ICSE Class 7 Physics · Selina Solutions Concise Physics Class 7 ICSE · 39 solved questions
When we rub our hands :
mechanical energy changes into heat energy.
Reason — When we rub our hands we feel heat i.e. mechanical energy changes into heat energy.
A ball rolling on the ground possess :
kinetic energy
Reason — A body in motion (rolling ball) possess kinetic energy.
The energy stored in an electric cell is
chemical energy
Reason — The energy stored in an electric cell is chemical energy.
When a bulb lights up on passing current, the change of energy is :
from electrical energy to heat and light energy
Reason — When a bulb lights up, electrical energy changes into heat and light energy.
The correct statement is :
Both work and energy have the same units
Reason — The S.I. units of both work and energy is joule(J).
According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system :
remains the same
Reason — According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system remains constant.
Water is lifted from ground floor to overhead tank at the second floor. The work done is stored in the form of :
potential energy
Reason — When water is lifted from the ground floor to an overhead tank, work is done against gravity which gets stored in the water in the form of gravitational potential energy due to its increased height.
It is not kinetic energy because the water is not in motion when stored, and it is not chemical energy since no chemical change occurs.
Hence, the work done is stored in the form of potential energy.
Assertion (A): Mechanical energy of a freely falling body increases as it falls.
Reason (R): Potential energy is converted into kinetic energy during free fall.
A is false but R is true
Explanation
Assertion (A) is false because in the case of a freely falling body (neglecting air resistance), the total mechanical energy remains constant. As the body falls, its potential energy decreases while its kinetic energy increases by an equal amount, so there is no increase in total mechanical energy.
Reason (R) is true because during free fall, the body loses potential energy and gains kinetic energy; that is, potential energy is converted into kinetic energy.
Therefore, assertion is false but reason is true.
Assertion (A): A stretched bow has kinetic energy.
Reason (R): Stretching stores potential energy.
A is false but R is true
Explanation
Assertion (A) is false because a stretched bow is not in motion, and kinetic energy is associated only with motion. Instead, the stretched bow stores potential energy due to its deformation, not kinetic energy.
Reason (R) is true because when a bow is stretched, potential energy is stored in it due to its deformation (elastic potential energy).
Therefore, assertion is false but reason is true.
Fill in the blanks:
(a) An electric fan converts electrical energy into ............... energy.
(b) Cooking gas converts ............... energy into heat energy.
(c) Energy possessed by a compressed spring is ............... energy.
(d) The ability to do work is called ............... .
(e) The energy possessed by a body due to its position is called ............... energy.
(f) The energy possessed by a body due to its motion is called ............... energy.
(g) Green plants convert ............... energy into chemical energy.
(h) The S.I. unit of energy is ............... .
(i) An object falling freely from the roof of a multistorey building has ............... and ............... energy when halfway down the building.
(a) An electric fan converts electrical energy into mechanical energy.
(b) Cooking gas converts chemical energy into heat energy.
(c) Energy possessed by a compressed spring is potential energy.
(d) The ability to do work is called energy .
(e) The energy possessed by a body due to its position is called potential energy.
(f) The energy possessed by a body due to its motion is called kinetic energy.
(g) Green plants convert light energy into chemical energy.
(h) The S.I. unit of energy is joule.
(i) An object falling freely from the roof of a multistorey building has potential energy and kinetic energy when halfway down the building.
Write true or false for each statement:
(a) A man going up has potential energy and kinetic energy both.
(b) A gum bottle lying on a table has no energy.
(c) In an automobile engine, chemical energy changes into mechanical energy.
(d) Potential energy changes into kinetic energy when it is put to use.
(e) One form of energy cannot be converted into another form.
(f) There is always some loss of energy in conversion from one form of energy to another form, so the total energy is not conserved.
(g) The energy of flowing water can be converted into electric energy (electricity).
(a) True
(b) False
Correct Statement — A gum bottle lying on a table has potential energy.
(c) True
(d) True
(e) False
Correct Statement — One form of energy can be converted into another form.
(f) False
Correct Statement — The total energy is always conserved in each transformation of energy.
(g) True
Match the following:
| Column A | Column B |
|---|---|
| (a) Running water | (i) heat energy |
| (b) Burning | (ii) vibrations |
| (c) Energy | (iii) atom bomb |
| (d) Sound energy | (iv) kinetic energy |
| (e) Nuclear energy | (v) joule |
| Column A | Column B |
|---|---|
| (a) Running water | (iv) kinetic energy |
| (b) Burning | (i) heat energy |
| (c) Energy | (v) joule |
| (d) Sound energy | (ii) vibrations |
| (e) Nuclear energy | (iii) atom bomb |
Define the term energy.
Energy of a body is defined as its capacity to do work.
State the unit of energy and define it?
The S.I. unit of energy is joule (J).
A body is said to possess an energy of one joule if one joule work is done to bring the body in that state i.e. a force of 1 newton moves the body by a distance of 1 metre in the direction of force.
Name five different forms of energy.
The five different forms of energy are:
What are the two kinds of mechanical energy?
The two kinds of mechanical energy are:
What is potential energy? State its unit.
Potential energy of a body is the energy possessed by it due to its state of rest or position. It is equal to the work done in bringing the body to that state of rest or position.
Its unit is joule(J).
Give one example of a body that has potential energy, in each of the following:
(i) due to its position,
(ii) due to its state.
(i) A hammer at a height has potential energy stored in it because of its position at a height.
(ii) A wound up watch spring has potential energy because of the wound up state of its coils.
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?
Body B has greater potential energy.
Reason — Both bodies, A and B are at the same height above the ground. Value of g is also same for both. As the mass of body B (20 kg) is greater than that of body A (10 kg) therefore potential energy of body B is greater.
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?
The bucket kept on second floor has greater potential energy.
Reason — The two buckets are identical containing the same amount of water so their mass is same. Value of g is also same for the two buckets. As the bucket on second floor is at a greater height hence its potential energy is also greater.
Define the term kinetic energy. Give one example of a body which possesses kinetic energy.
Kinetic energy of a body is the energy possessed by it due to its state of motion.
Example : A fast moving stone has kinetic energy which has the capacity of breaking a window pane when it strikes on it.
State two factors on which the kinetic energy of a moving body depends.
The two factors on which the kinetic energy of a moving body depends are:
Two toy-cars A and B of masses 500 g and 200 g respectively are moving with the same speed. Which of the two has greater kinetic energy?
The toy car A has greater kinetic energy than toy car B.
Reason — The two toy cars A and B are moving with the same speed. As the toy car A has greater mass (500 g) than toy car B (200 g) thus its kinetic energy is also greater.
Hence, car A has greater kinetic energy.
A cyclist doubles his speed. How will his kinetic energy change: increases, decreases or remains the same?
Kinetic energy of cyclist will increase as speed is doubled because more the speed of the body, more is its kinetic energy.
Name the form of energy which a wound up watch spring possesses.
Potential Energy.
Name the type of energy (kinetic or potential) possessed by the following:
(i) A moving cricket ball.
(ii) A stone at rest on the top of a building.
(iii) A compressed spring.
(iv) A moving bus.
(v) A bullet fired from a gun.
(vi) Water flowing in a river.
(vii) A stretched rubber band.
(i) A moving cricket ball has kinetic energy due to its state of motion.
(ii) A stone at rest on the top of a building has potential energy due to its raised position.
(iii) A compressed spring has potential energy due to its compressed state.
(iv) A moving bus has kinetic energy due to its state of motion.
(v) A bullet fired from a gun has kinetic energy due to its state of motion.
(vi) Water flowing in a river has kinetic energy due to its state of motion.
(vii) A stretched rubber band has potential energy because of its stretched position.
State the energy changes that occur in the following:
(i) The unwinding of a watch spring.
(ii) Burning coal while operating a steam engine.
(iii) Lighting of a torch bulb.
(iv) An electric generator (or dynamo).
(i) During the unwinding of a watch spring, the potential energy changes into kinetic energy.
(ii) In a steam engine chemical energy of coal first changes into heat energy which is transferred to steam. Then the heat energy of steam changes into mechanical energy which causes the train to move.
(iii) When torch bulb is lighted, chemical energy of batteries change into light and heat energy.
(iv) In an electric generator (dynamo) mechanical energy changes into electrical energy.
A ball is made to fall freely from a height. State the kind/kinds of energy possessed by the ball when it is:
(a) at the highest point
(b) just in the middle
(c) at the ground.
(a) Potential energy
(b) Potential energy and kinetic energy
(c) Kinetic energy
State two factors on which the potential energy of a body at a certain height above the ground depends.
The two factors on which the potential energy of a body at a certain height above the ground depends are:
Mass of the body — Greater the mass of the body, greater is the potential energy of the body.
Its height above the ground — Higher the height of the body above the ground, more is the potential energy.
Can a body possess energy even when it is not in motion? Explain your answer with an example.
Yes, a body can possess energy even when it is not in motion.
Example — A stone at rest placed at a height above the ground has potential energy. Even though the body is not in motion (kinetic energy = 0), but when a body is raised to a height say h possess potential energy because of its raised position.
Give one example to show the conversion of potential energy to kinetic energy when put in use.
A stone placed at a top of building has potential energy due to its height and when it is dropped from the top of the building it begins to fall. When it begins to fall the potential energy stored in it changes into kinetic energy. So the falling stone has kinetic energy.
Energy can exist in several forms and may change from one form to another. Give two examples to show the conversion of energy from one form to another.
The two examples that show the conversion of energy from one form to another are:
In an electric fan, the electrical energy changes into mechanical energy that rotates the blades of a fan.
In a glowing bulb, the electrical energy changes into heat energy and light energy.
Give one relevant example for each of the following transformations of energy:
(i) Electrical energy to heat energy
(ii) Electrical energy to mechanical energy
(iii) Electrical energy to light energy
(iv) Chemical energy to heat energy
(v) Chemical energy to light energy.
(i) Electric iron
(ii) Electric motor
(iii) Glowing bulb
(iv) Burning of wood
(v) Bursting of crackers
What do you mean by conservation of mechanical energy? State the condition when it holds.
Whenever there is an interchange between the potential and kinetic energy, the total mechanical energy remains constant. This is the principle of conservation of mechanical energy.
The condition when it holds is that there should be no frictional forces.
Give one example to show that the sum of potential energy and kinetic energy remains constant if friction is ignored.
During the vertical fall of a ball downwards from a height, the sum of potential energy and kinetic energy remains constant at each point of its path, if friction is ignored.
State the changes in form of energy while producing hydro-electricity.
Energy changes while producing hydro-electricity are:
Potential energy (Water stored in dam) ⟶ Kinetic energy (Water from dam falls on turbine) ⟶ kinetic energy (of blades of turbine) ⟶ Electrical energy (Rotating turbine rotates the generator)
A truck starts from rest on a plane road. What are the possible energy changes taking place while the truck is in motion?
The possible energy changes taking place while the truck is in motion are:
Read the clues across and clues downwards and fill up the blank squares.
Across :
Down :
The solved crossword puzzle is given below:
Sushila goes to a park and rides a swing. When she reaches the highest point, she pauses for a fraction of a second before coming down. At the lowest point she moves the fastest. She keeps swinging without anyone pushing again and again for some time. She does not know the reason.
Answer the following :
(i) Which mechanical energy is maximum at the highest point ?
(ii) Which mechanical energy is maximum at the lowest point ?
(iii) Is mechanical energy generated again when she swings back up ?
(iv) Give one condition when the swing may finally stop with reason.
(v) What information is given by the passage about energy ?
(i) At the highest point, potential energy is maximum because the height of the swing is greatest, while the speed is momentarily zero.
(ii) At the lowest point, kinetic energy is maximum because the speed of the swing is greatest and height is minimum.
(iii) No, mechanical energy is not generated again. It is only converted from kinetic energy to potential energy and from potential energy to kinetic energy during the motion.
(iv) The swing will finally stop due to air resistance and friction at the pivot. These forces gradually dissipate mechanical energy as heat, reducing the motion.
(v) The passage shows that mechanical energy is continuously transformed between potential and kinetic energy and is gradually lost due to external resistive forces.
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