Tag: different forms of energy

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Multiple choice physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

Work done by 100 calorie of heat is __________.

  1. 418.4 J

  2. 4.184 J

  3. 41.84 J

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
For an isothermal expansion of an ideal gas, the change in internal energy is zero.

According to the first law of thermodynamics, 

Change in internal energy U = Q-W = 0

So, all the heat energy is utilized to do work. 

Q = W

We know that, one calorie is equal to 4.184 J

Therefore, Work done by 100 calorie of heat in an isothermal expansion of any ideal gas will be 4.184 * 100 =  418.4 Joule


Multiple choice physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

A person of weight 60 kg wants to loose 5 kg by going up and down 10m high stairs. Assume he burns twice as much fat while going up than going down. If 1 kg of fat is burnt on expending 7000 kcal. How many times must he go up and down to reduce his 7 kg weight? (Take $  g=10 \mathrm{ms}^{-2} )  $

  1. $ 1.8 \times 10^{3} $ times
  2. $ 2.4 \times 10^{3} $ times
  3. $ 1.7 \times 10^{3} $ times
  4. $ 2.1 \times 10^{3} $ times
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Energy used to go up $=mgh=60\times 10\times 10=6000\,J$

Energy used to come down $\dfrac{6000}{2}=3000\,J$

Energy used in one round trip $=9000\,J$

$1\,cal=4.5\,J$

$1\,J=\dfrac{1}{4.2\,cal}$

$9000\,J=\dfrac{9000}{4.2}=2142.85\,cal$

$7000\,kilo\,cal$ is required to burn $1\,kg$ mass

To reduce $5\,kg$ mass, energy required $=7000\times 5=35000\,kilo\,val$

Number of trip $=\dfrac{35000\times 1000}{2142.85}=1.7\times 10^{3}$
Multiple choice physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

A steel ball of mass $5$ ${ g }$ is thrown downward with velocity $10$ ${ ms } ^ { - 1 }$ from height $19.5$ ${ m }$ . It penetrates sand by $50$ ${ cm }$ . The change in mechanical energy will be ( ${ g } = 10$ ${ ms } ^ { - 2 }$ )

  1. $1$ ${J}$
  2. $1.25$ ${J}$
  3. $1.5$ ${J}$
  4. $1.75$ ${J}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$\begin{array}{l} The\, \, change\, \, in\, \, mechanic\, \, energy\, \, \Delta U=mg\left( { h+x } \right) +\frac { 1 }{ 2 } m{ v^{ 2 } } \ here\, \, m=5g=0.00\, 5kg\cdot h=19.5\, mx=50cm=0.5m,v=10\, m/s \ So,\, \Delta U=0.005\times 10919.5+0.5+\frac { 1 }{ 2 } \times 0.005\times { \left( { 10 } \right) ^{ 2 } }=0.005\times 10\times 20+\frac { 1 }{ 2 } \times 0.005\times 100=1.25J \end{array}$

Hence,
option $(B)$ is correct answer.

Multiple choice physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

A block of ice at 0 C whose mass is initially 50.0 kg slides along a horizontal surface starting at a speed of 5.38 m/s and finally coming of ice melted as a result of the friction between the block and the surface will be

  1. 2.16 g

  2. 4.0 g

  3. 1 g

  4. 50 g
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Initial KE = 0.5 * 50 * 5.38^2 = 723.6 J. Heat = 723.6 J. Mass melted = Heat / Latent heat = 723.6 / 334,000 J/kg = 0.00216 kg = 2.16 g.

Multiple choice physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

An ice block is falling from a height. If $75\%$ of the block melts then $h = (g = 10\ ms^{-2})$.

  1. $10.2\ Km$
  2. $12.8\ Km$
  3. $25.2\ Km$
  4. $26.8\ Km$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When an ice block falls and 75 percent of it melts due to friction and thermal conversion, the heat required for melting (Q = m_melt * L_f) comes from the loss of potential energy (m * g * h). Since 75 percent melts, 0.75 * m * L_f = m * g * h. Using the latent heat of fusion of ice (approx 3.34 * 10^5 J/kg) and g = 10, solving for height h yields approximately 25.2 Km.

Multiple choice physics the essence of change different forms of energy energy conversions energy transformations and energy transfers

A lead ball moving with velocity v strikes a wall and stops. I 50% of its energy is converted into heat, then what will b the increase in temperature? (Specific heat of lead is s)

  1. $\dfrac { 2{ v }^{ 2 } }{ Js } $
  2. $\dfrac { { v }^{ 2 } }{ 4Js } $
  3. $\dfrac { { v }^{ 2 }s }{ J } $
  4. $\dfrac { { v }^{ 2 }s }{ 2J } $
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

0.5 * (0.5 * m * v^2) = m * s * delta T * J. 0.25 * m * v^2 = m * s * delta T * J. delta T = v^2 / (4 * J * s).