Work Energy and Power: Let us understand the concept of SI units

Understanding the Concept, Formulae
& S.I Units of Work Energy and Power


Work results when a force acts upon an object to cause a displacement (or a motion) or, in some instances, to hinder a motion. We can say that when constant force F, acts on a particle while it moves through a displacement S, then the force is said to do work W on the particle given by.

work power and energy
Equation- W= F.S = FS cos

Where is the smaller angle between F and S.
Important points related to work-
• If the force and the displacement are in the same direction = 0 degree, therefore- W= FS
• If the force and displacement are perpendicular to each other, = 90 degree, then W= FScos90 degree.
• Work is a scalar quantity
• In SI system the absolute Standard Unit of Work is Joule whereas in CGS system the Unit of Work is Erg
• Work done by a constant force in moving from one point to another depends only on the initial and final positions and not on the actual path followed between two points.
• If the work done by a force during a round trip of a system is zero, the force is said to be conservative, otherwise it is called non-conservative. Force of friction is called non conservative force.


Energy can be defined as the capacity of the body for doing work, i.e. it is the total amount of work stored up in the body. Since energy of a body represents the total amount of work done, so it’s a scalar quantity.

Important points related to Energy-

• Energy can exist in different forms such as mechanical energy, heat energy, sound energy, electrical energy, light energy etc.
• Mechanical energy is of two types- kinetic energy and potential energy.
• Kinetic energy of a body is further the energy acquired by the body by virtue of its motion. If the body of mass M moves with velocity V, its kinetic energy is given by KE=1/2 m
• Potential energy of a body is the energy possessed by the body by virtue of its position or configuration
• If a body of mass m be raised through a vertical height against gravity then its gravitational potential energy will be PE= mgh
• Work done by a force acting on a body is equal to change in kinetic energy of a body. This is called as work-energy theorem of the body.


A small engine can do just as much work as a larger engine, but it takes longer to do it. Whereas the larger engine can do work at a faster rate. So, we can say that the rate at which work is done is called Power.
Power= Work done/ Time taken
Or Power= Energy transferred/ time taken

Power is measured in Joules per sec (J/s). or watts (W) If a body does 1J of useful work every second, it has a power output of 1W. If a body does 4000 J of work in 10 s, it has a power output of 400W. Larger powers are also given in kilowatts or megawatts.

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