9702/Oct Nov/13/2013/Q16

The graph shows how the total resistive force acting on a train varies with its speed.

Part of this force is due to wheel friction, which is constant. The rest is due to wind resistance.

What is the ratio wind resistance/wheel friction at a speed of 200 km h^–1?

A 4
B 5
C 8
D 10

Solution:

Answer: A

The wheel friction is constant and the wind resistance increases with speed.

When the speed is zero, the wind resistance is also zero. So, wheel friction = 8 kN.

At a speed of 200 km h^–1, the total resistive force is 40 kN.

Wheel friction = 8 kN

Wind resistance = 40 – 8 = 32 kN

Ratio = wind resistance / wheel friction = 32 / 8 = 4

Reference: PYQ - Oct/Nov 2013 Paper 13 Q16