VIDEO SUMMARY
CBSE Class 9 || Physics || Work and Energy || Animation || in English Sreducators.com
Objectives*Define work and energy
*Write different forms of energy
*Write the expression for kinetic and potential energy
*Understand the laws of conservation of energy
*Define power
*Write the units of work, energy, and power Work
We do lots of work like playing, studying, cycling, running, etc. All these works require a lot of energy. But the scientific definition of work may involve very little work. Let's see how. Look at this huge trunk. We work hard to push it, but the trunk does not move despite all the effort. We get tired, however, we have not done any work on the trunk as there is no displacement of the trunk. We can say that we did hard work, but in science, work is not done on the trunk. We use and define the term "work" differently in science. For example, if we climb up a cliff, then according to science, we have done lots of work.
Scientific conception of work: For work to be done, these two conditions should be satisfied. First, a force should act on an object. Second, the object must be displaced. Let's take a look at a few situations:
If we pull a table and the table moves through a distance, we have exerted a force on the table and it is displaced. So both the conditions are satisfied here, therefore work is done.
Similarly, while playing cricket, if we hit a ball with a bat, then work is done because force is exerted on the ball and the ball is displaced.
Work done by a constant force
To understand the concept of work in science, let us consider a case where the force is acting in the direction of displacement. Let a constant force F act on an object. Let the object be displaced through a distance s in the direction of the force. Let W be the work done. We define work to be equal to the product of the force and displacement. Work done is equal to force multiplied by displacement, or W = F x s. Thus, work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force. Work has only magnitude and no direction. If F = 1 N and s = 1 m, then the work done by the force will be 1 Nm. The unit of work is Newton meter or Joule. Thus, one Newton meter or one Joule is the amount of work done on an object when a force of one Newton displaces it by one meter along the line of action of the force.
Example: Let's now find the work done when force applied and displacement is given. A student lifts a school bag from the ground and puts it on his shoulders 1.5 meters above the ground. A force of 5 Newton is acting on the object. Calculate the work done by him on the school bag. We can see that here force applied is 5 Newton and displacement s is 1.5 meters. We know that work done is equal to force multiplied by displacement. Therefore, we get work done is equal to 7.5 Newton meter or 7.5 Joule.
Energy
We now know that to do any kind of work, we need energy. We must have energy to accomplish work. But the question is, where does this energy come from? The sun is the biggest natural source of energy to us. Many of our energy sources are derived from the sun. Energy sources are classified into two groups: nonrenewable and renewable. Most of our energy comes from nonrenewable energy sources. Coal, petroleum, natural gas, propane, and uranium are nonrenewable energy sources. They are used to make electricity, to heat our homes, to move our cars, and to manufacture all kinds of products. These energy sources are called nonrenewable because their supplies are limited. Renewable energy sources include biomass, geothermal energy, hydropower, solar energy, and wind energy. They're called renewable energy
Potential Energy
Potential energy is the energy stored in an object as a result of its vertical position or height. Let's find out the expression for the gravitational potential energy of an object at a height.
Expression for Gravitational Potential Energy
An object of mass m raised through a height h from the ground gains energy equal to the work done on it.
The work done on the object against gravity is given by w = force × displacement. The minimum force required to raise the object is equal to its weight, which is mg. Therefore, the work done is w = mg × h. Since work done on the object is equal to mgh, the object gains potential energy of mgh units.
Also, the work done by gravity depends on the difference in vertical heights of the initial and final positions of the object, not on the path along which the object is moved.
Conversion of Energy
One form of energy can be converted into another form. For example, when we switch on a bulb, electric energy is converted into light energy. Similarly, when we plug on the TV, electric energy converts into heat energy, sound energy, and light energy.
The law of conservation of energy states that whenever energy gets transformed, the total energy remains unchanged. Energy can neither be created nor destroyed. The total energy before and after the transformation always remains the same.
Example: Free Fall of an Object
Let's consider a simple example of an object of mass m falling freely from a height h.
At the start, the potential energy is mgh and the kinetic energy is zero because the velocity is zero.
As the object falls, its potential energy decreases while its kinetic energy increases.
When the object is about to reach the ground, the potential energy is the least (zero) and the kinetic energy is the largest.
The sum of the potential energy and kinetic energy of the object remains constant at all points: mgh + 1/2mv2 = constant.
During the free fall of the object, the decrease in potential energy at any point in its path is equal to the increase in kinetic energy at that point.
Rate of Doing Work
Power is defined as the rate of doing work or the rate of transfer of energy. If an agent does work w in time t, then power is given by p = w/t.
The unit of power is watt (W). One watt is the power of an agent that does work at the rate of one joule per second.
Commercial Unit of Energy
The unit of energy is joule (J), but for large quantities of energy, we use a bigger unit called kilowatt-hour (kWh).
One kilowatt-hour is the energy used in one hour at the rate of 1000 joules per second (one kilowatt). It is equal to 3.6 × 10^6 joules.
Did You Know?
James Prescott Joule, an outstanding British physicist, is best known for his research in electricity and thermodynamics. He formulated a law for the heating effect of electric current and verified experimentally the law of conservation of energy. The unit of energy and work called joule is named after him.
Summary
Here's a summary of what we've learned:
Work done on an object is the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force.
An object possessing the capability to do work is said to possess energy.
An object in motion possesses kinetic energy, given by 1/2mv².
Potential energy is the energy possessed by a body due to its position is given by mgh