# Is there a minimum gravity

1. May 13, 2006

### disregardthat

If you are in a spaceship in the middle of nowhere in space. (minimum gravity) Nothing would weigh anything, right? So if someone punches you in the face with their (wieghtless) fist, will it hurt?

2. May 13, 2006

### Curious3141

First of all, I hope you know that just because you're in space does not mean that gravitational forces acting on you are zero, or even minimal. You need to be very far away from anything massive for that to apply. (I think you understand that from the wording of your question).

To answer your question, yes, it will hurt like a mother. $$F = ma$$ still applies and there is still momentum transfer as a fist decelerates rapidly in an inelastic collision against your face.

3. May 13, 2006

### ZapperZ

Staff Emeritus
A mass is defined independent of whether it has any weight. This means that it will still have both inertia and momentum when it moves. Just ask the astronaut who had to stop a statellite from spinning while trying to repair it. So a cannon ball coming at you 100 km/hour will still hurt like crazy whether it is on earth, or in space with almost no gravity.

Zz.

4. May 13, 2006

### Rach3

In particular - it's defined by Newton's second law, as the ratio of how much it accelerates to how much force is applied in order to accelerate it:

$$m\equiv\frac{F}{a}$$

The same fist will take the same force to decelerate in your face, as anywhere else in space or on earth; because its inertial mass is always the same.

5. May 14, 2006

### disregardthat

Bit i heard that hammering a nail in a piece of wood, would be much harder on the moon due to its low gravity. would this mean that it would be almost impossible at a place with extremely low gravity? Is the reason then that the hammer is weightless, or something else?

6. May 14, 2006

### Curious3141

Think about what happens when you hammer something on earth in the usual orientation - e.g. when driving a nail into the floor.

Then consider hammering something in a weird orientation on earth, like hammering a nail up into the ceiling.

One task will be made harder, the other easier in zero g. Can you figure out which is which and why? (Of course, we're only considering localised forces here and assuming everything is anchored down during the work, if everything and everyone are floating around, things will become harder no matter what).

7. May 14, 2006

### pervect

Staff Emeritus
A point I want to add to the previous discussion.

An object in a weightless environment may not have any "weight" (net force due to gravity), but it will certainly still have inertia.

8. May 15, 2006

### skywolf

wouldnt the force then matter how far away they are from you, since there is no longer a floor that they can push of off to punch you, they now need a part of their body to equally accelerate in the other direction?

9. May 17, 2006

### Farsight

Not if they get in close and get a hold of your hair:

Smack!

Rattle, clatter. Them's your teeth pinging around the airlock.

10. May 17, 2006

### disregardthat

i dont understand this! inertia???

11. May 17, 2006

### Neohaven

What is it you don't understand about inertia, specifically?

12. May 17, 2006

### Staff: Mentor

The best way I've seen to notice inertia decoupled from weight is with a grocery cart or other similar cart (like a cart at Ikea). At Ikea, all 4 wheels are castors, so they swivel. You aren't supporting the weight of what is in the cart, yet the more you put in the cart, the harder it is to accelerate/decelerate it. And with one of those carts at Ikea, if you have a bunch of unassembled furnature in it, get some decent speed, then go to turn, you'll find that it will spin, but it will not turn unless you orient yourself perpendicular to its direction of motion. That resistance to change in velocity is inertia.

Last edited: May 17, 2006
13. May 18, 2006

### Farsight

Weight is kinda like inertia pointing down because of gravity, Jarle. Similar to that shopping trolley, only Mr Gravity keeps on pulling it down. Anything that's got a mass has always got inertia, which is its resistance to moving. It's the same as momentum, which is the resistance to not moving, and moving depends on your point of view - is it you who's moving or the other thing? Anyhow, inertia is basically how we define something as having mass.

So a mass has always got inertia, even if there's no gravity around. So, smack, it hurts.

14. May 18, 2006

### pervect

Staff Emeritus
Inertia is the tendency of bodies that are in motion to remain in motion.

The amount of inertia a body has is measured by its inertial mass and its momentum. Inertial mass is just the quantity 'm' in Newton's law of motion

Force = (m)ass * acceleration.

while momentum = mass * velocity.

To give an example of the importance of inertia

A hypothetical space construction worker, working in "0 g", would be able to lift extremely heavy beams, beams he could not possibly lift on Earth, due to the weightless environment.

He would have to be careful how he handled them, though. If the worker applies a force to make the beam move, the beam will move very slowly, because of it's high inertial mass (the m in Newton's law, the acceleration will be given by force/mass).

If he applies a large force, or a small force for a very long time, he could make the beams move very rapidly. But once the beams are moving so rapidly, they would be very hard to stop - because of their inertia. They could seriously injure a person who got hit with them, or could damage other parts of the construction project if an object was hit rather than a person.

15. May 21, 2006

### disregardthat

Now i do understand ;) thanks.