Work and EnergyWork and Energy

Two interrelated quantities

Word Bank for Work & Energy Word Bank for Work & Energy Unit Unit

Force Energy Momentum Conserved quantities Work Joule Total energy Kinetic energy

Translational kinetic energy

Potential energy Gravitational potential

energy Reference level for PEg

Conservative/Dissipative forces

Law of Conservation of Energy

Power

Definition of WorkDefinition of Work

Work done on a particle by a constant force (constant both in magnitude and direction) is defined as the dot (scalar) product of force and displacement i.e., the product of magnitude of the displacement and the component of force parallel to the displacement.

W = F·d = (Fcosθ)d = Fdcosθ SI unit of work is the Joule where 1 J = 1N·m Other units centimetre-gram-second system (cgs)

1erg = 1 dyne·cm (105 dyne = 1 N) Imperial system (IS? Not SI) foot-pound

If the displacement is along the +x axis, neither the normal force nor the gravitational force will do work. They are both perpendicular to the displacement

ie θ = 90º so cosθ = 0Refer to example 6-1, 6-2 in Giancoli p.126

Work done by a varying forceWork done by a varying force

For the work done by a variable force, graphical techniques may be used. An exact answer may be found from integral calculus.

Fcosθ vs distance

Energy –one of the most imp concepts in science. It can be defined as the ability to do work (useful definition but not so precise).

Moving objects can do work on other objects e.g. tackler and quarterback, hammer and nail, baseball and glove.

Energy of motion is called “kinetic” energy.

The Work-Energy TheoremThe Work-Energy Theorem

W = Fd = (ma)d = m [(v2 – vo2)/2d]d

W = ½ mv2 – ½ mvo2

Translational KE = ½ mv2 So W = ΔKEThe net work done on an object is equal to

its change in kinetic energy.NB this is not a wireless network