• Physics 9702 Doubts | Help Page 196

Question 949: [Work, Energy and Power]

An electric railway locomotive has maximum mechanical output power of 4.0 MW. Electrical power is delivered at 25 kV from overhead wires. Overall efficiency of the locomotive in converting electrical power to mechanical power is 80 %.

What is the current from the overhead wires when the locomotive is operating at its maximum power?

A 130 A                      B 160 A                      C 200 A                      D 250 A

Reference: Past Exam Paper – November 2006 Paper 1 Q18 & November 2009 Paper 11 Q16 & Paper 12 Q15

Solution 949:

Answer: C.

Efficiency = 80%

Output power / Input power = 80 / 100

Input power, P = 4×10⁶ × (100/80) = 5×10⁶

Power = VI

Current I = P / V = (5×10⁶) / (25×10³) = 200 A

Question 950: [Forces > Equilibrium]

Two rigid rods, XZ and YZ, are fixed to a vertical wall at points X and Y.

Load of weight W is hung from point Z.

The load is not moving.

Which diagram shows the forces acting at point Z?

Reference: Past Exam Paper – June 2008 Paper 1 Q13

Solution 950:

Answer: A.
Since the load is not moving, the system is in equilibrium.
The weight W of the load is downwards.

Consider the rod XZ. There should be a tension along this rod from Z to X for the system to be in equilibrium. [B and C are incorrect]

Now, consider the force in XZ and the weight. The weight acts downwards and the force in XZ has a component vertically down. So, we need a force with having a component vertically up, so that the vertical forces on the system is in equilibrium. Thus, the force should be from Y and Z. [D is incorrect]

If the forces were as shown in choice B, the rod would bend. The resultant vertically downward force would be greater than the vertically upward force since the magnitudes of W and the force in YZ (represented by their lengths) would be greater.











Question 951: [Forces > Couple]
Diagrams all show a pair of equal forces acting on a metre rule.
Which diagram shows forces that provide a couple and zero resultant force?

Reference: Past Exam Paper – June 2011 Paper 12 Q15



Solution 951:
Answer: B.
For the resultant force to be zero, the directions of the 2 vectors should be exactly opposite to each other. [A and D are incorrect]

A couple consists of 2 forces, equal in magnitude, acting in opposite directions on the object so that they produce a resultant turning effect.
But for the forces to produce a resultant turning effect, they should act at different points. Else, the moments use to each of the forces would cancel each other. [C is incorrect]










Question 952: [Waves > Electromagnetic spectrum]
What do not travel at the speed of light in a vacuum?
A electrons
B microwaves
C radio waves
D X-rays

Reference: Past Exam Paper – June 2005 Paper 1 Q23 & November 2012 Paper 11 Q27



Solution 952:
Answer: A.
All components of the electromagnetic spectrum travel at the speed of light in a vacuum.

From the list, only the electron is not from the electromagnetic spectrum.