1. What is the empirical formula for the tractive force required to overcome curve resistance? (W-the weight of the body, R – radius of curvature)
A. 710×W÷R
B. 700×W÷R
C. 720×W÷R
D. 750×W÷R
Answer: B
Fc= 700×W÷R is the tractive force required to overcome curve resistance where W is the weight of the body in Kg and R is the radius of curvature in meters.
2. Force resisting the upward motion of a body on an inclined plane is given by (alpha – the angle of inclination, W- the weight of the body).
A. F = W×sin(alphA.
B. F = W×cosec(alphA.
C. F = W×sec(alphA.
D. F = W×cos(alphA.
Answer: A
When a body is moving upward on an inclined plane its weight can be resolved into two perpendicular components that are
W×sin(αA)
W×cos(αA)
W×cos(αA). is the component that is opposite to the normal of the inclined plane
W×sin(αA) is the component that opposes the upward motion of the body.
3. The unit of the torque is ______
A. N-m
B. N-m2
C. N-m/sec
D. N-Hz
Answer: A
Torque is defined as the vector product of displacement and force. The unit of the force is Newton(N) and the displacement is a meter (m) so the unit of torque is N-m.
4. Calculate the value of the torque when 10 N force is applied perpendicular to a 10 m length of rod fixed at the center.
A. 200 N-m
B. 300 N-m
C. 100 N-m
D. 400 N-m
Answer: B
Torque can be calculated using the relation
T = (length of rod) × (Force applied)
= r×F×sin90.
F is given as 10 N and r is 10 m then torque is 10×10 = 100 N-m. (the angle between F and r is 90 degrees)
5. What is the dimensional formula for torque?
A. [ML2T-2]
B. [MLT-2]
C. [M1L2T-3]
D. [LT-2]
Answer: A
Torque is a vector product of force and displacement.
Dimensional formula for force is [MLT-2] and displacement is [L] so dimensional formula for torque is [MLT-2] [L] = [ML2T-2].
6. Buck converter is used to _________
A. Step down the voltage
B. Step up the voltage
C. Equalize the voltage
D. Step up and step down the voltage
Answer: A
The output voltage of the buck converter is Vo = D×Vin. The value of the duty cycle is less than one which makes the Vo < Vin. The buck converter is used to step down voltage. Vin is a fixed voltage and Vo is a variable voltage.
7. If the starting torque of the motor is less than the load torque, the motor will fail to start.
A. True
B. False
Answer: A
J×d(w)÷d(t) = Motor torque – Load torque is the dynamic equation of the motor.
If starting torque (motor torque) is less than the load torque then d(w)÷d(t) <0, acceleration <0 so the motor will decelerate and fails to start.
8. Torque is a scalar quantity.
A. True
B. False
Answer: B
A scalar quantity has only magnitude whereas a vector quantity has both directions and magnitude. Torque is a force applied to a body perpendicularly. As the force is a vector quantity, the torque must be treated as a vector quantity.
9. 250V, 15A, 1100 rpm separately excited dc motor with armature resistance (Ra) equal to 2 ohms. Calculate back emf developed in the motor when it operates on half of the full load. (Assume rotational losses are neglected.
A. 210V
B. 240V
C. 230V
D. 235V
Answer: D
Back emf developed in the motor can be calculated using the relation
Eb = Vt-I×Ra.
In question, it is asking for half load, but the data is given for full load so current becomes half of the full load current
= 15÷2 = 7.5 A. 250V is terminal voltage it is fixed so
Eb = 250-7.5×2 = 235V.
10. Duty cycle (D. is _______
A. Ton÷Toff
B. Ton÷(Ton+ Toff)
C. Ton÷2×(Ton+ Toff)
D. Ton÷2×Toff
Answer: B
Duty cycle (D. is defined as the ratio of time for which a system is active to the total period. It is also known as the power cycle. It has no unit.
