[2 Mark Question Answer]

Question 12 Marks

A spaceship is moving in space with a velocity of $60 kms ^{-1}$. It fires its retro engines for 20 second and velocity is reduced to $55 kms ^{-1}$. Calculate the distance travelled by the spaceship in 40 s , from the time of firing of the retro- rockets.

Answer

Initial velocity of spaceship $= u =60 kms ^{-1}$
Final velocity of spaceship $=v=55 kms^{-1}$
Case -1:
It decelerates for 20 s
$
\begin{array}{l}
\Rightarrow t=20 s \\
v=u+a t \\
55=60+a(20) \\
20 a=55-60=-5
\end{array}
$

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Question 22 Marks

From the diagram given below, calculate
1. acceleration
2. deceleration
3. distance covered by body.

(i) Acceleration $=$ Slope of $v$-t graph from O to A
$
=\frac{AC}{OC}=\frac{15}{10}=1.5 ms^{-2}
$

Answer

(ii) Deceleration $=$ Slope of $v$. .t. graph from A to B
$
=\frac{A C}{C B}=\frac{15}{16-10}=\frac{15}{6}=2.5 ms^{-2}
$
(iii) Distance covered $=$ area under $v$.t. graph $=$
area of $\triangle OAC +$ area of $\triangle ABC$ $=\frac{1}{2} \times OC \times AC +\frac{1}{2} \times BC \times AC$ $=\frac{1}{2} \times 10 \times 15+\frac{1}{2} \times 6 \times 15$
$
\begin{array}{l}
=5 \times 15+3 \times 15 \\
=75+45=120 m
\end{array}$

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Question 32 Marks

A racing car is moving with a velocity of $50 m / s$. On applying brakes, it is uniformly retarded and comes to rest in 20 seconds. Calculate its acceleration.

Answer

Initial velocity of car $=u=50 m / s$
Final velocity of car $=v=0 \quad[\because$ As car comes to rest $]$
Time $=t=20 s$
Acceleration $=a=$ ?
$
\begin{array}{l}
v=u+a t \\
0=50+a(20) \\
-20 a=50 \\
a=\frac{-50}{20}=-2.5 ms^{-2}
\end{array}
$

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Question 42 Marks

Copy the following table and fill in the blank spaces.

Quantity	S.I. Unit	Scalar or Vector
Displacement	______________________	______________________
____________________	kg m^-3	________________________

Answer

Quantity	S.I. Unit	Scalar or Vector
Displacement	metre (m)	Vector
Density	kg m^-3	Scalar

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Question 52 Marks

(a) Distinguish between scalar and vector quantities,
(b) State whether following are scalar or vector quantities
1. speed
2. force
3. acceleration
4. energy

Answer

(a) self
(b)
1. Speed is a scalar quantity because it has magnitude and no direction.
2. Force is a vector quantity because it has both magnitude as well as direction.
3. Acceleration is a vector quantity.
4. Energy is a scalar quantity because it has magnitude and no direction.

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Question 62 Marks

What is the relation between distance and time when
1. body is moving with uniform velocity
2. body is moving with variable velocity?

Answer

1. When body moves with uniform velocity, then body covers equal distance in equal interval of time in a specified direction.
2. When a body moves with variable velocity, then body covers unequal distance in equal interval of time in a specified direction.

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Question 72 Marks

Give an example of a body which covers a certain distance, but its displacement is zero.

Answer

Yes, displacement of a body can be zero even if the distance covered by it is not zero. For example, if a body moves in a circle, then displacement of the body in one rotation is zero but the distance covered by it in one roation = 2tr, where r is the radius of the circle in which the body is moving.

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Question 82 Marks

Define the term acceleration due to gravity. State its value in C.GS. as well as in S.I. system. When is acceleration due to gravity
1. positive
2. negative?

Answer

Acceleration due to gravity: The acceleration of a freely falling body, under the action of gravity of earth, is called acceleration due to gravity. It is represented by 'g'.
In S.I. system, g = 9.8 ms; In C.GS. system, g = 980 cms If a body falls towards the earth, then value of acceleration due to gravity is positive.
If the body rises vertically upwards, then value of acceleration due to gravity is negative.

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Question 92 Marks

Pick out the scalar and vector quantities from the following list:
1. mass
2. density
3. displacement
4. distance
5. momentum
6. acceleration
7. temperature
8. time

Answer

1. Scalar quantities: Mass, density, distance, temperature, time.
2. Vector quantities: Displacement, momentum, acceleration.

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Question 102 Marks

By giving an example, prove that rest and motion are relative terms.

Answer

Rest and motion are relative terms. It means a body at rest in one situation at certain time can be in motion in another situation at the same time.
A person sitting in the compartment of a moving train is in the state of rest with respect to the surrounding of the compartment yet he is in state of motion, if he compare himself with the surroundings outside the compartment.

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PHYSICS [2 Mark Question Answer]

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