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Science Question Answer (3 Marks)

Work and Energy · Maharashtra Board STD 9 (MSBSHSE - Semi English Medium). 24 questions.

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Question Answer (3 Marks)

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24 questions · self-marked practice — reveal the answer and mark yourself.

Question 13 Marks
A body of mass $2\ kg$ is thrown vertically upwards with an initial velocity of $20\ m / s$. What will be its potential energy at the end of $2\ s$ ? (Assume $g =10 m / s ^2$ ).
Answer
Mass of the body, $m =2 kg$
Initial velocity, $u=20 m / s$
Acceleration due to gravity, $g=10 m / s ^2$
Height reached $=h$
Time, $t =2 s$
Using second equation of motion
$h =ut+\frac{1}{2} gt^2$
$=20 \times 2+\frac{1}{2}(-10) \times 2^2$
$=20 m$
PE after $2 s= m \times g \times h$
$=2 \times 10 \times 20=400 J$
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Question 23 Marks
A man drops a $10\ kg $rock from the top of a $5\ m$ ladder. What is its speed just before it hits the ground? What is its kinetic energy when it reaches the ground? $\left(8=10 m / s ^2\right)$
Answer
Mass of rock $=10 kg$
Height of ladder, $h =5 m$
Initial velocity of rock, $u=0$
Final velocity v
$g=10 m / s^2$
using third equation of motion
$v^2-u^2=2 g h$
$v^2-0^2=2 \times 10 \times 5$
$v=10 m / s$
$\text{KE}=\frac{1}{2}\text{mv}^2$
$=\frac{1}{2}\times10\times10^2=500\text{J}$
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Question 33 Marks
Explain how, the total energy a swinging pendulum at any instant of time remains conserved. Illustrate your answer with the help of a labeled diagram.
Answer
Initially the pendulum is at rest. The bob is pulled to one side to position B to give it potential energy due to higher position of B and then released, the bob starts swinging:
  1. When the bob is at position B, it has potential energy but no kinetic energy.
  2. As the bob starts moving down from position B to A, its potential energy starts decreasing and kinetic energy starts increasing.
  3. When the bob is at position A, it has maximum kinetic energy and zero potential energy.
  4. As the bob starts moving down from position A to C, its kinetic energy starts decreasing and potential energy starts increasing.
  5. On reaching the extreme position C, the bob stops for a very small instant of time and bob maximum potential energy and zero kinetic energy.
Therefore at extreme positions B and C bob has only potential energy and at A it has only kinetic energy and at other intermediate positions bob has both kinetic
and potential energy. Thus the total energy of the pendulum is same for any instant of time (conserved).
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Question 43 Marks
A man weighing 500N carried a load of 100N up a fighter of stairs 4m high in 5 seconds. What is the power?
Answer
Weight of the man = 500N
Weight of the load = 100N
Total weight = 600N
Height of stairs = 4m
Time taken = 5s
Work done = mg × h
= weight × h
= 600 × 4
$\text{Power}=\frac{\text{Work done}}{\text{Time taken}}$
$=\frac{600\times4}{5}=480\text{W}$
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Question 53 Marks
A weightlifter is lifting weights of mass $200\ kg$ up to a height of $2$ metres. If $g = 9.8\ ms^{-2}$​​​​​​​, Calculate:
  1. Potential energy acquired by the weights.
  2. Work done by the weightlifter.
Answer
Mass, $m=200\ kg$
Height, $h =2\ m$
Acceleration due to gravity, $g=9.8 m / s ^2$
Potential energy $= m \times g \times h$
$=200 \times 9.8 \times 2=3920\ J$
Work done is against gravity = Potential energy gained by the weights, therefore
Work done $W = m \times g \times h$
$=200 \times 9.8 \times 2=3920\ J$
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Question 63 Marks
What is the difference between gravitational potential energy and elastic potential energy? Give one example of a body having gravitational potential energy and another having elastic potential energy.
Answer
Gravitational potential energy: The potential energy due to the position of the body above the ground is gravitational potential energy.
Example: Water in the overhead tank has gravitational potential energy due to its height above the ground.
Elastic potential energy: The potential energy due to change in shape and size of the body is elastic.
Example: A stretched rubber has elastic potential energy due to change in its shape.
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Question 73 Marks
If the work done by a force in moving an object through a distance of 20cm is 24.2J, what is the magnitude of the force?
Answer
Work done, W = 24.2J
Distance, s = 20cm = 0.2m
Force, F
W = F × s
24.2 = F × 0.2
$\text{F}=\frac{24.2}{0.2}=121\text{N}$
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Question 83 Marks
A ball falls to the ground as shown below:
  1. ♦ Potential energy = 80 J, Kinetic energy = 0
  2. ♦ Kinetic energy = 48J
  3. ♦ Potential energy = 0
  1. What is the kinetic energy of ball when it hits the ground?
  2. What is the potential energy of ball at B?
  3. Which law you have made use of in answering this this question?
Answer
Since from point A total energy = PE + KE = 80J
And according to the law of conservation of energy Total energy remains constant.
  1. PE = 0
Total energy = PE + KE = 80J

KE = 80J

PE = 80J
  1. At point B KE = 48J
Total energy = PE + KE = 80J

PE = 80J

KE = 80 - 48 = 32J
  1. Law of conservation of energy.
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Question 93 Marks
Name the energy transfers which occur when:
  1. An electric bell rings.
  2. Someone speaks into a microphone.
  3. There is a picture on a television screen.
  4. A torch is on.
Answer
  1. Electrical energy to sound energy.
  2. Sound energy to electrical energy.
  3. Electrical energy to light (and heat) energy.
  4. Chemical energy to electrical energy to light energy (and heat energy).
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Question 103 Marks
Give one example each in which a force does (a) positive work (b) negative work, and (c) zero work.
Answer
  1. Positive work: Work done by the force applied by a person on a ball that is thrown upwards.
  2. Negative work: Work done by gravitational force of earth on a ball thrown upwards.
  3. Zero work: Work done by gravitational force of earth on a box that is sliding horizontally on the ground.
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Question 113 Marks
A horse and a dog are running with the same speed. If the weight of the horse is ten times that of the dog, what is the ratio of their kinetic energies?
Answer
$\text { Speed horse }=\text { Speed of dog }=v$
$\text { Weight of dog }=m_d g$
$\text { Weight of horse }=m_n g$
$=10 \times m_d g$
Where, $m_n$ and $m_d$ are the masses of horse and dog respectively.
$\text{KE}_\text{h}=\frac{1}{2}\text{m}_\text{h}\text{v}^2$
$\text{KE}_\text{d}=\frac{1}{2}\text{m}_\text{d}\text{v}^2$
$\frac{\text{KE}_\text{h}}{\text{KE}_\text{d}}=\frac{\text{m}_\text{n}\text{v}^2}{\text{m}_\text{d}\text{v}^2}=\frac{\text{m}_\text{n}}{\text{m}_\text{d}}=\frac{\text{m}_\text{h}\text{g}}{\text{m}_\text{d}\text{g}}=\frac{10\times\text{m}_\text{d}\text{g}}{\text{m}_\text{d}\text{g}}=\frac{10}{1}$
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Question 123 Marks
Explain by an example what is meant by potential energy. Write down the expression for gravitational potential energy of a body of mass m placed at a height h above the surface of the earth.
Answer
Potential energy: The energy of a body due to its position or change in its shape is known as its potential energy.
Example: A stretched rubber has potential energy due to change in its shape and water in the overhead tank has potential energy due to its height above the ground.
PE = m × g × h
where, PE is the potential energy of the body.
m is the mass of the body.
g is the acceleration due to gravity.
h is the height above the surface of earth.
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Question 133 Marks
When do we say that work is done? Write the formula for the work done by a body in moving up against gravity. Give the meaning of each symbol which occurs in it.
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Question 143 Marks
Derive the relation between commercial unit of energy $(kWh)$ and SI unit of energy $(joule)$.
Answer
$1 \text { kilowatt-hour }=1 kW \text { for } 1 \text { hour }$
$=1000 W \text { for } 1 \text { hour }$
$\text { But } 1 \text { watt }=\frac{1 \text { joule }}{1 \text { second }}$
$1 \text { kilowatt-hour }=\frac{1000 \text { joule }}{1 \text { second }} \text { for } 1 \text { hour }$
$=1000 \frac{\text { joule }}{\text { second }} \text { for } 3600 \text { seconds }$
$=1000 \times 3600=3.6 \times 10^6 \text { joule }$
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Question 153 Marks
A ball of mass $0.5\ kg$ slows down from a speed of $5\ m/s$ to that of $3\ m/s$. Calculate the change in kinetic energy of the ball. State your answer giving proper units.
Answer
Mass of ball, $m = 0.5\ kg$
Speed $v_1 = 5\ m/s$
Speed $v_2 = 3\ m/s$
$\text{KE}_1=\frac{1}{2}\text{mv}_1^2=\frac{1}{2}\times0.5\times5^2$
$\text{KE}_2=\frac{1}{2}\text{mv}_2^2=\frac{1}{2}\times0.5\times3^2$
Change in $\text{KE}=\Big(\frac{1}{2}\times0.5\times5^2\Big)-\Big(\frac{1}{2}\times0.5\times3^2\Big)$
$=\frac{1}{2}\times0.5\times(5^2-3^2)$
$=0.25\times16=4\text{J}$
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Question 163 Marks
What are the various forms of energy?
Answer
Various forms of energy are:
  1. Kinetic energy.
  2. Potential energy.
  3. Chemical energy.
  4. Heat energy.
  5. Light energy.
  6. Sound energy.
  7. Electrical energy.
  8. Nuclear energy.
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Question 173 Marks
Find the momentum of a body of mass 100g having a kinetic energy of 20J.
Answer
$\text{KE} = 20\text{J}$
$\text{Mass} = 100\text{g} = 0.1\text{kg}$
$\text{KE}=\frac{1}{2}\text{m}\times\text{v}^2$
$20=\frac{1}{2}\times0.1\times\text{v}^2$
$\text{v}^2=400$
$\text{v}=20\text{m}/\text{s}$
$\text{Momentum} =\text{m}\times\text{v}$
$= 0.1 × 20 = 2\text{kg}\ \text{m}/\text{s}$
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Question 183 Marks
What is the meaning of the symbol kWh? What quantity does it represent?
Answer
The unit kWh stands for kilowatt-hour. One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having power of 1 kilowatt is used for 1 hour. It represents the amount of electrical energy consumed in 1 hour.
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Question 193 Marks
What is the difference between potential energy and kinetic energy?
Answer
 
Kinetic energy
Potential energy
1.
Kinetic energy of a body is due to motion of the body.
Potential energy is due to position or change in shape of the body.
2.
Kinetic energy is zero for a still body.
Potential energy may or may not be zero for a still body.
3.
Kinetic energy of a body is directly proportional to its speed
Potential energy is directly proportional to the height to which the body is above the ground.
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Question 203 Marks
How much electric energy in kWh is consumed by an electrical appliance of 1000 watts when it is switched on for 60 minutes?
Answer
Power = 1000W = 1kW
Time = 60 minutes = 1 hour
Energy (kWh) = 1 × 1 = 1kWh
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Question 213 Marks
When is the work done by a force on a body: (a) positive, (b) negative, and (c) zero?
Answer
Work done by a force applied on a body is:
  1. Positive work: Positive when the force acts in the direction of motion of the body.
  2. Negative work: Negative when the force acts in the direction opposite to the direction of motion of the body.
  3. Zero work: Zero when the force acts at right angle to the direction of motion of the body.
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Question 223 Marks
A car weight 20000N climbs up a hill at a steady speed of 8 m/s, gaining a height of 120m in 100s. Calculate:
  1. Work done by the car.
  2. Power of engine of car.
Answer
Weight of the car = 20000N = 20kN
Speed = 8m/s
Distance s = 120m
Time = 100s
  1. Work done W = f × s
= 20 × 120 = 2400kJ
  1. $\text{Power}=\frac{\text{Work done}}{\text{Time}}$
$=\frac{2400}{100}=24\text{kW}$
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Question 233 Marks
How much is the mass of a man if he has to do $2500$ joules of work in climbing a tree $5\ m$ tall? $\left(8=10 ms^{-2}\right)$
Answer
Mass of man, $m$
Height of tree, $h = 5\ m$
Work done, $W = 2500\ J$
Acceleration due to gravity, $g = 10m/s^2$
$W = m \times g \times h$
$2500 = m \times 10 \times 5$
$\text{m}=\frac{2500}{10\times5}=50\text{kg}$
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Question 243 Marks
What do you understand by the term “transformation of energy”? Explain with an example.
Answer
Transformation of energy: The change of one form of energy into another form of energy is known as transformation of energy.
Example: In a cell chemical energy is transformed into electrical energy.
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Question Answer (3 Marks) - Science STD 9 Questions - Vidyadip