Que-Ans (Each of 2 Mark )

Question 12 Marks

List any three distinguishing features between the models of an atom proposed by J.J. Thomson and Ernest Rutherford.

Answer

J. J. Thomson Model of Atom	Rutherford's Model of Atom
1. Positive charge forms a kernel.	1. Nucleus (dense positive charge) is in the centre of the atom.
2. Electrons are present (embedded in positive charge) throughout the atom.	2. Electrons revolve around the nucleus in orbits.
3. No space inside the atom is empty. Thomson likened an atom to a pudding or a watermelon.	3. According to Rutherford, most of the space inside the atom is empty.

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Question 22 Marks

When a force of $40 \ N$ is applied on a body it moves with an acceleration of $5 \ ms^{-2}$. Calculate the mass of the body.

Answer

Let $m$ be the mass of the body
Given: $F=40 N, a =5 ms^{-2}$
From the relation : $F=$ ma we have $40= m \times 5$
or $m =\frac{40}{5}=8 \ kg$

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Question 32 Marks

Using second law of motion, derive the relation between force and acceleration. A bullet of mass $10 g$ strikes a sand bag with a velocity of $103\ m/s^1$ and gets embedded after travelling $5 \ cm$. Calculate
$i$. the resistive force exerted by the sand bag on the bullet.
$ii.$ the time taken by the bullet to come to rest.

Answer

$\text { i. } m=10 g=\frac{10}{2000} \ kg,$
$ u=10^2 m / s, v=0, s=\frac{5}{100} m$
$v^2-u^2=20 s$
$\Rightarrow 0-\left(10^3\right)^2=2 \cdot a \cdot \frac{s}{100}$
$\Rightarrow a=\frac{1000-1000}{2 \times 5} \times 100$
$--10^7 ms^2$
$\Rightarrow F=ma=-10^5 N$
$ii. v=u+a t$
$=0=\left(10^7\right) \cdot 10^7 t$
$\Rightarrow 10^7 t=10^3$
$\Rightarrow t=\frac{10^2}{10^7}-10^{-4}s$

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Question 42 Marks

Why is ice at 273 K more effective in cooling than water at the same temperature?

Answer

While melting ice absorbs latent heat of melting from the surroundings and gets changed into water that makes the cooling effect more intense as compared to water at same temperature.

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Question 52 Marks

Why do we see light first and hear the sound later during thunderstorm?

Answer

We see the lightning before we hear the thunder because light travels faster thansound. The light from the lightning travels to our eyes much quicker than the soundfrom the lightning, so we hear it later than we see it.

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Question 62 Marks

Give reasons for the following observation:
The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.

Answer

Since hot sizzling food has temperature higher than the cold food and at higher temperature diffusion rate (movement) of particles is very fast due to this the smell of hot sizzling food reaches us from several metres away but to get the smell from cold food you have to go close.

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Question 72 Marks

Why will a sheet of paper fall slower than one that is crumpled into a ball?

Answer

Sheet crumpled into a ball has small surface area as compared to the similar unfolded sheet. Therefore unfolded sheet will experience more friction due to air as compared to the sheet crumpled into a ball in spite of same force of gravity acts upon them. It is larger friction of air which slows down the unfolded sheet and therefore it falls slower as compared to sheet crumpled into a ball.

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Question 82 Marks

A body moves along a circular path. How much work is done in doing so? Explain.

Answer

In case of a body moving along a circular path, the force (centripetal) is always along the radius while displacement is tangential. Hence work done $W=\Gamma s \cos 90^{\circ}-0$ as angle between E and sis $90^{\circ}$.

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Science Que-Ans (Each of 2 Mark )

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