Lesson Notes By Weeks and Term - Junior Secondary School 2
POTENTIAL AND KINETIC ENERGY
POTENTIAL AND KINETIC ENERGY
TERM: 2nd TERM
SUBJECT: BASIC SCIENCE
CLASS: JSS 2
REFRENCE
WEEK TWO DATE: ……………..
TOPIC: POTENTIAL AND KINETIC ENERGY
Kinetic energy is energy possessed by a body by virtue of its movement. Potential energy is the energy possessed by a body by virtue of its position or state. While kinetic energy of an object is relative to the state of other objects in its environment, potential energy is completely independent of its environment. Hence the acceleration of an object is not evident in the movement of one object, where other objects in the same environment are also in motion. For example, a bullet whizzing past a person who is standing possesses kinetic energy, but the bullet has no kinetic energy with respect to a train moving alongside.
Definition
The energy of a body or a system with respect to the motion of the body or of the particles in the system. Potential Energy is the stored energy in an object or system because of its position or configuration.
Relation to environment
Kinetic energy of an object is relative to other moving and stationary objects in its immediate environment. Potential energy is not relative to the environment of an object.
Transferability
Kinetic energy can be transferred from one moving object to another, say, in collisions.
Potential energy cannot be transferred.
Examples, Flowing water, such as when falling from a waterfall. Water at the top of a waterfall, before the precipice. SI Unit is Joule (J)
INTERCONVERSION OF KINETIC AND POTENTIAL ENERGY
The law of conservation of energy states that energy cannot be destroyed but can only be transformed from one form into another. Take a classic example of a simple pendulum. As the pendulum swings the suspended body moves higher and due to its position potential energy increases and reaches a maximum at the top. As the pendulum begins its downward swing, the stored potential energy is converted into kinetic energy.
When a spring is stretched to one side, it exerts a force to the other side so it can come back to its original state. This force is called restoring force and acts to bring objects and systems to their low energy level position. The force required to stretch the spring is stored in the metal as potential energy. When the spring is released, the stored potential energy is converted into kinetic energy by the restoring force.
When any mass is lifted, the gravitational force of the earth (and the restoring force in this case) acts to bring it back down. The energy needed to lift up the mass is stored as potential energy due to its position. As the mass is dropped, stored potential energy is converted to kinetic energy.
Types of Kinetic Energy and Potential Energy
Kinetic energy can be classified into two types, depending on the type of objects:
Translational kinetic energy
Rotational kinetic energy
Rigid non rotating bodies have rectilinear motion. Thus translational kinetic energy is kinetic energy possessed by an object moving in a straight line. Kinetic energy of an object is related to its momentum (product of mass and velocity, p= mv where m is mass and v is velocity). Kinetic energy is related to momentum through the relation E = p2 / 2m and hence translational kinetic energy is calculated as E = ½ mv2. Rigid bodies which rotate along their center of mass possess rotational kinetic energy. Rotational kinetic energy of a rotating body is calculated as the total kinetic energy of its different moving parts. Bodies at rest also have kinetic energy. The atoms and molecules in it are in constant motion. The kinetic energy of such a body is the measure of its temperature.
Potential energy is classified depending on the applicable restoring force.
Gravitational potential energy – potential energy of an object which is associated with gravitational force. For example, when a book is placed on top of a table, energy required to raise the book from the floor and energy possessed by the book due to its elevated position on the table is gravitational potential energy. Here gravity is the restoring force.
Elastic potential energy – energy possessed by an elastic body like the bow and catapult when it is stretched and deformed in one direction is elastic potential energy. The restoring force is elasticity which acts in the opposite direction.
Chemical potential energy – energy related to arrangement of atoms and molecules in a structure is chemical potential energy. Chemical energy possessed by a substance due to the potential it has to undergo a chemical change by taking part in a chemical reaction is chemical potential energy of the substance. When fuel is used, for example, chemical energy stored in fuel is converted to produce heat.
Electrical potential energy – energy possessed by an object by virtue of its electric charge is electrical potential energy. There are two types – electrostatic potential energy and electrodynamic potential energy or magnetic potential energy.
Nuclear potential energy – potential energy possessed by particles (neutrons, protons) inside an atomic nucleus is nuclear potential energy. For example, hydrogen fusion in the sun converts potential energy stored in solar matter into light energy.
Applications
EVALUATION
CALCULATING KINETIC AND POTENTIAL ENERGY
For moving objects, we can easily calculate kinetic energy using the formula:
KE = (mass x velocity2)/2 or 1/2 mv2
Although mass and velocity both have great effects on kinetic energy, it is velocity more significantly determines kinetic energy.
Sample Problem
First we identify the information we are given in the problem:
mass = 45 kg
velocity = 13 m/sec
Next, we place this information into the kinetic energy formula:
KE = 1/2 mv2
KE = 1/2 (45 kg)(13 m/sec)2
Solving the equation gives a kinetic energy value of 3802.5 J
Note: the unit for energy is the same as for work: the Joule (J)
The kinetic energy of a boat is calculated at 52,000 J. If the boat has a mass of 39,000 kg, with what velocity is it moving?
We identify the information given in the problem:
KE = 52,000 J
mass = 39,000 kg
We now place the information into the kinetic energy formula:
KE = 1/2 mv2
52,000 J = 1/2 (39,000 kg)(v)2
52,000 J/(1/2 x 39,000 kg) = v2
Solving the equation gives a velocity value of 1.63 m/sec
Potential energy is the energy possessed by a body or an object due to its position. Potential energy, on the other hand, is energy of position, not of motion. The amount of potential energy possessed by an object is proportional to how far it was displaced from its original position. If the displacement occurs vertically, raising an object off of the ground let's say, we term this Gravitational Potential Energy. We can calculate the gravitational potential energy of an object with this formula:
GPE = weight x height
An increase in the weight of an object or the height to which it is raised will result in an increase in the potential energy the object possesses. Once the object is dropped, the potential energy begins to decrease due to reduced height, but we also now see an increase in kinetic energy because the velocity is also increasing.
A 37 N object is lifted to a height of 3 meters. What is the potential energy of this object?
Identify the information given to you in the problem:
weight = 37 N
height = 3 meters
Insert the information into the gravitational potential energy formula:
GPE = weight x height
GPE = 37 N x 3 meters
Solving the problem gives a potential energy value of 111 J.
Evaluation
GENERAL EVALUATION
WEEKEND ASSIGNMENT
THEORY
READING ASSIGNMENT
Precious seed BASIC SCIENCE FOR JUNIOR SECONDARY SCHOOLS BOOK 2 PAGE 118-122
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