Equal Forces - Bullet and train (opposite directions)

In summary, the conversation revolves around a theory about a man firing a gun from the back of a train traveling at 100mph, and what a bystander would see happen to the bullet. The general conclusion is that the bystander would see the bullet fall to the floor, as motion is always relative to some frame. The conversation also includes discussions about frames of reference, vectors, and the concept of energy required to throw a ball on a moving train versus a stationary train. A video demonstrating a similar experiment is shared with a link provided.
  • #1
bennos
2
0
Hi I've been having an argument with a few friends about a theory of mine (based on what i can remember from my physics lessons and common sense (i think lol).

My theory is this.

There is a train traveling at 100mph in one direction

There is a man with a gun standing at the back of the train pointing the gun exactly horizontal in the opposite direction to the trains direction of travel (ie pointing out the back of the train). the gun that he has fires bullets out at exactly 100mph

The man manages to fire his gun at the exact point at which the train passes through a station and passes a by-stander standing on the platform.

My question is - what does the by-stander see happen to the bullet? (ignoring wind resistance, and any spinning of the bullet caused)

My answer is, the by-stander would just see the bullet fall to the floor.

Can anyone confirm that this is true?

Regards

Ben
 
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  • #2
You are correct.
 
  • #3
:-) any idea what i can say to my 'doubters' that will proove I am right and that they can't argue with?

Cheers
 
  • #4
Do you know anything about frames of reference ? How about adding vectors ?
 
  • #5
Tell them the burden of proof is on them. Tell them they must demonstrate why the bullet would do anything other than drop to the ground.

I suggest, BTW, that, rather than a gun, you switch the scenario to a baseball pitcher, who can throw at 100mph. They will likely have a much easier time grasping the results intuitively. In fact, it may well lead them to perform a scaled down experiment. You'll be a lot closer to winning them over.
 
  • #6
There's was a previous thread about this, and there was a link to a video that was 90% introduction and 10% actual example of the experiment, but basically a ball was shot off the back of an open pickup truck and the ball basically just drops down.

If someone here can remember the thread and/or the link to that video.
 
  • #7
Jeff Reid said:
There's was a previous thread about this, and there was a link to a video that was 90% introduction and 10% actual example of the experiment, but basically a ball was shot off the back of an open pickup truck and the ball basically just drops down.

If someone here can remember the thread and/or the link to that video.

There is some thing I don't understand here. If I were to throw a ball to someone at 20m/hr inside a train moving at 20m/hr (the direction of throw is opposite to the direction of the train) you mean to say the ball would not move?

From experience when inside an airplane if I were to toss something to a passanger at the back, it difnetly does reach them!
 
  • #8
arul_k said:
There is some thing I don't understand here. If I were to throw a ball to someone at 20m/hr inside a train moving at 20m/hr (the direction of throw is opposite to the direction of the train) you mean to say the ball would not move?
It will not move relative to the ground, so a bystander on the ground will just see the ball fall down. Of course it moves relative to the train!

Motion is always relative to some frame.
 
  • #9
Doc Al said:
It will not move relative to the ground, so a bystander on the ground will just see the ball fall down. Of course it moves relative to the train!

Motion is always relative to some frame.

But if the person I throw the ball to were to catch the ball what would the by stander see? Would he see the ball fall or the person catch the ball?
 
  • #10
arul_k said:
But if the person I throw the ball to were to catch the ball what would the by stander see? Would he see the ball fall or the person catch the ball?
He'd see both! From the bystander's viewpoint, one person (the thrower) moved away from the ball while another person (the catcher) moved towards the ball. From his viewpoint, the ball itself didn't move (except to fall down a bit).
 
  • #11
There could only be one outcome and that is the other person catches the ball, but to the stationary observer it is the catcher who (along with the train) moves towards the ball.
 
  • #12
So if I'm directly behind the train on the ground, and someone fires a gun from the back of the traveling train at the same speed, I won't get hit by the bullet...

Interesting.
 
  • #13
scarecrow said:
So if I'm directly behind the train on the ground, and someone fires a gun from the back of the traveling train at the same speed, I won't get hit by the bullet...

Interesting.

Yes. Thats the general idea.

However, the scenario that you just mentioned implies the bullet is moving just as fast as the train (specify these things or ur going to get shot) :biggrin:.
 
  • #14
I get the picture.

However I have another doubt. Will the energy required to throw the ball inside the train at 20m/hr (since it is moving along with the train) be the same as the energy required if the train were stationary?
 
  • #15
arul_k said:
However I have another doubt. Will the energy required to throw the ball inside the train at 20m/hr (since it is moving along with the train) be the same as the energy required if the train were stationary?
Yes, and consider the fact that if this were at the equator, then the Earth is rotating at 1040mph, and that the Earth is orbiting the sun at 65000+mph, and the sun is orbiting in the galaxy at 486000 mph.
 
  • #16
There's was a previous thread about this, and there was a link to a video that was 90% introduction and 10% actual example of the experiment, but basically a ball was shot off the back of an open pickup truck and the ball basically just drops down.

If someone here can remember the thread and/or the link to that video.

Found a link to the aforementioned video.

http://www.spikedhumor.com/articles/109706/Physics-Experiment-With-Baseball.html"
 
Last edited by a moderator:
  • #17
elzekeio said:
Found a link to the aforementioned video.

http://www.spikedhumor.com/articles/109706/Physics-Experiment-With-Baseball.html"

Oh, very cool! All in Japanese, but still very clear indeed.

I especially like the building drama and excitement as the little truck speeds along the race track at the apparently enormously exciting speed of 100 km/hr. The thrill and danger is emphasized with the the driver wearing a whopping big crash helmet...

... but with his mate sitting bare-headed on the tray outside.
 
Last edited by a moderator:

1. What is the concept of equal forces in the context of a bullet and train moving in opposite directions?

The concept of equal forces refers to the fact that when a bullet and a train are moving in opposite directions, they exert an equal amount of force on each other. This is due to Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. Therefore, the force exerted by the bullet on the train is equal to the force exerted by the train on the bullet.

2. How does the mass of the bullet and train affect the equal forces?

The mass of the bullet and train does not affect the equal forces in this scenario. This is because according to Newton's Second Law of Motion, the force exerted by an object is directly proportional to its mass and acceleration. Since the acceleration of the bullet and train is the same (due to their opposite directions), the mass does not have an impact on the equal forces.

3. Can the equal forces between a bullet and train be observed in real life?

Yes, the equal forces between a bullet and train can be observed in real life. This phenomenon can be seen in train tracks when a high-speed train passes by and causes small vibrations in the tracks. These vibrations are a result of the equal forces exerted by the train and the tracks on each other.

4. Is there a maximum limit to the equal forces between a bullet and train?

No, there is no maximum limit to the equal forces between a bullet and train. As long as the bullet and train are moving in opposite directions, they will exert equal forces on each other. However, the magnitude of these forces can be affected by external factors such as air resistance and friction.

5. How does the direction of the forces change if the bullet and train are moving in the same direction?

If the bullet and train are moving in the same direction, the forces exerted on each other will be in the same direction as well. This is because the objects are now interacting with each other, rather than opposing each other. In this scenario, the forces will not be equal as the mass and acceleration of the bullet and train will come into play.

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