Energy Semester Review

You can disregard topics related to springs in the video.

Work

Work is a force acting over a distance that changes energy. \(W=Fdcos \Theta\)

You need to be able to determine if work is positive or negative based on the angle between the force and distance, or weather it is trying to add or remove energy from the system.

You also need to be able to incorporate work into conservation of energy equations.

Kinetic Energy

Kinetic Energy \(KE=\frac{1}{2} mv^2\) is the energy due to motion.

Remember that the net work, or the work done by the net force on an object is always equal to a change in kinetic energy.

Gravitational Potential Energy

Gravitational Potential Energy, \(U_g=mgh\).

The height can be measured from any arbitrary point as long is it consistently used.

Thermal Energy

Thermal energy is sometimes called internal energy and comes from the random motion of the molecules that make up a substance. Because it is caused by motion it is a type of kinetic energy. Dissipative forces like friction and air resistance cause mechanical energy to turn into thermal energy.

Mechanical Energy and The conservation of Mechanical Energy

Mechanical Energy is the sum of the kinetic and potential energies of a system. It is conserved if there is isn’t any dissipative forces like friction or air resistance, or other forces from outside the system doing work.

You should be comfortable setting up LOL charts like we did with this assignment. There is an interactive practice from the physics classroom here.

When Not to use Conservation of Energy Ideas.

Mechanical energy in not conserved if there are are forces doing work on your system from the outside. It is difficult to use conservation of energy if you don’t have enough information to calculate the work done by outside forces.

In collisions, it is very difficult to calculate the energy lost to heat through friction. You should not use conservation of energy strategies unless a collision is elastic.

Force position (or distance) Graphs

The area under a force position graph is equal to work or a change in energy. Force time graphs are usually a clue that you should be using energy ideas to solve a problem.