Can anyone explain why this FL question answer is wrong (PHYSICS/IMPULSE)

[MCAT guy]

• Why do the soles of running shoes have more rubber than a typical athletic shoe?

• CORRECT: D. Additional rubber increases the shoe's contact time with the ground, so that the average force of impact decreases.
• WRONG: B. Additional rubber increases the shoe's contact time with the ground, so that the impulse decreases.

• D is the best answer. Impulse equals change in momentum experienced by an object as the result of a collision. In the case of shoes, the impulse of concern occurs when the wearer strides and bounces off the surface on which they are moving. The sole of a runner's shoe experiences a greater impulse than the sole of a walker's shoe when it strikes the ground, so the goal in designing running shoes is to minimize the impact on the runner. This means that the goal is to reduce the average force during the impact period. Impulse can be calculated by multiplying the average force exerted on the object during the momentum change by the period of time that force is acting on the object (J = m∆v = Favg∆t). In order to reduce the average force during impact, the shoes must either reduce impulse or increase contact time.
  
  Heavier shoes in motion have greater momentum than lighter shoes, which would result in an increase in momentum change upon their contact with the ground. Heavier shoes experience an increase in impulse, so choice A is a factually true statement. However, the goal of a running shoe's design is not to increase impulse, but rather to decrease the impact through cushioning. Choice A is thus eliminated. Additional rubber in the shoe causes a greater compression of the sole, which extends the time during which the shoe is in contact with the ground. This must be true, given that it is stated in each of the three remaining answer choices. Choice B should be eliminated, because the additional rubber only slightly increases the mass of the shoe, resulting in an increase in momentum and impulse. Choice C can be eliminated, because the additional contact time with the ground does not increase the impulse, but instead reduces the force of impact. The impulse is in fact slightly greater with the slight increase in mass, but choice C is still not a valid statement. The additional rubber provides cushion, which extends the duration of the shoe's collision with the ground over a greater period of time, thereby reducing the average force felt during that collision. Choice D is the best answer, assuming that the impulse is only slightly greater with the additional rubber in the sole of the shoe.

I thought that the wrong answer above was correct, Choice B. Maybe I'm not understanding definitions correctly. The answer logic is above, but as I understand, impulse = F delta T, and therefore if I am extending the length of the contact is the impulse therefore increasing? Intuitively I would think if I add cushion and extend the average force over time, then the impulse has DECREASED... is this wrong physical thinking?

I guess it says increasing the rubber increases the impulse, but this seems wrong

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[sizillyd]

The way I would reason this is that Impulse is equal to F delta t. And that is also equal to delta mass times velocity. So we can reason that:

F delta t= delta mass times velocity.

Mass and velocity are constant here so the only things that can change are F and t. We can see that if momentum (mv) is constant, then if t increases, F has to decrease. Because if t increases, F has to decrease to compensate for the gain.

 

[collegestud2013]

for all intent purposes u have to assume the person's impulse remains constant because J = (m)(deltav) and since J = (F)(deltat) and for J to stay constant while deltat increases, F must decrease.