Forms of Energy Notes

The Many Forms of Energy

Focus Question

• What are some forms of energy?

New Vocabulary

• Translational kinetic energy
• Rotational kinetic energy
• Potential energy
• Gravitational potential energy
• Reference level
• Elastic potential energy
• Thermal energy

Review Vocabulary

• System: Object or objects of interest that can interact with each other and the external world.

Changing Forms of Energy

• The work-energy theorem states that work causes a change in the energy of a system.
• Work is the process that transfers energy between a system and the external world.
• Energy can be changed from one variety to another.

Translational and Rotational Kinetic Energy

• Energy due to changing position is called translational kinetic energy.

• Translational Kinetic Energy equation:

• Kinetic energy can also be due to rotational motion.

• The kinetic energy of a spinning object is called rotational kinetic energy.

• Rotational kinetic energy can be calculated using the system’s moment of inertia $(I)$ and its angular velocity $(ω)$.

• $KE_{rot} = \frac{1}{2}Iω^2$

Potential Energy

• Energy that is stored due to interactions between objects in a system is called potential energy.
• The stored energy due to the gravitational force between objects is called gravitational potential energy, or GPE.
• The height to which an object has risen is determined by using a reference level, the position where GPE is defined to be zero.

Gravitational Potential Energy

• The gravitational potential energy of an object-Earth system depends on the mass of the object $(m)$, its distance above the reference level $(h)$, and the strength of the gravitational field $(g)$.

• Gravitational Potential Energy formula:

• Changes in reference levels affect the values of GPE.

Problem Example: Work and Gravitational Potential Energy

• Problem: How much work does a bricklayer do to carry $30.2 kg$ of bricks from the ground up to the third floor (height $= 11.1 m$) of a building under construction? What is the gravitational potential energy of the brick-Earth system when the bricklayer reaches the third floor?
• Known:
  • $h = 11.1 m$
  • $m = 30.2 kg$
  • $g = 9.8 N/kg$
• Unknown:
  • $W = ?$
  • $GPE = ?$
• Solution:
  • The work done is equal to the weight of the bricks times the distance they were carried.
  • Use the relationship among gravitational potential energy, mass, height, and the gravitational field.

Elastic Potential Energy

• The stored energy due to an object’s change in shape, like a drawn bowstring, is called elastic potential energy, which is often stored in rubber balls, rubber bands, slingshots, and trampolines.

Other Forms of Energy

• Chemical energy: released from the burning of fossil fuels and during digestion
• Nuclear energy: released when the structure of an atom’s nucleus changes
• Thermal energy: the sum of the kinetic energy and potential energy of the particles in a system
• Radiant energy: carried by electromagnetic waves
• Electrical energy: associated with charged particles
• Mass: apparent only when large amounts of energy is involved, such as nuclear explosions; called rest energy, $E<em>0$: $E</em>0 = mc^2$