Railway Science (Physics-Work, Energy and Power) (Part-III)

Total Questions: 50

21. If the speed of a cycle becomes two times, then the kinetic energy becomes ________. [RRB Group D 22/09/2018 (Evening)]

Correct Answer: (b) 4 times
Solution:

22. Find the energy possessed by a stationary object at a height of 6 m and with a mass of 50 kg. (g = 10 m/s²) [RRB Group D 22/09/2018 (Evening)]

Correct Answer: (a) 3000 J
Solution:

Given, Height (h) = 6 m, Mass (m) = 50 kg,
Acceleration due to gravity (g) = 10 m/s².
Potential Energy of object (P.E)
= m × g × h
⇒ P.E = 50 × 10 × 6 = 3000 J.

23. A boy weighing 50 kg climbs 45 steps of stairs in 9 s. If the height of each step is 15 cm, find its power. (Take g = 10 ms⁻²) [RRB Group D 23/09/2018 (Morning)]

Correct Answer: (d) 375 W
Solution:

Given the mass of the body (m) = 50 kg,
Height (h) = 45 × 15 = 675 cm = 6.75 m,
time (t) = 9 sec, g = 10 ms⁻² . Potential
energy = mgh = (50 ×10 ×6.75) = 3375 J.
Power = 𝐸𝑛𝑒𝑟𝑔𝑦/𝑇𝑖𝑚𝑒 = 3375/9 = 375 W.

24. An object of mass 1 kg has a potential energy of 2 J relative to the ground, when it is at a height of: take g=10 m/s² [RRB Group D 23/09/2018 (Afternoon)]

Correct Answer: (a) 0.2 m
Solution:

Given, Mass = 1 kg, Potential Energy = 2 J.
As we know that, Potential Energy = mgh
According to question,
Required height, h = 𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦/𝑚×𝑔
= 2/1 × 10 = 0.2 m.

25. Kinetic energy will become/remain ________when the momentum of the body is doubled. [RRB Group D 23/09/2018 (Afternoon)]

Correct Answer: (b) Four times
Solution:

26. A porter picks up a 20 kg luggage from the ground and places it on his head 2 meters above the ground. Calculate the work done by him on the goods. (g = 10ms⁻²) [RRB Group D 24/09/2018 (Morning)]

Correct Answer: (c) 400 J
Solution:

The work = Force × distance (which is height in this case) = mg × h = mgh. Work done by the porter, W = m × g × h = 20 × 10 × 2 = 400 J.

27. If the velocity of an object doubles its initial velocity, then how many times will its kinetic energy increase? [RRB Group D 24/09/2018 (Afternoon)]

Correct Answer: (b) 4
Solution:

28. A mobile phone was dropped from a balcony. If the mass of the mobile phone is 0.5 kg and the mobile phone has been dropped from a height of 100 m. g = 10 m/𝑠², then what is the potential energy of a mobile phone ? [RRB Group D 24/09/2018 (Afternoon)]

Correct Answer: (d) 500 J
Solution:

Given : Mass of the mobile phone (m) = 0.5 kg,
Height (h) = 100 m, Acceleration due to gravity (g) = 10 m/s²
Now, Potential energy (PE) = mass × gravity × height
⇒ PE = 0.5 kg × 10 m/s² × 100 m = 500 J.

29. A body holds a school bag of 4kg for 30s, the amount of work done by him in joules is _________. [RRB Group D 24/09/2018 (Evening)]

Correct Answer: (c) zero
Solution:

In physics, work is the transfer of energy due to a force applied over a distance. In the scenario of holding a bag. Force: The bag's weight (mass × gravity) acts downward. The bag remains stationary, so there's no displacement in the direction of the force. Thus, no work is done: Work (W) = Force (F) × Distance (d). Since distance (d) is zero, work (W) is zero.

30. The energy possessed by an object of mass 10 kg, when it is at a height of 6 m above the ground, is _________ (take g = g = 9.8 ms⁻²) [RRB Group D 25/09/2018 (Afternoon)]

Correct Answer: (b) 588 J
Solution:

Given, Mass of object m = 10 kg, Height h = 6 m, Acceleration due to gravity g = 9.8 m/s² . Potential Energy of object P.E = m × g × h ⇒ mgh = 10 × 9.8 × 6 = 98 × 6 = 588 J.