How does mass and velocity affect the amount of damage?

Question

I’m not a science-y person, but I love sci-fi. In a novel I read near a decade ago (Larry Niven’s Known Space series), he mentions occasionally kinetic weapons. I’ve seen this online as well with tungsten-based projectiles being discussed. So my question is how does mass and velocity affect the amount of damage? If I had a nickel-sized object, how fast would it need to go to cause city-wide devastation (would it be possible or would the damage output be capped based on either size or velocity)? Conversely, If I launched something at the speed of sound, would the damage output be the same if it were different-sized objects?

In: 5

Answer 1

Think about putting a weight on a scale. The 10lb weight will read 10lbs if it is just sitting there. But lift it a few inches and drop it, and an analog scale will have a spike above 10lbs. This is because when you accelerate the mass of the object, you are putting energy into it. When the weight is stopped by the scale, it “dumps” that energy into the scale, and the scale reads it as a spike above 10lbs. In this case you lift the weight, creating potential energy and dropping it releases that energy via gravity. But that’s another topic all together.

The higher your lift and drop the weight, the more energy is in it, and the higher that needle will move. Of course it won’t be long until that 10lb weight will have enough energy to destroy/break the scale. The same is true for cars, planes, bullets, people. To make a mass move, you have to give it energy. The equations the other posters wrote down define that for you, but the high level idea is, if you double the speed of a given object, you quadruple the energy. THat’s why a car crash at 60mph can be catastrophic, and one at 5mph may do nearly 0 damage. It’s also why getting hit with a baseball bat will break you, but getting hit with a cardboard tube at the same speed might not leave a mark. The faster and/or more massive something is, the more energy it has. When you make that stop you have to use up that energy, and that’s why bends, breaks, and blows things up.