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Science One word answer.

Work And Energy · Gujarat Board (GSEB) STD 9 (GSEB - English Medium). 70 questions.

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Question 11 Mark
Define average power.
Answer
When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power.
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Question 21 Mark
Define 1 watt of power.
Answer
A body is said to have power of 1 watt if it does work at the rate of 1 joule in 1 s, i.e., 1 W = 1J/1s.
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Question 31 Mark
What is power?
Answer
Power is the rate of doing work or the rate of transfer of energy. IfWis the amount of work done in time t, then power is given by the expression,
$\text{Power}=\frac{\text{Work}}{\text{Time}}$
$=\frac{\text{Energy}}{\text{Time}}$
$\text{P}=\frac{\text{W}}{\text{T}}$
It is expressed in watt (W).
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Question 41 Mark
Write an expression for the kinetic energy of an object.
Answer
If a body of mass m is moving with velocity v, then its kinetic energy Ek is given by the expression,
$\text{E}_\text{k}=\frac{1}{2}\text{mv}^2$
Its S.I. unit is Joule (J).
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Question 51 Mark
What is the kinetic energy of an object?
Answer
Kinetic energy is the energy possessed by a body by the virtue of its motion. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.
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Question 61 Mark
Define 1 joule of work.
Answer
1 joule: When a force of 1 newton moves a body through a distance of 1 meter in its own direction, then the work done is known as 1 joule.
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Question 71 Mark
When do we say that work is done?
Answer
Work is done whenever the given conditions are satisfied:
  1. A force acts on the body.
  2. There is a displacement of the body caused by the applied force along the direction of the applied force.
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Question 81 Mark
An electric heater is rated 1500W. How much energy does it use in 10 hours?
Answer
Energy consumed by an electric heater can be obtained with the help of the expression,
$\text{p}=\frac{\text{W}}{\text{T}}$
Where,
Power rating of the heater, p = 1500W = 1.5kW
Time for which the heater has operated, T= 10h
Work done = Energy consumed by the heater.
Therefore, energy consumed = Power × Time
= 1.5 × 10 = 15kWh
Hence, the energy consumed by the heater in 10h is 15kWh.
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Question 121 Mark
Name two units of power bigger than watt.
Answer
Megawatt and kilowatt are the units of power bigger than watt.
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Question 141 Mark
Write an expression for the kinetic energy of a body of mass m moving with a velocity v.
Answer
Mass = m
Velocity = v
$\text{KE}=\frac{1}{2}\text{mv}^2$ 
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Question 151 Mark
Define the term ‘watt’.
Answer
Watt is the power of an appliance which does work at the rate of 1 joule per second.
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Question 161 Mark
A cell converts one form of energy into another form. Name the two forms.
Answer
A cell converts chemical energy into electrical energy.
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Question 181 Mark

What kind of energy is possessedby the following?

Water stored in the reservoir of a dam.

Answer
Potential energy.
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Question 191 Mark
Write the energy transformations which take place in an electric bulb (or electric lamp).
Answer
In an electric bulb, electrical energy is first converted into heat energy and then into light energy.
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Question 211 Mark
Name the device or machines which convert:
Chemical energy into kinetic energy (or mechanical energy).
Answer
Car engine.
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Question 221 Mark

What happens to the potential energy of a body when its height is doubled?

Answer
The potential energy is doubled when the height is doubled since potential energy is directly proportional to height, h to which body is raised.
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Question 231 Mark

Define the term 'energy' of a body. What is the SI unit of energy.

Answer
Energy: Energy is the ability to do work. SI unit of energy is Joule.
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Question 241 Mark

Which would have a greater effect on the kinetic energy of an object: doubling the mass or doubling the velocity?

Answer
Doubling the velocity would have a greater effect on kinetic energy.
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Question 251 Mark

A load of 100kg is pulled up by 5m. Calculate the work done. (8 = 9.8m/s)

Answer
Mass = 100kg
Height = 5m
g = 9.8m/s2
P.E. = m × g × h
= 100 × 5 × 9.8 = 4900J
Work done is equal to PE acquired by the body.
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Question 261 Mark

How much work is done when a force of 1N moves a body through a distance of 1m in its own direction?

Answer
Force, F = 1N
Distance, s = 1m
Work done W = F × s
= 1 × 1 = 1J
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Question 281 Mark

Is potential energy a vector or a scalar quantity?

Answer
Potential energy is a scalar quantity as it has magnitude only and it does not require any specification of direction.
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Question 291 Mark

A body is thrown vertically upwards. Its velocity goes on decreasing. What happens to its kinetic energy as its velocity becomes zero?

Answer
When the velocity becomes zero, the kinetic energy also becomes zero since kinetic energy is directly proportional to square of the velocity.
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Question 331 Mark
Calculate the work done when a force of 15N moves a body by 5m in its direction.
Answer
w = f × s
w = 15 × 5
w = 75
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Question 341 Mark
Is work a scalar or a vector quantity.
Answer
Work is a scalar quantity.
Work, as a physical quantity, requires only magnitude to be represented. Hence, it is scalar quantity.
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Question 351 Mark
Give the formula to calculate work done on an object if force applied and displacement in it is known.
Answer
Work = force × displacement 
W = F × S
Where,
W = work
F = force and
S = displacement 
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Question 361 Mark
In which of the following case the work done by a force will be maximum: when the angle between the direction of force and direction of motion is 0° or 90°?
Answer
Work done will be maximum when angle between the direction of force and direction of motion is 0° because,
$\text{W} =\text{F}\cos\theta\times\text{s}.$
And, $\cos\theta\ 0^\circ=1$ and $90^\circ=0$ 
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Question 371 Mark
What is the condition for a force to do work on a body?
Answer
The condition for a force to do work on a body is that it should produce motion in the body.
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Question 381 Mark
Relate watt to joule.
Answer
As we know that
$\text{Power}=\frac{\text{Work}}{\text{Time}}$
$\text{watt}=\frac{\text{joule}}{\text{second}}$
So if the time is taken as unity then we can define watt in terms of joule.
Therefore,
One watt power is equal to one joule work when the time taken is unity.
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Question 391 Mark
Which will have more impact on kinetic energy-doubling mass or velocity?
Answer
As kinetic energy of an object is directly proportional to its mass, so if mass is doubled (i.e. it becomes 2m), then kinetic energy will also get doubled. Hence, it is clear that doubling the velocity has a greater effect on the kinetic energy of an object.
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Question 401 Mark
When a ball is thrown vertically upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero?
Answer
Potential energy becomes maximum.
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Question 411 Mark
Name the device or machines which convert:
Mechanical energy into electrical energy.
Answer
Electric generator.
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Question 421 Mark
Define energy? Name and defin its SI unit.
Answer
Power is defined as the rate of doing work. It's SI unit is joule/ sec or watt.
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Question 431 Mark
Name the device which converts chemical energy into electrical energy.
Answer
A battery or cell is the device that converts chemical energy into electrical energy.
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Question 441 Mark
On what factors does the kinetic energy of a body depend?
Answer
The kinetic energy of a body depends on:
  1. Mass of the body, m.
  2. Square of the velocity of the body, v2.
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Question 451 Mark
What kind of energy transformations takes place at a hydroelectric power station?
Answer
At a hydroelectric power station, the potential energy of water is transformed into kinetic energy and then into electrical energy.
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Question 471 Mark
A body does 1200 joules of work in 2 minutes. Calculate its power.
Answer
Work done = 1200J
Time taken 2 minutes
= 2 × 60 = 120s
$\text{Power}=\frac{\text{Work done}}{\text{Time taken}}=\frac{1200}{120}=10\text{W}$
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Question 501 Mark
Name the device or machines which convert:
Heat energy into kinetic energy (or mechanical energy).
Answer
Steam engine.
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