**FORCE:** A force is that, which can change the motion or can change the velocity of motion i.e. accelerate of an object with mass. It may be pushing or pulling force.

As force has both magnitude and direction it is vector quantity.

The S.I. unit of force is Newton as the definition originates from Newton’s second law of motion, F=ma. Where F= force, m=mass, a= acceleration speed.

Now if, m=1 kg, a=1m/s^{2}, then F=1 Newton. Hence one Newton force means that force which accelerates the speed 1m/s^{2} of a 1kg mass object.

Gravitational unit of force is Kilogram-weight (kg-wt), it is that force for which acceleration of 9.8m/s^{2}of a 1kg mass object.

1 kg-wt=9.8N.

**MASS: **The mass of a body is nothing but the quantity of matter of that body. This is the fundamental property of a body for which it resists the net force.

The S.I. unit of mass is Kilogram (kg).

**WEIGHT**: Due to gravity, when an object feels a force which pulls downward.

Unit of weight is Newton (N). Gravitational unit of weight is kg-wt/kg-f.

W= m X g

So, the weight of 1 kg mass object is 1X9.8= 9.8 Newton.

W= m X g

**WORK:** When an object displaced by a force, we can say work done has occurred upon the object.

If the object moves in the direction of force, work is done by the force. If a move opposite the direction, work is not done by that force, done by another external force.

Work done = Force X Distance moved in direction of force.

FORCE = F

If an object moves from A to B by a force F, then work done = F X S. But if force, F, move the object with an angle, ϴ, then,

Work done = F X S cosϴ.

The S.I. unit of work done is Joule.

If F=1, S=1 then work done = 1 N-M/ J. So, 1 joule means 1 Newton force moves an object to 1 meter.

Gravitational unit of work is m-kg wt or m-kg.

So, 1 m-kg = 9.8 N-m or joule.

**ENERGY:** Energy is nothing but the capacity of doing work.

As per physics, energy never created or destroyed, it gets converted from one form to another or transferred to other objects. The basic energy source in this planet is from the sun rays.

The S.I. unit of energy is joule or N-m same as work done.

Mainly the energy is categorized by two types, i) potential energy and ii) kinetic energy.

i) **Potential energy:*** There are two types of potential energy namely a) gravitational potential energy and b) elastic potential energy.
a) Gravitational potential energy: This is the stored energy of an object when lifted against gravity.
b) Elastic potential energy: This is that stored energy in elastic materials of their stretching or compressing, like rubber, spring etc.
ii) Kinetic energy:* The energy due to motion is called kinetic energy. It may be vertical or horizontal motion.

Suppose a weight is lifted against gravity by external force. The work done by this external force gets stored as potential energy (gravitational potential energy).

If that weight falls to do some work, the energy is kinetic energy.

**POWER:**Power is nothing but the rate of doing work or energy transferring, that is, work done in a specified time.

Power = work done/taken time or, P = W/T

The S.I. unit of power is Watt, denoted as W.

Instantaneous power, P = dw/dt j/s or watt.

Needs to be remember, 1 W-H = 3600 joule and 1H.P. = 735.5 watt.