CONSERVATION OF ENERGY

• http://www.pbs.org/opb/circus/classroom/circus-physics/conservation-energy/

• A conservative force converts potential energy to other forms of mechanical energy when it does work.

• In other words, a conservative force does not change the mechanical energy of a system

• Remember that mechanical energy is potential energy + kinetic energy

Conservative Force

• So, what would be a nonconservative force???• Friction is most common example (air

resistance is a form of friction)• Work done by friction becomes microscopic

internal energy in the object, raising its temperature

• The internal energy cannot be recovered and turned back into kinetic energy

NonConservative Force

• When is thermal energy a desirable outcome and when is is undesirable?

Thermal Energy

• Energy cannot be created or destoyed, only changed from one form to another

• Ei = Ef

• KEi + PEi = KEf + PEf + HE

• ½ mv2 i + mghi = ½ mv2 f + mghf + HE

Law of Conservation of Energy

Example

• While jumping over the Great Wall of China an 82 kg skateboarder needs to leave the ramp traveling 21.7 m/s. How much potential energy does he need to start with? And what is the minimum height of ramp he should use?

Example

• A trampoline dunk artist is bounces to a maximum vertical height of 4.8 m before launching himself towards the hoop. At the top of his arc he is 3.2 m above the ground. How fast is he traveling at this point?

Example

• A 65 kg snowboarder starts at rest, travels down a hill into a gulley and back up the other side as shown. Find his speed at top of the 2nd hill.

Example

• A 5.0 kg block of wood is pushed down a ramp with a velocity of 6.0 m/s. The ramp is 1.5 m tall and 3.5 m long. At the bottom of the ramp it is traveling at 7.5 m/s. How much thermal energy is generated due to friction? Determine the force of friction and the coefficient of friction.

Example

• Net work = change in kinetic energy.

• This means that the net work on a system causes a change in speed of the system

Work-Energy Theorem

netW KE

• If you know how much energy you want a system or object to have, then you can calculate how much work you need to perform to get it there. Conversely, if you know how much energy a system has, you can calculate how much work that energy can perform. This has literally MILLIONS of real-life examples:

• The energy of wind movement performs work when it turns a Wind Turbine• The chemical energy in gasoline performs work on a piston, which in turn

performs work on a vehicle to create kinetic energy.• Work is performed on air as it enters a Jet Engine to speed up the air, which

results in higher kinetic energy of the air particles, which pushes the airplane• Stirring a pot of water performs work in the form of heat transfer, which results

in a higher temperature in the water (higher temp = higher energy)• When you throw a water balloon at someone’s face, their face performs work

on the balloon, which then increases in pressure (higher pressure = higher energy) until it pops.

Work-Energy Theorem

• A 60 kg sprinter exerts a net force of 260 N over a distance of 35 m. What is his change in kinetic energy? If started at rest, what is speed at finish?

Example

• A student pushes a 25 kg crate which is initially at rest with a force of 160 N over a distance of 15 m. If there is 75 N of friction, what is the final speed of the crate?

Example