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If the action force is always equal to the reaction force, why is it that when we punch a boxing bag, our hands rarely get hurt, but when we punch a wall made of high-quality materials, our hands get hurt much more? Or the carnivorous dinosaurs of the T-Rex family, which can bite violently to kill large size prey, or the Triceratops, which can hit animals of equal size to kill, rarely get hurt from the reaction force?and why people has higher survival rate form car ramming than fall form high?

Qmechanic
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Title Charnpradit
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Boxing bag v/s High-quality wall

There are two reasons here. The first is that the boxing bag compresses as you punch it, which increases the amount of time your hand is in contact with it. The force you exert on it is not about how fast your hand was moving, but instead about how quickly your hand stops moving. With the case of the wall, your hand comes to a sudden stop the moment you make contact with it. This means that you are actually punching the wall with a harder force than you are punching the boxing bag, even though your hand is moving at the same speed. The second reason is that the boxing bag slightly deforms when you punch it, which increases the area over which your hand feels the reaction force. This means that your hand will feel less pressure, and it would hurt lesser.

T-Rex Bite

A dinosaur's tooth is quite sharp, so while it might seem like it is biting its prey with a large force, what is actually happening is that it is biting its prey with a large pressure. Pressure is force per area, and a sharp tooth has a very small area of contact with the prey's skin. Hence, even with a small force the T-Rex can kill its prey, so it doesn't feel that much of a reaction force.

Triceratops Tackle

When a large dinosaur tackles a smaller one, it still feels the same reaction force on itself. The difference is that the same force is more lethal to the small dinosaur than it is to the big one. This is somewhat related to how acceleration by a force is inversely proportional to the mass of the body, i.e., larger masses are more resistant to being moved by forces.

Amogh
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