4.3 Conservation Laws

Kinetic Energy

• Kinetic energy is the energy associated with motion.

• KE = ½ mv2

• m = mass, v = velocity

Potential Energy

Potential Energy is stored energy:Gravitational PE depends on:– object’s mass (m)– Acceleration due to

gravity (g)– distance object could

fall– PE = mgh

PE

• In space, an object or gas cloud has more PE when it is spread out than when it contracts.

A contracting cloud converts gravitational potential energy to thermal energy.

Radiative Energy – energy from light

Energy can change forms

KE & GPE for an orbiting body:

• Thermal energy is a kind of kinetic energy• Related to temperature but it is NOT the

same

Thermal energy depends both on temperature AND density. For example:

• Mass is a form of potential energy•A small amount of mass can release a great deal of energy• E = mc2

Conservation of Energy

• Energy can’t be created or destroyed.

• It can change form or be exchanged between objects.

• The total energy content of the Universe was determined in the Big Bang and remains the same today.

Momentum

• Momentum: a property of moving objects

• Momentum = mass velocity

• A net force changes the momentum of an object

Conservation of Momentum

• The total momentum of a system can’t change unless an external force is applied

• Interacting objects exchange momentum through equal and opposite forces

• Spinning or orbiting objects have angular momentum

• Angular momentum = mass x velocity x radiusL = m v r

m x v x R = m x V x r

Conservation of angular momentum: the angular momentum of an object can’t change unless an external twisting force is applied

This explains why objectsrotate faster as they shrink in radius:

For an orbiting body, this means if r is small, v is big and if r is big, v is small