Impulse (qualitative)

[Insulinshock]

When a box is hit, he or she is often advised to "ride the punch," that is, to move his or her head backward during contacnt with the opponent's fist. Which would be a reasonable explanation for this advice?

A. The impulse received by the head will be less during the collision.
B. The impulse received by the head will be greater during the collision.
C. Increasing the time of collision will decrease the force of contact.
D. Riding the punch may throw off the opponents timing.

This if from NOVA. It said the correct answer was C, but I didn't get it. They reasoned that the longer you are subjected to a force, the less the contact force would be. But I was under the assumption that force remained constant, and by extending the time you just distributed that force over a longer period?

I said answer B, but the correct answer is C. Any advice?

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[StarryNights]

I think of it as J = Ft, so F = J/t. Therefore if you increase t at the same impulse then your force actually decreases. In my old physics class, my teacher threw an egg (raw) at the wall and made a big mess, then repeated it against a sheet that moves back with the egg to increase the contact time (keeping it perfectly intact) to make his point.

 

[Isoprop]

The force isn't the same between the 2 punches.

Impulse is the change in momentum of an object. So, assuming the same kind of punch is used, the impulse of your head should be the same whether you are "riding it out" or not.

Like the above poster said, J = Ft. Since we just said J was constant, F must vary with contact time.