Discussion
A two-wheel drive tractor weighing 18 kN has a wheel base 1.8 m. Its centre of gravity is located 600 mm ahead of the rear axle centre, under static condition, on a level ground. When this tractor is used to pull a disc plough hitched at a height of 390 mm from the ground, the draft observed is 6 kN. The change in reaction on rear wheels of the tractor due to pull in kN is
Answer: A
The change in rear-wheel reaction is 1.30 kN.
The draft acting at the hitch height creates a moment that lifts the front of the tractor and transfers weight to the rear wheels. Taking moments about the front wheel contact, the extra rear reaction is the draft times hitch height divided by the wheel base:
\[ \Delta R = \frac{6 \times 0.39}{1.8} = 1.30\ \text{kN} \]
The tractor's weight and the position of its centre of gravity set the static reactions but do not enter the change caused by the pull. 3.90 kN divides by the 0.6 m from the rear axle to the centre of gravity and 1.95 kN by the 1.2 m from the centre of gravity to the front axle; neither is the wheel base that the moment acts over.
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