# I Bull charging and you charge the bull

1. Apr 28, 2017

### TheWonderer1

Would you be better off standing your ground? I don't actually know how massive a bull is but let's say about 1,700 kg. It's coming at you with a velocity of 16 m/s. You got a human who is a decent 93 kg and runs in the opposite direction at 12.517 m/s (he's an olympian 100m runner). Would this decrease the impact rather? I'm thinking about conservation of momentum.

The total momentum of the system changes from about 25,600 to 24,600 (compared to standing). It's not that significantly different if he stood there with no momentum but I'm just thinking a bit.

Last edited: Apr 28, 2017
2. Apr 28, 2017

### TheWonderer1

Apologies, I didn't think about kinetic energy and relative speed.

3. Apr 28, 2017

### Staff: Mentor

It isn't the momentum or KE with respect to earth that matters, it is with respect to you that matters.

4. Apr 28, 2017

### Ibix

One way to think of it is this: by your argument crashing your car into an identical, but stationary, car should trash your car but a head-on collision at speed shouldn't even scratch your bumpers. As Russ says, it's the momentum and energy in your rest frame that matters in determining how much the initial impact will hurt. The Earth frame only becomes important in thinking about how much hitting the ground will hurt.

5. Apr 28, 2017

### 256bits

What if the bull stood still in indifference, and the human ran at it?
What would you then say about the impact?

6. Apr 28, 2017

### Ibix

Are we saying that the choice of frame of reference is a moooot point?

7. Apr 28, 2017

### TheWonderer1

I think to answer the question it would hurt far less to run at a bull that was standing still because there is no momentum coming at you. However, it would still hurt.

I'm not sure what the reference point bit is getting at since you just mention that it matters only in your own reference point.

8. Apr 28, 2017

### jbriggs444

What if the bull was charging at you from the east? The earth is rotating eastward and by a non-rotating standard there is no momentum coming at you at all.

9. Apr 28, 2017

### TheWonderer1

Ok but you would still feel that bull pretty badly.

Was I correct in my assumption that you running at bull would hurt less than the opposite?

10. Apr 28, 2017

### Ibix

Fair enough - I didn't say that well. Reference frame doesn't matter to the physics. But it's easier to solve particular problems in some frame, and in this case your rest frame has helpful properties. Particularly because you initially chose to work in it for the first half of the problem statement.

11. Apr 28, 2017

### jbriggs444

No.

12. Apr 28, 2017

### TheWonderer1

Why not? The overall momentum experienced by you would be less and same as your KE. I understand it's about what you experience so that's the involvement.

13. Apr 28, 2017

### jbriggs444

Because both are descriptions of the exact same collision. The movement (or not) of the earth beneath the feet of the person or of the bull is irrelevant.

14. Apr 28, 2017

### TheWonderer1

I don't see that since there are different amounts of momentum and energy involved.

Are we comparing the same scenario? If you had a bull charging at you vs. if you charged the bull?

15. Apr 28, 2017

### FactChecker

Your change in velocity and the associated acceleration and force would be greater if you run toward the bull.

16. Apr 28, 2017

### TheWonderer1

Ok there is some important aspect I'm missing. The frame of reference stuff changed my understanding and I get that pretty well. I see why the earth moving meaning no momentum would be still a hurtful scenario. However, you charging at the bull who is at rest seems different completely. This is still stumping me as I see the bull sitting there in my head. I run at it and the bull feels very little. I feel my momentum bounce me backwards. However, if that bull decided to charge me, I would bounce much further than if I charged or stayed still hurting me a lot more. I'm going to test it :)

17. Apr 28, 2017

### FactChecker

Force causes damage. F=MA. Acceleration is what counts. Whether you run into the bull at speed v or the bull runs into you at speed v doesn't matter. The acceleration is the same so the damage is the same. If you and the bull are running toward each other, that increases the velocity of the collision and increases the acceleration (=> force => damage) that you would suffer.

18. Apr 28, 2017

### houlahound

I would take one step to the side just before estimated time of impact or throw a red rag on the ground and run up a tree.

Fun fact: bulls run faster up hills than down hills.

19. Apr 28, 2017

### TheWonderer1

Ok just to get to the meat of what you are saying, running at a bull and the bull runs at you would mean greater acceleration than the bull at speed v OR the human at speed v. Therefore, you get hurt much worse.

20. Apr 28, 2017

### houlahound

Also consider the weight of the human, a big guy will get more impact from the same bull than a lighter guy.