NCERT Class 9 • Science • Chapter "Work and Energy"

Work, Energy and Power

In science work is done only when a force produces a displacement in the direction of that force. A porter standing still with a heavy load does no work in the scientific sense, however tired he becomes, because there is no displacement in the direction of the force.

Work and Forms of Energy

• Work equals force multiplied by displacement in the direction of the force, and its SI unit is the joule.
• Work is zero when there is no displacement, or when the force is perpendicular to the displacement.
• Work is negative when the force acts opposite to the displacement, as friction does.
• Energy is the capacity to do work, and its unit is also the joule.
• Kinetic energy is the energy of motion and equals ½mv², so it rises with the square of speed.
• Potential energy is stored energy due to position or configuration; gravitational potential energy equals mgh.
• Other forms include heat, light, sound, chemical, electrical and nuclear energy.

Conservation and Power

• The law of conservation of energy states that energy can neither be created nor destroyed, only transformed from one form to another.
• In a freely falling body, potential energy keeps converting into kinetic energy while the total stays constant.
• A swinging pendulum, a hydroelectric dam and a burning fuel are all examples of energy transformation.
• Power is the rate of doing work or transferring energy, and its SI unit is the watt.
• One watt equals one joule per second; one kilowatt equals one thousand watts.
• The commercial unit of energy is the kilowatt hour, the energy used by a one kilowatt device in one hour.
• Electricity bills are charged in kilowatt hours, which are popularly called units by consumers.
• Energy efficient appliances do the same work while drawing less power, so they cost less to run.

Power — the rate at which work is done or energy is transferred, measured in watt.