How can smaller animals brush off seemingly dangerous high velocity impacts?

 

Lately, our family welcomed a pair of kittens into our home. They've been a joyous bunch bouncing off of the walls playing until they're dead tired and curl on top of each other. When they play, they play hard, and often times when going full sprint for a toy they lose all sense of self-preservation and come crashing into the wall hard, or fall off of the table with a gleeful sense of abandon. Many times we audibly shudder, and my parents have wondered how they can shrug off such high-velocity impacts that if we were to experience would leave us bruised and in pain. Well, luckily this was something that physics could answer. The law of conservation of momentum states that the total momentum before a collision is equal to the total momentum after a collision. While this doesn't exactly translate into an inelastic collision with a wall, the principle holds true. Momentum can be broken down into mass x volume. If we were to impact a wall at a high speed, and a cat was to impact the same wall at the same speed, we would experience a greater momentum than the cat as our mass component of the momentum would be many magnitudes greater (a cat weighs generally around 9 pounds while the average American weighs generally around 160 pounds). As the collision with the wall greatly reduces our velocity we experience a change in momentum that's felt through the wall exerting momentum on us which usually results in a lot of pain. This change in momentum is so much less for a typical cat that they can just keep going whereas we would need a moment to recover.