# eli5 How does inertia and impact work? Does moving make a stronger or weaker impact for say punching something?

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eli5 How does inertia and impact work? Does moving make a stronger or weaker impact for say punching something?

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to move something, you have to put energy into it, giving it a velocity in a given direction.

inertia is literally that energy. your trying to move it in a new direction, but the object still has energy pushing it in the original direction, and you need to counteract and overcome that to change the direction of travel.

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in terms of impact, its not really possible to have a impact without inertia, as that would imply no movement. So, you could say its vital to impact happening.

Speed=energy.

The faster something moves compared to what it hits, the harder the impact. If two cars are going 100km/h side-by-side and accidentally tap each other, you’ll just get a scratch. If a car going 100km/h hits a parked car, that’s 100km/h of energy (very, very simplified version of the equation, mass also plays a huge role) so there will be a strong impact. If two cars go 100km/h in opposite directions and crash into each other, that’s 200km/h of energy in the collision.

The same principle works for punching.

To really calculate the force of impact, you’ll also need to factor in mass and a few more variables

Inertia and the effects of an impact are mostly *relative*. If you are running towards something while also punching towards it, that punch would hit harder because you can punch your fist forward from your body just as hard as you could normally, but your body is also moving forward, so these things combined to an overall faster punch if you compare to something that is stationary. If you are backing away from something while punching towards it, that punch would hit weaker.

You could have a lot of speed but little or no relative speed. For example 2 people on an airplane moving 500mph fighting each other are going to notice no difference compared to fighting on the ground because they are not moving compared to each other.

Also when you are moving down a highway at 70mph, if a car next to you switches lanes and gently bumps you, that crash *might* not be catastrophic because you are going nearly the same speed. The crash itself doesn’t involve a lot of energy. Of course, it would be easy for a small bump and change of direction to cause you to lose control of your car and hit something like a tree which could be catastrophic, so this is still dangerous.