# I Understanding forced motion in space

#### Greg654

Summary
Understanding forced motion in space
Hello,

Can you please explain the analogy oft quoted to explain the concept of applied motion to objects in space, which goes as follows :

1. You are standing on a skateboard or sitting in a boat floating on the water, holding a bowling ball.

2. You throw the bowling ball towards the back of the skateboard (or stern of the boat).

3. As a result of the action of throwing the heavy weight, you (and the skateboard or boat) move in the opposite direction (reaction).

However in non-gravitational space the bowling ball has no weight and is therefore equal, in terms of weight, to that of a feather on Earth.

in non gravitational space : bowling ball = feather = 0 Kg (though both objects evidently possess very different masses)

In what way does the above analogy apply to the context of non-gravitational space in which the object being pushed has no weight ?

edit : for clarity

Last edited:
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#### andrewkirk

Homework Helper
Gold Member
In what way does the above analogy apply to non-gravitational space in which the bowling ball holds no weight and thus equals, in terms of weight, that of a feather :
It has no weight but it still has mass. Weight and mass are different things. That's why they are measured in different units: kilograms for mass and newtons for weight. Mass is defined in terms of inertia, which is the degree of resistance to acceleration. Its definition says nothing about weight, which is to do with gravity. It is Newton's law of gravitation, not his laws of motion, that says mass also affects gravity as well as inertia.

It's important to understand that the effect of mass on weight (gravity ) and the effect on inertia are two different things. A graphic example of that is that, on the Moon, if you are dropped from a height of twenty metres, you will probably survive, but on Earth you would be killed or horribly maimed. But if you crash into a cliff while travelling at 80 kph in a vehicle, you will be killed or maimed regardless of whether on the Moon or on Earth. That's because the first experiment is driven by weight, which is the effect of mass on gravity, while the second is about inertia - the effect of mass on acceleration.

#### A.T.

In what way does the above analogy apply to the context of non-gravitational space in which the object being pushed has no weight ?
You can simulate what happens in space on slippery ice. The weight here is balanced by the normal force of the ice, but the inertial mass is still there, so you can push off from a massive object to propel yourself.

#### PeroK

Homework Helper
Gold Member
2018 Award
Hello,

Can you please explain the analogy oft quoted to explain the concept of applied motion to objects in space, which goes as follows :

1. You are standing on a skateboard or sitting in a boat floating on the water, holding a bowling ball.

2. You throw the bowling ball towards the back of the skateboard (or stern of the boat).

3. As a result of the action of throwing the heavy weight, you (and the skateboard or boat) move in the opposite direction (reaction).

However in non-gravitational space the bowling ball has no weight and is therefore equal, in terms of weight, to that of a feather on Earth.

in non gravitational space : bowling ball = feather = 0 Kg (though both objects evidently possess very different masses)

In what way does the above analogy apply to the context of non-gravitational space in which the object being pushed has no weight ?

edit : for clarity
You're example must use mass, not weight. Momentum is mass x velocity. Weight is irrelevant.

If you are in space it's just as hard to throw a bowling ball as it is on Earth. In space:

Bowling ball = 3kg (or whatever it is). The same as on Earth. There is, however, no force on the ball due to gravity. It's that force that we call weight. So, weight = 0 Newtons. A Newton being the SI unit of force. The kg is the SI unit of mass.

#### zanick

[
Summary: Understanding forced motion in space

Hello,

Can you please explain the analogy oft quoted to explain the concept of applied motion to objects in space, which goes as follows :

1. You are standing on a skateboard or sitting in a boat floating on the water, holding a bowling ball.

2. You throw the bowling ball towards the back of the skateboard (or stern of the boat).

3. As a result of the action of throwing the heavy weight, you (and the skateboard or boat) move in the opposite direction (reaction).

However in non-gravitational space the bowling ball has no weight and is therefore equal, in terms of weight, to that of a feather on Earth.

in non gravitational space : bowling ball = feather = 0 Kg (though both objects evidently possess very different masses)

In what way does the above analogy apply to the context of non-gravitational space in which the object being pushed has no weight ?

edit : for clarity
you have confused weight with mass. (they are different) the bowling ball and feather have no weight in space, but they both have mass. (bowling ball is more). so, If you go back to newtons 2nd law, it will say, a=F/m....... the acceleration of you pushing the bowling ball out the back will depend on its mass and the force applied to it and for how long.. kg is mass...... Newtons are a unit measure of force.

"Understanding forced motion in space"

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