Work, Energy and Power

Is the student doing work in pushing against the wall?

Is the girl doing work in pushing the cart?

Is the man doing work in carrying the load across the street?

Is the lady doing work while holding the weights above her head?

Is work done in lifting the box?

Is work done in putting down the box?

Work If a constant force F acts on an object as it undergoes a displacement d, the work done by the force on the object during the displacement is W = Fdcos where W = work done in Joules (J) F = force in N d = displacement in m = angle (180 or less) between the direction of F and the direction of d Note: 1 J = 1 Nm

Work

Requirements in order for work to be done 1. Force need to be exerted 2. There must be a displacement 3. The force must be exerted in such a way that it has a component that is in the same direction or opposite to the direction of the displacement.

Energy The capacity of a physical system to do work.

Kinetic Energy (KE)

Gravitational Potential Energy (PE g )

Elastic Potential Energy (PE s )

Work-Energy Calculation When a system gains or loses energy from its environment because of work done on the system by forces origination in the environment, then the change in the systems energy is W = E f – E i Rearranging and substituting for the different types of energy results to KE i + PE gi + PE si + W = KE f + PE gf + PE sf

Law of conservation of energy In a closed, isolated system, energy is not created or destroyed, but rather, is conserved. KE i + PE gi + PE si = KE f + PE gf + Pe sf

Power