quiz eight - physics 1200

when energy is tranferred between objects:

work is being done

law of conservation of energy

total energy is never increased or decreased in any process - it can be transformed into another form of energy or transferred between objects

if there is a non-conservative force:

the mechanical energy will not be conserved

most common type of non-conservative force

friction

friction and mechanical energy

mechanical energy decreases when friction does work on a system

work done by friction

-F(friction)(d)

dissipative force

force that causes mechanical energy to decrease - the lost mechanical energy is converted to thermal or internal energy

absolute value of the work done by friction equals

change in internal energy

power

rate at which work is done or the rate at which energy is transformed

SI unit for power

J/s = Watt

1 horsepower

746 Watt

efficiency

rate of useful power output to power input; some energy is always lost in any real machine; e will always be less than 1

momentum

mass times velocity

SI unit for momentum

Kg times m/s

internal forces

forces that act between objects in a system

external forces

forces that act on objects in a system by objects outside of a system

law of conservation of momentum

total momentum in an isolated system (where there is no external force acting on objects in a system) - remain constant

• total momentum before collision equals total momentum after collision

collision

interaction between objects occuring over a very short time interval and involving internal forces that are much larger than any external forces on a system

• momentum is conserved

elastic collision

momentum and kinetic energy are both conserved

important for elastic collision

Va-Vb = -(V’a - V’b)

inelastic collision

momentum is conserved, but kinetic energy is not conserved

• kinetic energy converted to thermal energy, sound energy, and potential energy (if the material is permanently deformed)

completely inelastic collision

momentum is conserved, kinetic energy is not conserved AND THE OBJECTS STICK TOGETHER