# Are objects at high speeds subject to friction (in described fashion)?

In this video quicksilver moves plastic bullet's with his finger from a different time reference. Needless to say, if bullets were in contact with an ordinary hand in the same manner, it would have some effect of friction and heat etc. I'm just wondering whether for quicksilver (a fast moving object), do these effects differ like in the movie, or not?

## Friction

Friction appears when there is some impending or happening relative motion between the surfaces of two bodies. When there's no relative motion between two surface, there can still be friction to prevent any impending relative motion. This friction is known as static friction. When two bodies' surfaces are moving relative to each other, the friction at play is kinetic friction.

The scene is not too wrong with respect to physics (of course, assuming you could travel at that speed and cope with any relativistic effects ;-)). In the scene, Quicksilver is moving at a high speed (though you only see him moving at a normal speed just because the time scale has been slowed down). In fact, he is travelling at a speed more than the bullets. So when he picks the bullets in his frame of reference of reference, his hand is essentially moving at an velocity really close to that of the bullets, and as we discussed above, since there is no relative motion between the two surfaces, thus there is no friction between them.

If we want to be more accurate, then we should say that there is no kinetic friction between his hand ad the bullet. However, there will be some static friction. But, the heat that you're talking about, can only be generated by kinetic friction. In the case of static friction only, the energy of the system is conserved and there is no loss of energy as heat. So this allows us to conclude that Quicksilver doesn't really feel any heat or burns when he touches those bullets and physics also supports this.

## Who wants to be a Quicksilver?

Let's extend our discussion above to think of an exciting and marvellous thought experiment. Imagine yourself in a cannon, There's a loaded gun just beside you at the same level. Now, if you were shot simultaneously with the gun (at the same speed as the bullet's speed), even you could touch the bullet in mid air, just like Quicksilver did!! (Of course, we are neglecting gravity, air drag and anything which might change the relative velocity between you and the bullet) So, who's ready to go into the cannon?

• Great answer. Though, out of interest, would he have been able to punch the guards with a touch as he did? – yolo Jun 11 at 19:27
• @yolo Definitely. Since he is going at a really fast speed, he doesn't really need to impart much momentum to his arm in order to give athe guard a blow. – FakeMod Jun 11 at 20:21

In the real world, friction forces occur in all instances like these. In the movie world, where screenwriters are english majors and not physics majors, effects like this can be safely ignored for the sake of a good story line.

• In a way it's like asking how much momentum can be transferred with a shorter contact time – yolo Jun 11 at 17:11

At first I was a bit skeptic about physics of this "bullet pushing" with fingers process. So I made some calculations. Typical bullet mass is $$0.0042 \,\text{kg}$$ and speed $$\approx 1000 \,\text{m/s}$$. Given mass-energy equivalence principle this gives relativistic mass increase by $$\Delta_+ m = 2.3\,\times 10^{-14} \text{kg}$$. So in principle this is doable (neglecting human abilities to run at such high speeds comparable to bullet speed).

BUT..., there is some subtle moment. In order to move bullet for some distance away in about $$1 \,\text{second}$$ you need to give $$\approx 1000 \,m/s$$ speed to your arm of mass lets say 2 kg, which gives a force of $$2000 \,N$$ or equivalently $$200 \,\text{kg}$$ punch. Typical elite-boxer punches with about $$350 \,\text{kg}$$ equivalent force. The problem is that QuickSilver doesn't look like he is putting some real effort into this action, it is so easy for him as about eating potato chips,... too easy.

• So what you're saying is, that an increase in speed sacrifices strength? – yolo Jun 12 at 12:12
• No. What I'm saying is that for changing bullet direction you have a very short time window, if you don't want bullet reaching target fast. Thus you need to accelerate your arm from a stationary position $0 \,\text{m/s}$ to almost a bullet speed, thus generating momentum change for your arm $\Delta p = m_{arm}\,v_{bullet}$. This huge acceleration in short time window produces a big punching force. Thus it must be seen from puncher that he/she puts a lot of effort in changing bullet direction. But it is not the case for QuickSilver. Unless you are hulk of course. – Agnius Vasiliauskas Jun 12 at 12:56