Review questions Basic Science Jss 2 Second Term

Subject :

Basic Science

TERM

SECOND TERM

Week

Week 9

CLASS

JSS 2 / Basic 8

WORK, ENERGY AND POWER

Energy is used up when work is done. The energy stored up in our muscular system is made available when we are to do  work. The stored up energy  (potential energy) is converted to kinetic energy as work is being done.

MEANING OF WORK, ENERGY AND POWER

Work

Work is done whenever a force is applied to cause a body to move. For instance when you lift a bucket from the ground to your head and stand at a point for several hours ; you have only done work by lifting the bucket of water from the ground  to your head ,you have done no work in carrying the bucket of water as long as you remain on the spot.

Energy

Energy is the ability to work by exerting a force to move an object through a distance. Anything that is capable of doing work has energy. For instance,a boy running a race of 100 meter has energy because he is able to move some distance. Work and energy are measured in the same unit as the JOULE.

Power

Power is defined as the time rate of doing work. If two boys   of the same weight are to climb a flight of stairs of the same height, the boy that gets to the top first is said to have the greater power. This is because he has done the work of moving his weight through that height at a shorter time.

Hence, in mathematical terms, power is expressed as: power=  workdone  or energy expended/time taken.

If the workdone  is in joules and the time is in seconds, then the power is watts.

1 watt=1 joule per second

Power can also be commonly expressed in kilowatts(KW), megawatt(MW) and horse power(h.p)

1 KW=1,000W

1MW=1,00 0,000W

1 h.p=746W

EVALUATION

1. Which of the following is not a source of energy? A. Coke   b. Coal   c. Smoke       d .Water
2. ————- is done when a force moves some things through distance.
3. Work  b. Energy  c. Movement   d. Effort
4. The energy obtained is cassava is ———————-
5. Mechanical   b. Chemical   c. Heat   d. Power
6. Abody which exerts force has ——————–
7. Strenght   b.pressure  c.Energy   d.Power
8. The light energy from the sun is called————–

6.The unit of energy is—————————–

SECTION B

1.Define the following

i.Work  ii Energy iii.Power

2.Give the unit of measurement of the following.

i.Work  ii Energy  iii Power

CONCEPT OF WORK,ENERGY AND POWER.

Work is the product of force and  distance moved in a given direction,and the quantity of workdone is always equal to the quantity of energy put in.

In science ,work is said to be done when a force can produce movement in a measured  direction ,i.e work=force(f)x distance(d) moved in the direction of the force(fxd).

Work can simply be defined as the product of distance moved and the force applied in the direction of movement.

Generally for any workdone, there must be energy input since energy is the capacity of any system or a body to do work. Both work and energy are measured in units called joules,named after a Scientist P.joules who carried out early studies on energy.

Force is that which changes a body’s state of rest or uniform motion in a straight line.It can be expressed as:

Force=Mass x acceleration

i.e F=m x a

where F =force,m =mass and a =acceleration.The unit of force is Newton (N)

If force=mass x acceleration

Work=mass x acceleration xdistance

SUN AS THE PRIMARY SOURCE OF ENERGY

The sun is the primary source of energy available for man. It does not  deplete, and cannot be used up.

The sun transmits solar energy to the earth in the form of light and heat and this solar energy can be transformed to other forms.

Solar energy is used by green plants to manufacture their food through the process of photosynthesis. The energy stored up in green plants is made available to man and animals when they eat plant products like yam, rice ,cassava, beans grasses and so on.

In addition, solar energy can be converted to other energies such as to chemical and electrical energy for heating in homes, boiling water and for lighting our homes and streets.

Hence, the sun is the primary source of  energy for life, all other sources are secondary as they are obtained from the sun.

Examples of secondary sources are

1. Food
2. Petrol
3. Wood,
4. Water

5.Chemical,

6.Magnet and

7.Battery.

FORMS OF ENERGY

There are many forms of energy which can be converted from one form to the other.These are some of the forms of energy.

1.Mechanical Energy

2.Kinetic Energy

3.potential energy

4.chemical energy

5.Atomic energy

6.sound energy

7.Electrical energy

8.Light energy

9.Heat energy and

10.Magnetic energy

EVALUATION

1.Mathematical expression of power is ——————

a.Newton/Distance   b.Force/Time   c.Workdone/Time   d.Mass/Density

2.Energy obtained in food is ——————–

a.Power  b.Food    c.Chemical   d.Solar   e.Nuclear

3.Mathematical expression for work done by a body is represented below as ——–a.W  =fxd   b.W=n xt  c.S=d xt  d.W=m xv  e.W=f Xj

4.The S.I unit of force is ————

a.N  b.NM   c.M   d.G

SECTION B

1.Name five different sources of energy.

2.Explain why the sun is the primary source of energy?

3.Name the different forms of  Energy.

POTENTIAL AND KINETIC ENERGY

Potential energy is the type of energy possessed by a body due to its position.A stone on the ground does not have energy so long as it is lying on the ground.The stone cannot be seen doing any work.However,if a stone is n a table and it placed on a table and it falls off,it can break a lamp on which it falls,the stone has done some work by virtue of its position.Therefore,when the stone is on the table,it has energy stored up as a result of its position.This energy increases as the height  of the table  increases and it decreases  as it falls towards the ground.When it reaches the ground ,it has zero potential energy.

KINETIC ENERGY

This is the energy possessed by a moving body.For example,a moving car, a man running, a falling orange, a fired bullet,a rolling ball and so on all possess kinetic energy.Both forms of energy are examples of a more general forms of energy called MECHANICAL ENERGY.

KINETIC ENERGY

Kinetic energy(K.E) is the energy possessed by an object due to its motion i.e movement energy √K.E=1/2mv2

Where M is the mass of  the moving object

V is the velocity of the object.

Potential energy is the energy possessed  by an obect by virtue of its position at rest ( its height).Potential  energy is stored energy.

P.E=m.g.h

M is mass of the object;

g is acceleration due to gravity; and

h is the height of the object from the ground.

Example1

If the mass of an object is 6kg and the height is 5m.Calculate the  potential energy .(Assuming g=10m/s2)

Solution

P.E=m.g.h

M=6k.g

h=5m

g=10m/s2

P.E=6x5x10

=300J

Example 11

If the kinetic energy of a ball moving at a velocity of 10m/s2 is 50KJ.What is the mass of the ball?

Solution

K.E=1/2mv2

K.E=50KJ=50,000J

Velocity=10m/s2

50,000=1/2x mx102

i.e 50,000=1/2x 100m

=1/2x100m

50,000=50m

50m==50,000

M=50,000/50

=1000Kg

EVALUATION

1.Energy is measured in ————— while distance  is measured in ———-

2.Mgh is a mathematical formula in basic science that is used to calculate ———–in a body

3.The energy possessed by a body by virtue of its position is called

a.potential  b.Kinetic   c.Food  d.Muscular

SECTION B

1.Explain the meaning of the  following.

i.Potential Energy   ii. Kinetic Energy.

2.What type of energy does the sun transmit  to the earth?

b.If a force of 50N is used to move an object through 20metres within 5seconds,Calculate the power of the  object.

2c. A man of mass 90kg is  moving at a constant velocity.He has kinetic energy of 2205J.Calculate the velocity at which he moves.