# Suppose you are far into deep space

1. Apr 8, 2013

### Forge

You are accelerating at constant acceleration without anything physically touching you. Do you feel anything? What would an accelerometer read if the same force is applied to the accelerometer without anything physically touching the accelerometer? What would an accelerometer read if the no force is applied to the accelerometer and you are physically holding the accelerometer while you are accelerating at constant acceleration?

Now you accelerate at increasing acceleration without anything physically touching you. Do you feel anything? What would an accelerometer read if the same force is applied to the accelerometer without anything physically touching the accelerometer? What would an accelerometer read if the no force is applied to the accelerometer and you are physically holding the accelerometer while you are accelerating at increasing acceleration?

2. Apr 8, 2013

### mikeph

If an inexplicable uniform gravitational field is giving me a weight, then I don't feel a thing because each element of mass in my body is subjected to proportional amount of force, and each bit of mass is accelerating due to this force by the same amount - there is no compression.

I don't see any reason the case would be different with a time-varying uniform gravitational field, but I never really thought about this before so I'm not certain.

I think an accelerometer would agree, since they are effectively measuring the same thing- proper acceleration.

3. Apr 8, 2013

### Staff: Mentor

Depends on what is causing you to accelerate. If it is gravity, then no. Anything else must be physically touching you to accelerate you though.
Same answer: if it is gravity doing the accelerating, then it reads nothing.
You can't accelerate something without applying a force to it.

4. Apr 8, 2013

### Forge

So would you be technically correct in saying that you are weightless when you are in orbit with Earth despite that your weight causes you to stay in orbit?

5. Apr 9, 2013

### technician

It depends on what the definition of weight is.
If it is the force of gravity on an object then technically you can never be weightless.
If it is what can be measured on bathroom scales then you will have no weight in free fall.
Sometimes, in exam questions, an object in free fall is said to experience 'apparent weightlessness'
In an exam you would be expected to explain.

6. Apr 9, 2013

### ImaLooser

How about being accelerated by an electromagnetic field?

7. Apr 9, 2013

### Staff: Mentor

Can a person be accelerated that way?

8. Apr 10, 2013

### technician

The Maglev is a train propelled by electromagnetic effects. If you were on it I suppose you could say that electromagnetic fields were responsible for the acceleration you experience.

9. Apr 10, 2013

### Forge

Wouldn't it be friction?

The electromagnetic fields would be responsible for accelerating the magnets under the train. But the friction between the magnets and the train is responsible for accelerating the train. The friction between your feet and the train is responsible for accelerating you.

10. Apr 10, 2013

### phinds

That's pretty indirect. What is causing your acceleration is the back of your seat. THAT's what is touching you.

11. Apr 10, 2013

### technician

Indirect but not wrong

12. Apr 10, 2013

### phinds

agreed

13. Apr 10, 2013

### Staff: Mentor

Disagree. Two reasons:

1. The magnetic field accelerates the train. You are accelerated by the chair you are sitting in. "Would be responsible" is carefully selected wording to try to avoid this disconnect.
2. The question was asking how you can be accelerated without feeling a force. If you are on a maglev train that is accelerating, you most definitely feel a force.