Question

A car driver prefers to hit something soft ( like hay) than a wall if his car goes out of control while driving. give reason

Answer 1

This has all to do with change in momentum. When a vehicle hits a wall the change in momentum is rapid because the velocity is reduced instantly to zero as compared to when the vehicle hits a soft obstacle like hay, the momentum change is less rapid since the velocity gradually reduces to zero.
The implication this has is that when momentum changes rapidly, a higher force is generated, from Newton's second law of motion; force is change in momentum per unit time. With a higher force from a rapid change in momentum, the damage would be more hence the driver avoids such collisions. 

Answer 2

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This is to decrease the rate of change of momentum by increasing the time.

According to Newton's second law of motion, the car exerts lesser force & hence the impact of the reaction force on the car is small.

0n the other hand, the impact of reaction force imparted by the concrete wall would be very large which might cause serious incident, thereby damaging the car & hurting the driver.

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