Forms of Energy
Work, Kinetic Energy and Potential Energy.
Outline
•	Newton’s 2nd and 3rd Laws
•	Work
•	Concepts of work
•	Work and energy
•	The energy of motion (kinetic energy)
•	The energy of position(potential energy)
•	Lost energy 
•	Energy conservation
Understanding the 2nd Law
•	F = ma
•	The more Mass, the harder it is to accelerate. (Demonstration)
•	Units of mass (kilograms or slugs?)
•	The Force due to gravity (Weight)
•	W = m g
•	If m = 5 kg, W = 5 kg x 9.8 m/s2 = 49 Newtons
•	Convert units:
•	49 N x (0.225 lbs / N) = 11 lbs
•	Also: 5 kg -> 11 lbs OR 2.2 lbs per kilogram.
Understanding the 3rd Law
•	Every push is met by a push of identical strength in the opposite direction!
•	The difference is mass
•	Demonstration.
Work and Energy
•	Work = Force x distance
•	Force applied over a distance changes the motion of the object.
•	Work is a measure of HOW MUCH the motion has been changed (in units of N-m or JOULES)
•	Energy is the ability to create change, so….
•	ENERGY IS THE ABILITY TO DO WORK.
Concepts of work
•	Work = Force x distance
•	There must be a FORCE applied
•	There must be a DISPLACEMENT 
•	The applied force must act in the DIRECTION of the displacement.
•	Example: How much work to lift up a 5 kg box 2 meters off the ground?
•	The force needed is the weight of the box
•	F = 5 kg x 9.8 m/s2 = 49 N
•	Then Work = 49 N x 2 m = 98 Joules.
The energy of motion
•	Things in motion can do work.
•	Kinetic Energy = Ek
•	Ek = ½ mv2
•	Example: A cart carrying 2 kg of mass is moving with v = 2m/s. What is its Kinetic energy?
•	Ek = ½ (2 kg)(2m/s)2 = 4 J
•	As the cart does work, it loses energy and slows down.  The maximum work it can do is 4 Joules.
•	Work done = - change in Ek

Energy of position
•	An object may not be in motion, and yet have the potential to do work because of its position = Potential Energy = EP
•	Gravitational Potential Energy:  EP = m g h
•	Example:  A 5 kg mass is at a height of 1 meter above the ground
•	EP = 5 kg x 9.8 m/s2 x 1 m = 49 J
•	Work done by the mass = - change in EP
Lost Energy?
•	Usually, some energy is lost in the conversion from kinetic or potential to work
•	A car may have 100,000 Joules of energy, but if it strikes a wall and the wall does not move, not all of its energy did work.
•	The car did work against friction, and the energy was lost as HEAT to the surroundings.
Energy Conservation
•	Objects have energy because of their motion, or their position
•	That energy enables them to do work on something else.
•	The energy they lose in doing work is gained by the thing they did work on.
•	Energy is not lost, but only transferred and transformed.
Energy Conservation - a picture