Selina solutions concise physics class 8 icse Solutions for Chapter 4 energy

Exercises in Chapter 4 energy Grade 8

Numericals

Objective Questions

Short/Long Answer Questions

Questions in Numericals

Q1) A force of 30 N acts on a body and moves it through a distance of 5 m in the direction of force. Calculate the work done by the force.

Q2) A man lifts a mass of 20 kg to a height of 2.5 m. Assuming that the force of gravity on 1 kg mass is 10 N, find the work done by the man.

Q3) A body when acted upon by a force of 10 kgf moves to a distance 0.5 m in the direction of force. Find the work done by the force. Take 1 kgf = 10 N

Q4) Two bodies of same masses are placed at heights h and 2h. Compare their gravitational potential energy.

Q5) Find the gravitational potential energy of 2.5 kg mass kept at a height of 15 m above the ground. The force of gravity on mass 1 kg is 10 N.

Q6) The gravitational potential energy stored in a box of weight 150 kgf is 1.5 $\times$ $10^4$ . Find the height of the box. Take 1 kgf = 10 N

Q7) The potential energy of a body of mass 0.5 kg increases by 100 J when it is taken to the top of a tower from ground. If force of gravity on 1 kg is 10 N, what is the height of the tower?

Q8) A body of mass 60 kg is moving with a speed 50 $m\ s^{-1}$ . Find its kinetic energy.

Q9) A truck of mass 1000 kg, increases its speed from 36 $km\ h^{-1}$ to 72 $km\ h^{-1}$ . Find the increase in its kinetic energy.

Q10) A car is moving with a speed of 15 $km\ h^{-1}$ and another identical car is moving with a speed of 30 $km\ h^{-1}$ . Compare their kinetic energy.

Q11) A pump raises water by spending $4\times10^5\ J$ of energy in 10 s. Find the power of pump.

Q12) It takes 20s for a girl A to climb up the stairs while girl B takes 15 s for the same job.

(a) Compare the work done.

(b) Compare the power spent by then.

Questions in Objective Questions

Q1) Write true or false for each statement:

(a) A coolie does no work against the force of gravity while carrying luggage on a road.

(b) The energy stored in water of a dam is the kinetic energy

(d) work done by a boy depends on the time in which he does work

(e) power spent by a body depends on the time for which it does work.

Q2) Fill in the blanks

(a) Work is said to be done by a forte only when ____.

(b) Work done = force $\times$ ____.

(d) The SI. unit of energy is ____.

(e) The potential energy of a body is due to its _____.

(f) Gravitational potential energy U = massforce of gravity on unit mass $\times$ _____.

(g) Kinetic energy = ½ mass $\times$ _____

(h) Power P = ______ / time taken.

(i) The SI. unit of power is _____.

(j) I.H.P = _____

Q3) Match the following:

Column A	Column B
(a) A stone at aheight	(i) Power
(b) A moving ball	(ii) joule
(c) Energy	(iii) work done in 1 s
(d) Power	(iv) potential energy
(e) watt	(v) kinetic energy

Q4) Select the correct alternative

(a) the S.I. unit of work is

second
metre
joule
newton

(b) no work is done by a force if the body ...............................

moves in direction of force
Does not move
moves in opposite direction
none of these

same by both coolies
Is more by coolie A than by B
is less by coolie A than by B
nothing can be said

(d) the expression of power P is .................

P = mgh
P = P = 1/2 $Mv^2$
P = F $\times$ d
P = F $\times$ d/t

(e) I H.P. is equal to .............................

1 W
1 J
746 J
746 W

(f) when a boy doubles his speed, his kinetic energy becomes

half
double
Four times
no change

(g) A boy lifts a luggage from height 2m to 4m. The potential energy will become

half
Double
one-third
one-fourth

Questions in Short/Long Answer Questions

Q1) Define work.

Q2) When does a force perform work?

Q3) State two conditions when no work is done by a force.

Q4) In which of the following cases is work being done:

(a) A boy pushing a rock

(b) A boy climbing up the stairs

(d) A girl moving on the road.

Q5) A coolie is moving on a road with a luggage on his head. Does he perform work against the force of gravity? Give reasons for your answer.

Q6) The moon is revolving around the earth in a circular path. How much work is done by the moon?

Q7) Write the expression for work done by a force.

Q8) State the S.I. unit of work and define it.

Q9) State two factors on which the work done on a body depends

Q10) Define the term energy.

Q11) State the S.I. unit of energy.

Q12) Define 1 Joule of energy.

Q13) How is work related to energy?

Q14) What are two kinds of mechanical energy?

Q15) What is potential energy? State its unit.

Q16) Give one example of a body that has potential energy in each of the following

(a) Due to its position at a height.

(b) due to its elongated stretched state.

Q17) State two factors on which the potential energy of a body at a certain height above the ground depends.

Q18) Two bodies A and B of masses 10 kg and 20 kg respectively are at the same height above the ground. Which of the two has greater potential energy?

Q19) A bucket full of water is on the first floor of your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy?

Q20) Write the expression for gravitational potential energy explaining the meaning of the symbols used.

Q21) A body of mass m is moved from ground to a height h. If force of gravity on mass of 1 kg is g newton,

(a) find the force needed to lift the body.

(b) find the work done in lifting the body

Q22) Define the term kinetic energy. Give one example of a body which possesses kinetic energy.

Q23) State two factors on which the kinetic energy of a moving body depends.

Q24) Two toy cars - A and B of masses 200 g and 500 g respectively are moving with the same speed. Which of the two has greater kinetic energy?

Q25) A cyclist doubles his speed. How will his kinetic energy change: increase, decrease or remain the same?

Q26) Write the expression for the kinetic energy of a body explaining the meaning of the symbols used.

Q27) A ball of mass m is moving with a speed v. What is its kinetic energy?

Q28) Name the form of energy stored in a wound up spring of a watch.

Q29) Can a body possess energy even when it is not in motion? Explain your answer with an example.

Q30) Name the type of energy (kinetic or potential) possessed by the following:

(a) A moving cricket ball

(b) A stone at rest on the top of a building

(d) A moving bus

(e) A bullet fired from a gun

(f) Water flowing in a river

(g) A stretched rubber band

Q31) Give an example to show the conversion of potential energy to kinetic energy when put in use.

Q32) State the energy changes that occur in a watch spring while it unwinds.

Q33) Give reasons for the following:

(a) No work is done if a man is pushing against a wall.

(b) Hammer drives a nail into the wood only when it is lifted up and then struck.

(d) A teacher moving around in the class is doing work but a child standing and reading a book is not doing any work.

Q34) State the energy changes in the following while in use:

(a) An electric bulb

(b) An electric oven

(d) A microphone

(e) An electric motor

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Chapters in this book

Matter

Physical Quantities and Measurement

Force and Pressure

Energy

Light Energy

Heat Transfer

Sound

Electricity