# Joules in zero-gravity in space

I started studying physics recently and I got a question.

So newtons are the force that mass exerts when it moves.

let's suppose I have a ball that weighs 5 kg. On earth, this ball would weigh 49N since 9.8m x 1^-2 x 5kg = 49N

In a zero-gravity in space, (where there is no gravitational pull from earth) this ball would weigh 0N since 0m x 1^-2s x 5kg = 0N.

So this means that if I want to push this ball 10m I would need 0J since 0N x 10m = 0J

This seems odd since you are creating motion without using energy (apart from the energy consumption in your body to push the ball)

Am I correct? probably not, again I started studying physics a couple of days ago.

No your assumption is wrong. Being in the vacuum of space does not mean there is no gravitational attraction. Gravity is still 'pulling' on the object. Gravity is a long range force and goes to zero only when distance approaches infinity. As a side note: space is not a perfect vacuum, there are still a few particles per cubic meter. Its closer than we can artificially produce but it's not a perfect vacuum.

In zero gravity if you do not apply a force on the object it will continue moving at constant velocity - Newton.

Suppose that you do apply a force $F$ on the object then the object will accelerate $F=ma$ and its velocity will change - Newton.

If you apply the force over a distance $x$ then the work that you do on the object is $Fx$ and this will result in the kinetic energy of the object changing.

Cease applying the force and the object will then continue moving at its new constant velocity with its new kinetic energy.

• ... And if you want the object to be at rest again you have to do work $-Fx$ to slow it down, making the total work zero. – sammy gerbil Aug 12 '17 at 11:15

Here are some important concepts:

1. Mass is a property of an object.
2. Weight is NOT a property of an object. Weight is a force which acts on an object.
3. Force is NOT a property of an object.
4. Forces do 2 things to objects: change momentum (and consequently result in accelerations), change kinetic energy.
5. In order to make an object which is at rest move, you must apply a force to it. In order to make an object change speed or direction, you must apply a force to it.
6. If the speed changes, you have also changed the energy of the object. (That's not the only sign that energy has changed, but it is one of them.)

So newtons are the force that mass exerts when it moves.

is totally incorrect. It has no conceptual foundation at all. Newtons are units of force. A force is the result of an interaction which can change momentum and energy of an object or system of objects. forces can exist whether object move or not.

Note that I will embed my answers within the original text to more directly answer the questions

I started studying physics recently and I got a question.

So newtons are the force that mass exerts when it moves.

A Newton is a unit of force. Forces are exerted on masses by other masses, but an individual mass does not exert a force on itself. Once an object is in motion at constant velocity, it does not need a force to keep it moving ... which is a major misconception that many people still have before they take a physics class.

let's suppose I have a ball that weighs 5 kg. On earth, this ball would weigh 49N since 9.8m x 1^-2 x 5kg = 49N

In a zero-gravity in space, (where there is no gravitational pull from earth) this ball would weigh 0N since 0m x 1^-2s x 5kg = 0N.

Weight is the force of gravity that the earth exerts on a particular object. That force can obviously vary, depending on where that object is, and how it is moving (e.g., as the object is moving in orbit, it is falling around the earth, and it is apparently weightless, but it STILL has its original mass.

So this means that if I want to push this ball 10m I would need 0J since 0N x 10m = 0J

This seems odd since you are creating motion without using energy (apart from the energy consumption in your body to push the ball)

A NET force is required to accelerate an object, whether it is in orbit or not. You will see, from Newton's 2nd Law, that the force required to accelerate an object follows the equation F=ma. This law applies even on the surface of the earth, which you can verify by measuring the force that it takes to accelerate a given mass, horizontally across the floor. If you do this experiment on a very low friction surface (e.g., the object is on an air track or on wheels), you will see that F=ma applies.

Am I correct? probably not, again I started studying physics a couple of days ago.

See above. Also, READ your text book, as many times as is required to learn the key concepts, and KEEP AN OPEN MIND. Many first year physics students have "pet" concepts in mind that are totally incorrect, and it is almost impossible to get many of those students to reject that pet concept in favor of Newtonian mechanics. If you fall into that group, you will be unteachable, and your grade in your physics class will suffer accordingly.

## protected by Qmechanic♦Aug 11 '17 at 18:18

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