Kinetic and Potential Energy Types of Energy Kinetic and Potential Energy

Types of Energy There are two main types of energy: * Kinetic energy * Potential energy

Kinetic energy is energy associated with an object in motion.

Kinetic Energy The work done on an object by a constant force equals the object’s mass times its acceleration times its displacement.

* Force must be constant Kinetic Energy Wnet = F∆x = (ma)∆x * Force must be constant

Kinetic Energy Previously we found that the following relationship holds when an object undergoes constant acceleration.

Kinetic Energy Vf2 = vi2 + 2a∆x a∆ = vf2 – vi2 2

Kinetic Energy So substituting this result into the equation Wnet = m* vf2 – vi2 2 Or ½mvf2 – ½mvi2

The quantity ½mv2 has a special name in physics: kinetic energy.

Kinetic Energy The kinetic energy of an object with mass m and speed v, when treated as a particle, is given by the following expression:

KE = ½ mv2 Kinetic energy = ½ x mass x (speed)2

Kinetic Energy Kinetic energy is a scalar quantity and the SI unit for kinetic energy is the Joule (J).

Kinetic Energy Wnet = ½mvf2 – ½mvi2 The net work done by a net force acting on an object is equal to the change in the kinetic energy of the object.

Kinetic Energy From this theorem, we see that the speed of the object increases if the net work done is positive.

Kinetic Energy The object’s speed decreases if the net work done is negative, because the final kinetic energy is less than the initial kinetic energy.

Potential Energy Think of a boulder. While the boulder remains balanced, it has no kinetic energy but potential energy.

Potential Energy If it becomes unbalanced, it will fall vertically to the desert floor and will gain kinetic energy as it falls.

Potential Energy As we have seen, an object in motion has kinetic energy. But a system can have other forms of energy.

Potential Energy Potential energy is present in an object that has the potential to move because of its position relative to some other location.

Potential Energy Unlike kinetic energy, potential energy depends not only on the properties of an object but also on the object’s interaction with its environment.

Potential Energy Going back to the example of the boulder, the force of gravity will cause the unbalanced boulder to fall.

Potential Energy The energy associated with an object due to the object’s position relative to a gravitational source is called gravitational potential energy.

Potential Energy Gravitational potential energy can be determined: PEg = mgh *Free fall acceleration (g) must be constant

Potential Energy The SI unit for gravitational potential energy is Joule (J).

Potential Energy Gravitational potential energy is a result of an object’s position, so it must be measured relative to some zero level.

Potential Energy The zero level is the vertical coordinate at which gravitational potential energy is defined to be zero.

Potential Energy Imagine you are playing with a spring on a tabletop. You push a block into the spring, compressing the spring, and then release the block. The block slides across the tabletop.

Potential Energy The kinetic energy of the block came from the stored energy in the stretched or compressed spring. This potential energy is called elastic potential energy.

Potential Energy Elastic potential energy is stored in any compressed or stretched object, such as a spring or the stretched strings of a tennis racket.

Potential Energy The length of a spring when no external forces are acting on it is called the relaxed length of the spring.

Potential Energy When an external force compresses or stretches the spring, elastic potential energy is stored in the spring.

Potential Energy The amount of energy depends on the distance the spring is compressed or stretched from its relaxed length.

Potential Energy Elastic potential energy can be determined using the following equation: PEelastic = ½kx2

Potential Energy The symbol k is called the spring constant, or force constant. Units for spring constant is newtons divided by meters (N/m)