# B The direction of Force?

1. Jan 8, 2017

### Matt G

I am a physics novice but I recently became a little confused about the direction of a force matters in terms of impact with another object This probably a poor example but lets say in the first case a human body is still relative to the earth and the bullet is traveling at 2,00 feet per second. But in the second case the bullet is still relative to the earth and the human body is now moving at 2,00 feet per second How would the results of the impact differ? Thanks

2. Jan 8, 2017

### Staff: Mentor

As long as the Earth does not take part in the impact (the human falling down afterwards or whatever), the two cases are exactly identical.

There is no "absolute speed" in physics. You can always decide to study a collision "as seen by the human" or "as seen by the bullet" or any other point of view ("as seen by someone passing in a train"), with the same results.

3. Jan 8, 2017

### Matt G

Thanks mfb Yes that is what I thought but is very counter-intuitive to the average person. Do you or anyone else have advice on how to explain it well I should add that you have already made an important point for me in regard to the relationship to the earth and perhaps that is the point i need to emphasize Additionally the point about the perspective of the bullet versus the perspective of the human body seem like key points as well. so after looking at this for a bit I believe you have summed it up perfectly so I kind of withdrawing the question, but if anyone has anything else i am ears

Last edited: Jan 8, 2017
4. Jan 8, 2017

### Staff: Mentor

Physics is the same for all (inertial) observers. That is one of the most fundamental observations we made. I don't think that concept is so counter-intuitive.

5. Jan 8, 2017

### Staff: Mentor

Many people do find it counterintuitive at first, but that's because our intuition comes from having lived our entire life on the surface of the planet earth, which is so massive and so dominates its immediate surroundings that we find it easy to think that being at rest relative to it is a uniquely privileged state, "at rest" at opposed to "moving".

The answer is to try to retrain your intuition by thinking about situations that aren't so naturally described in terms of motion relative to an enormous planet just underfoot. A few that you might try:
1) A spaceship is flying through empty space when it encounters a pea-sized meteoroid moving at ten kilometers per second relative to it. Does it matter whether the spaceship is moving at ten km/sec relative to the far-distant earth while the meteoroid is at rest relative to the earth? Or if the spaceship is at rest relative to that far-distant earth while the meteoroid is moving at ten kilometers per second? How about if we choose to describe the speeds relative to Mars, or Alpha centauri, or the Andromeda galaxy?
2) Mars is moving relative to the earth at some speed between 5 and 30 km/sec, depending on where the two planets are in their orbits. Your hypothetical bullet is moving at about 600 m/sec relative to the surface of the earth. So how does the shooting look to someone on mars, sitting in a comfortable chair and watching through a telescope? Depending on the time of year, we might have a bullet moving at 5.6 km/sec striking a person moving at 5.0 km/sec, or a bullet moving at 30.6 km/sec striking a person moving at 30 km/sec, or a person moving at 30.6 km/sec catching up to and smashing into a bullet moving at 30 km/sec, or anywhere in between. Now does your intuition tell you that the results of the bullet/person impact might be different?

Last edited: Jan 8, 2017
6. Jan 9, 2017

### sophiecentaur

IS it really counterintuitive? When you prepare to board a high speed train, do you plan a different set of actions for drinking and pouring your tea and tossing a biscuit to your friend across the table? It's the same on a plane - which is going a lot faster.
If you had never heard of modern transport methods then you might have a problem with that (people originally challenged Newton's ideas about motion) but you wouldn't need many train journeys to convince your body that things are just the same as on the ground.
The sums should convince you but Maths doesn't always do as much to correct misconceptions as a simple bit of physical experience.

7. Jan 9, 2017

### Staff: Mentor

I remember reading that when railroads were first invented c. 1830, people worried about the effects on the body, of traveling at the tremendous rate of 30 miles per hour or whatever.

8. Jan 9, 2017

### sophiecentaur

Oh yes, I remember reading that. That 'intuition' was based on having travelled on horseback or a coach / cart on bad roads. The basic fear was being thrown about and possibly out of the vehicle.