Kinetic energy is the energy of motion. Most kinetic energy you will work with is translational. Objects that are rotating (which you will study later) have rotational kinetic energy, possibly in addition to translational kinetic energy.
Thermal energy is a form of kinetic energy related to the internal motion of the individual molecules that make up an object or substance. An object’s thermal energy is related to its temperature: as an object grows warmer, it has more thermal energy. Internally, energy can transform from other forms into thermal energy, usually when there is a dissipative internal force like friction or air resistance. Externally, thermal energy can be transferred from one system to a colder system as heat.
Energy is measured in joules (J). Use the definition of kinetic energy to write a joule in terms of other units you are familiar with. Does this agree with other definitions of the joule you have seen?
Many systems include more than one object. In general, the different objects could have different masses and different speeds. How do you think you could find the kinetic energy for such a system? Write an equation for the total kinetic energy.
I estimated around \(576,000 \mathrm{~J}\text{.}\) (Many cars have a mass that is somewhere around \(1200 \mathrm{~kg}\text{,}\) and a typical highway speed limit is \(70 \mathrm{~mph}\text{,}\) which is about \(31 \mathrm{~m/s}\text{.}\))
Estimate the kinetic energy (in J) of a system consisting of two identical cars traveling toward each other, both traveling at the same speed. Is your answer greater than, less than, or equal to the kinetic energy for a single car you found in the previous activity? Explain why this makes sense.
