values of potential energy… Force Potential Energy Weight Gravity Spring

Conservation of energy Many consider this to be the most important principle in physics The key to use energy is to find a moment in time in which you know the total energy of the system If there are no external forces, the total energy is always the same

Resistive forces destroy mechanical energy Work done on an internal conservative force is released when the system returns to the original configuration This is why potential energy depends on the configuration of the system rather than the means whereby it is rearranged But friction opposes motion – it always does negative work