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

Total Questions: 50

31. If a person moves a trolley for a distance of 10 m with a force of 50 N, then the work done is : [RRB Group D 27/09/2018 (Morning)]

Correct Answer: (d) 500 J
Solution:

Given that, Force acting on the trolley (F) = 50 N.
Distance travelled by trolley (s) = 10 m.
Total work done, W = F × s = 50 × 10 = 500 J.

32. An object of mass 20 kg is moving with a constant velocity of 8 ms⁻¹ . The kinetic energy of the object will be: [RRB Group D 27/09/2018 (Afternoon)]

Correct Answer: (a) 640 J
Solution:

Given that, mass (m)= 20 kg, Velocity = 8 ms⁻¹ .
Kinetic energy = 1/2 × m × v² = 1/2 × 20× 8² =  640 J.

33. An object has a potential energy of 400 J with a mass of 20 kg and gravity of 10 N kg⁻¹. The height of the object is: [RRB Group D 28/09/2018 (Morning)]

Correct Answer: (d) 2 m
Solution:

Given that, Potential energy (U)= 400 J, mass of a body (m) = 20 kg, gravitational acceleration (g)= 10 N/kg or m/sec² , height (h)= ?
Potential energy (U) = mgh
400 = 20 × 10 × h ⇒ h = 2 m.

34. The kinetic energy of an object of mass 10 kg moving with a velocity of 6 ms⁻¹ is: [RRB Group D 28/09/2018 (Evening)]

Correct Answer: (a) 180J
Solution:

Given that, Mass (m) = 10 kg, Velocity (v) = 6 m/s.
The Kinetic Energy of a body is, K.E. = 1/2 mv² = 1/2 × 10 × 6² = 180 J.

35. An object moving with a uniform velocity of 4 ms⁻¹ has kinetic energy of 120 J. Find the mass of the object in it. [RRB Group D 01/10/2018 (Morning)]

Correct Answer: (a) 15 kg
Solution:

Given, 𝑣 = 4 ms⁻¹,  K.E. = 120 J
Now, mass (m) = 2 𝐾.𝐸./𝑣² = 2 × 120 / 4² = 15 kg.

36. If a factor W does work in time t, then its power (p) will be: [RRB Group D 01/10/2018 (Afternoon)]

Correct Answer: (c) Work/Time (W/t)
Solution:

Power is defined as the rate of doing work or the rate of transfer of energy. It is a scalar quantity. SI unit- watt (W). Dimensional Formula - [M¹L²T ⁻³].

37. The kinetic energy of an object of mass m moving with a speed of 5 ms⁻¹ is 25 J. What will be its kinetic energy if its speed doubles? [RRB Group D 01/10/2018 (Afternoon)]

Correct Answer: (a) 100 J
Solution:

Kinetic energy(KE) = 1/2 mv²
⇒ KE ∝ v² .
⇒ If velocity is doubled the kinetic energy becomes 4 times.
⇒ Kinetic energy = 25 × 4 = 100 J.

38. An object moving with a uniform velocity of 4ms⁻¹ possesses kinetic energy of 120 J. Find the mass of the object. [RRB Group D 01/10/2018 (Evening)]

Correct Answer: (c) 15 kg
Solution:

39. An object of mass 12 kg is at a certain height above the ground. If the potential energy of the object is 480 J. find the height at which the object is with respect to the ground. Given, g=10 ms⁻² [RRB Group D 03/10/2018 (Afternoon)]

Correct Answer: (c) 4 m
Solution:

Given that, the object of Mass (m) = 12 kg, Potential energy= 480 J, Acceleration of gravity (g)= 10 ms⁻², Height (h) = ?
We know that, Potential energy = mgh
= 12 × 10 × h.
⇒ 480 = 12 × 10 × h ⇒ h = 4 m.

40. A coolie lifts a load of 11 kg from the ground and places it on his head, 1.5 m above the ground. How much work is done by the coolie? (g = 10 ms⁻²)? [RRB Group D 03/10/2018 (Evening)]

Correct Answer: (c) 165 J
Solution:

Given that, m = 11 kg, h = 1.5 m,
g = 10 m/s² .
W = mgh = 11 × 1.5 × 10 = 165 J.