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Name: Sia
Status: student
Age: 10
Location: N/A
Country: N/A
Date: 8/28/2004


Question:
My 10 years old son wants to know If a person jumps (or throws an object straight up) inside of a moving object (plane, train, others) traveling fast will it land on the same spot theoretically? (Assume time in the air to be long).


Replies:
Sia -

Yes, if the vehicle is in straight-line, unaccelerated motion the object will land in the same spot from which it was launched. Try it in your car. Then try it with the car accelerating or braking.

If you think about it, we are moving on the surface of the earth. ( This motion is unaccelerated and almost straight line.) When you through a ball straight up it will come straight down to the original position.

This is related to Newton's first law of motion - inertia. You might want to read more about it.

Larry Krengel


The best thing is to try this yourself. After all, that is how people figured all this out in the first place. But to answer, if the car is going along steadily, then the ball will never "know" (windows rolled up of course). But if the driver hits the gas just as you throw it up, then things might be a bit different, the ball might fall behind, and seem to come back towards the rear of the car.

Steve Ross


Sia,

The object will land in the same spot within the vehicle. Before throwing the object upward, it has a forward velocity. It is traveling with the vehicle. When thrown upward, the object keeps its forward velocity. It keeps moving forward with the vehicle.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


Sia,

Thank our old friend Einstein for this one as this simple problem was the start of relativity. But to answer your question, Yes, if the object being tossed upward is not affected by any other force and the moving object maintains constant velocity, it will land in the same spot. In the moving object, the observer and all objects could be considered at rest and the rest of the world moving by them in the opposite direction. The laws of physics work the same way.

Bob Hartwell


Hi Sia,

Yes, if you jump straight up inside a moving train, plane, bus or other closed vehicle moving at constant speed, you will theoretically land on exactly the same spot. Experimentally, you will land very close to the same spot; how close depends on how straight up you jumped and how well you can measure the take off and landing spot. Also, if you rotate your body or stick out your arms your feet will land in a slightly different spot since it is your center of gravity that goes straight up and down.

This works because you are travelling just as fast as the train before, during, and after your jump. If the train accelerates (speeds up or slows down) while you are in the air, this is not true and you won't land in the same spot. Also, if you not in a closed cabin (for example standing on a train flat car), the wind will exert a force on you, you will slow down, and not land on the same spot. Inside a closed train, the air inside the train is travelling at the same speed as the train and so will not exert a force on you.

You can most easily check this by throwing up a ball while you are in a moving vehicle and see if it comes straight down as it does when you are not travelling.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Hi Sia,

It surely will land in the same spot. The thrown object is traveling at the same velocity as the car, plane, etc. So when you throw it up, it will come down where you expect it to, as if you are sitting still. If the motion of the vehicle should change while the object is in the air, then it might land in an unexpected place. (Sudden stop, it is in the back seat.)

I hope this explains things.

Martha Croll


Well, if the object stays inside the car so wind does not push it to the rear, and the car is going slow compared with an orbiting spacecraft, and the car is going a steady speed and not turning, then definitely Yes, the ball will land right on the same spot on the seat from which you threw it straight up. Just as it would if you were sitting on a "motionless" park bench.

Going a steady speed does not change anything that happens completely inside a box.

There is a scientific theory that says approximately this, that all non-accelerating frames of reference are equal, with respect to the results of an experiment. (One frame is watching any experiment while sitting still in a moving car, another frame is watching it while sitting still on motionless ground.) This principle was very important to Einstein when he figured out relativity theory.

I guess if the ball takes an hour to come back down there would be some differences. The earth is turning, after all. 15 degrees per hour, or 1/4 degree per minute. So your direction "Up/Down" when you through the ball would be tilted compared with when the ball finally came back down. If you could throw a ball 2,000 feet exactly straight up, I think it might come down a few inches to the west of you. A little bit "behind" you as the earth turns. (Hope I figured it correctly.) Too small a difference to matter, and hard to prove it if you tried. The car's speed going around the earth matters too, but much less. Because at 60 miles per hour, your car is going around the earth about 16 times slower than the earth turns.

Jim Swenson



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