Question

Trucks intended to carry heavy loads have eight tyres instead of four tyres. why ?

Answer 1

Trucks carrying heavy loads use eight tires to reduce the pressure on its tyres. As the surface area increases the force/load is spread over a larger area and hence reducing the pressure. This reduction in pressure allows the truck to move better and also protects the tyres from excessive wear and tear.
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Answer 2

They have eight tyres and each tyre is broader i.e, they have more area thereby increasing the area of contact with the ground. More area, less pressure. 8 broad tyres ensures that less pressure is exerted on the ground.