What is energy?--Ability to do work!

Forms of energyThermal Energy

the kinetic energy of atoms and molecules (remember, “heat” is the transfer of this energy between systems)

Chemical energyenergy associated with electronic structure of atoms and the electromagnetic force

Nuclear energyenergy associated with nuclear structure of atoms and the strong nuclear force

Electrical energyassociated with an electric current (kinetic energy of electrons in a conductor)

Radiant (light) energyenergy associated with photons of light

Mechanical energyassociated with the movement of position of physical bodies (kinetic and potential energy)

Energy can be categorized into two main classes:

Kinetic energy– energy a body has because it is moving

KE=

Kinetic energy is a scalar quantity with unit of Joules (Newton*Meter)

Since it involves velocity, it depends on the frame of reference

Potential Energy: energy that is stored in a body.

Gravitational potential energy: Energy associated with gravitational force. That is, energy that goes into lifting an object is stored in the object as gravitational potential energy.

PE=mghg is the acceleration due to gravity (9.81 )

h is the height (relative to the reference height)

It’s a scalar quantity with unit of Joules (Newton*Meter).

Gravitational potential energy is relative because h is relative. A reference height has to be set first.

Change of gravitational energy doesn’t depend on the path, only height. (conservative forces)

Practice

A ball of mass m collides into a stationary ball with the same mass at a velocity of v. Suppose the collision is elastic collision. a. What are the velocities of the two balls after the

collision? b. What is the total kinetic energy of the system

before collision?c. What is the total kinetic energy of the system after

collision?d. Is the total kinetic energy conserved?

Practice

A ball of mass m collides into a stationary ball with the same mass at a velocity of v. Suppose the collision is completely inelastic collision. a. What are the velocities of the two balls after the collision? b. What is the total kinetic energy of the system before

collision?c. What is the total kinetic energy of the system after collision?d. Is the total kinetic energy conserved?

Momentum conserved

Kinetic energy conserved

Elastic collision

Yes Yes

Inelastic collision

Yes No

Completely inelastic collision

Yes No

Practice

Drop a ball of 1 kg mass from a height of 10 meters. Compute the potential energy before it is dropped and the kinetic energy just before it hits the floor. Are they the same?