9th Class Chapter 6 Physics Long Questions

h= 6 m

t= 10s

power of the pump = p=?

Since

F= w

= mg

200 x 10

=2000n

As we know that

p=w/t =F x s/t

But putting the values , we have

p= 2000 x6/10

p=m12000/10

P = 1200 W Ans

Efficiency of a system is the ratio of required form of energy obtained from a system as output to the total energy given to it as input.

Example:

Electric motors may be used to pump water, to blow air , to wash clothes, to drill holes, etc for that they use electricenergy. How good a machine is depends how much output we obtain from it by giving certain input .The ratio of useful output to input energy is very important to judge the working of machine

Mathematical form:

Efficiency =Required form of output/Total input energy x100

Height of h= 16 cm

each step = 0.16m

n= 25

t = 20s

p= ?

Since

F = w

= mg

= 50 x 10

= 500N

Height = h= 0.16 x 25 = 4m

reached by man

As we know that

P =w/t = F x s/t

By putting the values

P= 500 x y/20

= 2000/20

= 100w Ans

Wind energy has used as a source of energy for centuries. It has powered sailing ships across the oceans . It has been used by windmills to grind grain and pump water. More recently , wind power is used to turn wind turbines. When many wind machines are grouped together on wind farms , they can generate enough power to operate a power plant. In the United stated , some wind farms generate more than 1300 MW of electricity a day. In Europe, many wind farms routinely generate hundred megawatts or more electricitya day

The energy possessed by a body due to its motion is called kinetic energy.

Derivation:

Let a body of mass m is moving with velocity V. An opposing force F actingthrough a distance S brings it to rest. The body possesses kinetic energy and is capable to do work against opposing force F until all of its kinetic energy used up.

K.E of the body = Work done by it due to motion

K.E - FS

Vi = V

Vf =0

F = ma

a =-f/m

Since motion is opposed, hence , a is negative. Using 3rd equation of motion

2as =Vf² - Vi²

2 (-F/m)S =0² - V²

FS = +1/2 mv²

As we know that K.F is equal to the work done.

K.E =1/2 mv² ( proved )

" work is done when a force acting on a body displaces it in the direction of force".

Examples:

Explanation: energy cannot be destroyed however it can be converted into some other forms . For example, rub your hands together quickly. You will feel them warm. You have used your muscular energy in rubbing hands as a result heat is produced. In the process of rubbing mechanical energy is converted into heat energy. Processes in nature are the results of energy changes. For examples, some of the heat from the sun is taken up by water in the oceans. This increases thermal energy. Thermal energy causes water to evaporate from the surface to form water vapors. These vapors rise up and form clouds. As they cool down, they form water drops and fall down as rain. Potential energy changes to kinetic energy as the rain falls . This rain water may reach a lake or a dam. As the rain water flows down, its kinetic energy changes into thermal energy while parts of kinetic energy of flowing water is used to wash away soil particles of rocks known as soil erosion. During the interconversion of energy from one form to other forms, the total energy at any time remains constant .

Uses; we use fossil fuels such as coal, gas to heat our houses and run industry. They are usually hydrocarbons.

Example:

Parts: a heating system consists of

E=mc²

Where c is the speed of light( 3*10⁸ ms^-1)

Mathematically:

Power=P= work done / time taken

P=W/t