General Physics

Force, Energy and Power

Force

Force is a push or a pull which changes a body's state of rest or of uniform motion in a straight line.

SI Unit

Its SI units are Newtons (N). A Newton is the force which when applied to a mass of 1 Kilogram (Kg) gives it an acceleration of 1 meter per second squared (m/s2).

1 KiloNewton (KN) =1000N (103 N)

1 MegaNewton (MN) = 1,000,000 N (106 N)

1 GigaNewton (GN) = 1,000,000,000 N (109 N)

1 Tera Newton (TN) = 1,000,000,000,000 N (1012 N)

1 miliNewton (mN) = 0.001 N (10-3 N)

1 microNewton (µN) = 0.000001 N (10-6 N)

1 nanoNewton (nN) = 0.000000001 N(10-9 N)

1 Pico Newton (PN) = 0.000000000001 N(10-12 N)

1 Ferre Newton (FN) = 0.000000000000001 N(10-15 N)

Force = mass x acceleration

F= ma

Example

Calculate the force required for a mass of 200 kg to be given an acceleration of 0.25 m/s2.

F= ma

F = 200 X 0.25

F = 50N

Moment of a Force and Torque

When loosening a tight nut, the magnitude of the force decreases with an increase in the length of the spanner used. That is, if you use a short spanner you will need to use more force than when you use a long one.

The Product of the force and the perpendicular distance of its line of action from the fulcrum/axis define the Moment of a Force.

The moment of a force measures Torque, the turning effect of a force.

When a force (F) in Newtons is applied to a spanner and it causes a turning effect on the nut, that action will be a circular movement. Considering the point of application of the force (F) to be Radius (r),

Torque (T) = force (F) x Radius (r)

T=Fr

The SI units are Newton metre (N m)

The Unit of Work and Energy

Work is when an applied force acts on a body and moves the body through a given  distance in the direction of force. It is found by multiplying force by the distance through which force moves.

wd=Fd

Energy is the ability to do work. It can be thought of as stored work.

The work done (wd) in one revolution = force (F) x 2πr where 2πr is the distance through which the point of application of the force travels in one revolution.

wd=F 2πr

wd=2πFr

Fr being Torque

wd=T2π

Types of Energy

Kinetic energy (Ke), possessed by moving objects.

Ke= ½(mass) x (velocity)2

Ke = ½mv2

Potential energy (Pe), this type of energy is possessed by objects above the ground. When the object falls, the energy is released and transformed into other types of energy.

Pe = mass(m) x acceleration due to gravity(g) x height (h)

Pe=mgh

Mechanical energy, a combination of Potential energy and Kinetic energy

Chemical energy, the energy locked up in a chemical compound.

Heat energy, the energy possessed by hot bodies.

Both Work and Energy have the same unit, the Joule.

A Joule is the work done when a point of application of a force of 1 N moves through a distance of 1 meter in the direction of force.

Units of Power

Power is the rate of doing work. The SI units of power are Watts.

1 Watt = 1 Joule per second

Sometimes a Watt is inconveniently small and the KiloWatt is used instead.

1 KW = 1000W

For larger powers, a MegaWatt is used.

1 MW = 106 W

It is also convenient to express the work in Kilo Watt Hour when dealing with large amount of Work or energy.

1 KW = 1000W

1 hour = 60 second x 60 minutes

1 KWH = 1000 x 60 x 60

1 KWH = 3,600,000 Joules

Power = work done/time taken

But Work done = Fd

Power=FdT

If Torque (T) in Newton meters is exerted on the shaft and n be the speed of the shaft in one revolution per second then,

from  Power = work done/time taken

Power = work done in one revolution x the number of revolutions

wd in one rev = F2πr = T2π

Power = 2πnT

But 2πn = omega (ѡ) and it is the angular velocity.

Therefore Power = ѡT