# The Impact of Force Direction on Collisions: A Physics Novice's Confusion

• B
• Matt G
In summary, if you are on a train and you drink and then throw a biscuit across the table to a friend, the results will be different depending on how fast the train is going.
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 let's 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

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.

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

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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.

Matt G said:
Thanks mfb Yes that is what I thought but is very counter-intuitive to the average person.
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?

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Cutter Ketch
Matt G said:
but is very counter-intuitive to the average person.
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.

sophiecentaur said:
If you had never heard of modern transport methods then you might have a problem with that
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.

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

## 1. How does force direction affect collisions?

Force direction plays a crucial role in determining the outcome of a collision. When two objects collide, the direction of the force exerted by each object determines the resulting velocity and direction of motion. If the forces are in opposite directions, the objects will collide and move in the same direction, while if the forces are in the same direction, the objects will collide and move in opposite directions.

## 2. What is the difference between a head-on collision and a rear-end collision?

In a head-on collision, the forces of the two objects are in opposite directions, resulting in a combined force that is equal to the sum of the individual forces. This means that the objects involved will move in the same direction. In a rear-end collision, the forces of the two objects are in the same direction, resulting in a combined force that is less than the sum of the individual forces. This causes the objects to move in opposite directions.

## 3. How does the angle of impact affect the outcome of a collision?

The angle of impact, also known as the angle of incidence, is the angle at which two objects collide. The steeper the angle, the greater the force of impact and the more likely there will be damage or deformation to the objects. In contrast, a more glancing angle of impact will result in less force and less damage.

## 4. Can the direction of force change during a collision?

Yes, the direction of force can change during a collision. This can happen if the objects involved are not rigid and can deform or if there are external forces acting on the objects during the collision. In these cases, the direction of force may change and affect the outcome of the collision.

## 5. How can we calculate the force direction in a collision?

The force direction in a collision can be calculated by using vector addition. This involves breaking down the forces into their individual components and then adding them together to determine the resulting direction of the force. It is also important to consider the angles of impact and the properties of the objects involved in order to accurately calculate the force direction.

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