Final answer:
Adding weight increases the friction between the tires and an icy road by increasing the normal force, which in turn increases the frictional force. This enhanced friction improves traction, similar to how winter tires or effective braking systems use friction to increase vehicle safety.
Step-by-step explanation:
Adding weight to the back of a pickup truck increases the friction between the tires and an icy road due to an increase in the normal force. The frictional force between two surfaces is given by the product of the coefficient of friction and the normal force. When weight is added, the normal force exerted by the truck on the road increases, and consequently, the frictional force also increases, allowing for better traction.
Tire selection is crucial for driving over slippery surfaces; for example, winter tires are designed to create more friction with the road compared to summer tires, improving safety in such conditions. Another example where more friction is desirable is during the operation of truck brakes on a downhill run, where friction is used to convert kinetic energy into heat, thus controlling the vehicle's speed and preventing accidents.