Joules

Conservation of Energy

Mr. Hawboldt

Open Ended

What does the word "conservation" or "conserved" mean in the context of energy?

Open Ended

A rubber ducky has 150 Joules of energy. Later the rubber ducky still has 150 Joules of energy. Was energy conserved? Explain

Open Ended

A tennis ball has 10 Joules of kinetic energy and 20 Joules of gravitational potential energy. Later, it has 20 Joules of kinetic energy and 10 Joules of gravitational potential energy. Was energy conserved? Explain

Open Ended

An egg has 10 Joules of gravitational potential energy and no kinetic energy when it is dropped from rest. It gains speed but loses height as it moves toward the ground. How much kinetic energy does the egg have right as it hits the ground (at height = 0 meters)? Explain

Open Ended

A box slides across the floor with a kinetic energy of 50 Joules. After a few seconds the box slows down and only has 40 Joules of kinetic energy. The friction with the floor warmed up the bottom of the box giving it additional 10 Joules of thermal energy. Was energy conserved? Explain

Conservation of Energy

• In a closed system, total energy cannot change

• energy can be transferred from one type of energy to another

• not all energy types are conservative

Conservative and Non-Conservative Energy

• Conservative Energies are: kinetic, gravitational potential, electrical potential, elastic, a few others

• Conservative Energies can be transferred between other types

• Non-Conservative Energies are: chemical, heat, sound, friction

• Non-Conservative Energies cannot be transferred back into other kinds

• We call non-conservative energies "Dissipated" since they are released and can't be retrieved

Conservation of Energy Equation

• Total Energy Before = Total Energy After + Dissipated Energy

•  $K_{initial}+U_{g\ initial}=K_{final}+U_{g\ final}+Dissipated$  

Multiple Choice

An object initially has 20 Joules of kinetic energy and 30 Joules of gravitational potential energy. After some time, that object has 10 Joules of kinetic energy and 20 Joules of gravitational potential energy. How much energy was dissipated?

• 10 Joules

• 20 Joules

• 30 Joules

• 40 Joules

Multiple Choice

A 100 kg high diver hits the water with 980 Joules of kinetic energy and no gravitational potential energy. If the diver started from rest on the diving platform, how much gravitational potential energy did they start out with?

• 980 Joules

• 9.8 Joules

• 98 Joules

• 9800 Joules

Multiple Choice

A 100 kg high diver hits the water with 980 Joules of kinetic energy and no gravitational potential energy. If the diver started from rest on the diving platform, from what height did the diver fall?

• 0.1 meters

• 1 meter

• 10 meters

• 100 meters

Summary

• In a CLOSED system, total energy remains constant

• energy can be transferred between kinetic and gravitational potential

• any energy that is dissipated (by heat, sound or friction) cannot be regained

•  $K_{initial}+U_{g\ initial}=K_{final}+U_{g\ final}+Dissipated$