Constant force exerted on the ball by the wall?

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A 2.5 kg ball strikes a wall at 8.5 m/s to the left and bounces off at 7.5 m/s to the right, resulting in a change in momentum. The momentum before the collision is calculated as -21.25 kg·m/s, while the momentum after is 18.75 kg·m/s. The total change in momentum is 40 kg·m/s. Given that the contact time with the wall is 0.25 seconds, the average force exerted on the ball by the wall can be determined by dividing the change in momentum by the time. This analysis illustrates the relationship between force, momentum, and time during a collision.
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a 2.5 kg ball strikes a wall with a velocity of 8.5 m/s to the left. The ball bounces off with a velocity of 7.5 m/s to the right. If the ball is in contact with the wall for .25s, what is the constant force exerted on the ball by the wall?
 
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Momentum to the left, before the collision, is (- 8.5m/s)(2.5 kg) (negative because it is to the left). Momentum to the right, after the collision is (7.5 m/s)(2.5kg). Change in momentum is the difference between those: (7.5 m/s)(2.5kg)+(8.5m/s)(2.5 kg).
Since that took place in 0.25 s, the average rate of change of momentum is that divided by 0.25 s. Force IS rate of change of momentum.
 
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