# Homework Help: Inertia/Change in Velocity

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1. Feb 2, 2016

### Balsam

1. The problem statement, all variables and given/known data
If an astronaut was in a weighless environment, sitting down and eating a meal, what would happen to the astronaut if the spacecraft decided at that moment to turn on the space craft's main engines and
a) the astronaut was facing the engines?
b) the astronaut's back was to the engine?

3. The attempt at a solution
How would you even figure this out since you don't know what direction the spacecraft starts moving? I'm just really confused. Would the astronaut move in the opposite direction?

2. Feb 2, 2016

### RUber

You don't need to know the direction of the movement, only the direction of the force applied. It seems natural to define this in terms relative to the engines.
The force of the engine is applied to the spacecraft, not the astronaut.

Then the question seems to ask you to think of the motion relative to the astronaut...since it tells you which way he/she is facing.

3. Feb 2, 2016

### PeroK

Does it matter that the astronaut is weightless? What would happen if she/he was sitting in the back of a truck?

In fact, that highlights the need for an assumption about how the engines work!

4. Feb 2, 2016

### Balsam

I think it would result in the same sort of movements as if the person were sitting in the back of a truck since inertia depends on mass and mass never changes, so the same feeling of inertia is experienced in space as on earth. I think this is wrong-- I'm not very good at physics, clearly.

5. Feb 2, 2016

### Balsam

Would the person always move in the direction of the force applied when an unbalanced force acts on them and they are at rest or moving at a constant velocity?

6. Feb 2, 2016

### RUber

The spaceship would move in the direction of the force applied, but the floating person has no force applied to him--until he hits a wall.

7. Feb 2, 2016

### Balsam

I think the question is still asking about the astronaut's movements since this is about inertia and I guess the astronaut was either at rest or moving at a constant velocity before the engines turned on? Which direction does an object at rest or moving at a constant velocity move when an unbalanced force acts on it?

8. Feb 2, 2016

### RUber

Right, from the astronaut's perspective things will look different.
However, the force is going to push the spacecraft forward and he will stay put. What will that look like to the astronaut?

9. Feb 2, 2016

### Balsam

He will hit the wall behind him because the spacecraft is moving forward while he is still moving at the velocity from when he was at rest?

10. Feb 2, 2016

### RUber

Right, but behind him is relative to which way he is looking. That's why there is an a and b.

11. Feb 2, 2016

### Balsam

I'm still confused-- if you know the direction of the force applied, where does the astronaut move relative to that?

12. Feb 2, 2016

### RUber

You had it right. He will move toward the back of the spacecraft, since the spacecraft will move forward and he says still. However, if he is looking toward the back of the spacecraft, he will feel like he is moving forward (in the direction he is facing).

13. Feb 2, 2016

### Balsam

So, when the engines are turned on, the force applied is towards the back of the space craft? What type of force do they apply to the space craft?

14. Feb 2, 2016

### dandy_stepper

The rocket engine will push gases downward (assuming the ship is about to take off) and these gases will exert an equal + opposite force upwards on the rocket. This can also be explained (perhaps more easily) through conservation of momentum

Last edited: Feb 2, 2016
15. Feb 2, 2016

### Balsam

That makes it easier to understand. Thank you!