# Weightlessness in space

1. Oct 14, 2007

### Boogeyman

The astronaut in space feels weightless, yet he has weight. Could you guys explain? I have an idea but not quite sure...

2. Oct 14, 2007

### Staff: Mentor

One must distinguish real weight (the pull of earth's gravity) with what is called apparent weight, which is the magnitude of the contact force supporting an object. "Weightlessness" is when the apparent weight equals zero.

The supporting contact force is what gives us the feeling of having weight. Remove the support--by jumping out the window, for example--and you'll feel "weightless". (Until you hit the ground, that is.)

Last edited: Oct 14, 2007
3. Oct 14, 2007

### Boogeyman

Does a spacecraft in orbit around the Earth need a force (i.e from the rockets) to maintain constant speed? Why?

I think the answer is yes but I can't give a good reason..my mind keeps telling me the spacecraft will keep constant velocity but the acc. due to gravity (3N/kg) obviously can't be ignored...

4. Oct 14, 2007

### cevdet.erturk

I think u r confused about the meanings of weight and mass.

weight=gravity.mass and it depends where u stand. Earth, mars, moon or smthn.

But mass is same everywhere. As it is in space.

Note: Astronaut has an accelaration? But feeling of jumping window (till touchin ground) contains an accelaration. I mean these should be different feelings from each other.

5. Oct 14, 2007

### Staff: Mentor

According to Newton's first law, the answer is no.

The acceleration due to gravity is perpendicular to the direction of motion, so it does not affect the speed, just the direction. That's what an orbit is.

6. Oct 14, 2007

### Boogeyman

Yeah I know mass is constant, but in this last question, I'm not sure if a spacecraft with a weight needs a force to keep constant speed.

7. Oct 14, 2007

### Boogeyman

Ah, good I was thinking along the lines of that. My mind's real rusty..

8. Oct 14, 2007

### mathman

The net force on an object (satellite, astronaut, moon) in orbit around the earth is zero. The earth's gravity is balanced by the centrifugal force resulting from being in orbit. If the object was not in orbit, it would fall to earth. If the earth's gravity stopped, it would go on a straight line tangent to the orbit (at the point it was when the earth disappeared).

9. Oct 14, 2007

### Staff: Mentor

No. If the spacecraft is in a stable orbit around the earth, it needs no rockets to maintain its speed.

(I'm getting slow; Looks like several others answered this question!)

10. Oct 14, 2007

### Boogeyman

How do I calculate speed of the spacecraft when all I'm given is mass(1000kg), radius(12000km) and gravitational field strength(3N/kg)? I know weight is 3000N, but I can't make the connection..

11. Oct 14, 2007

### Staff: Mentor

No. Gravity exerts a net force on the object and the object is accelerating as it moves in its orbit.

Centrifugal force is a "fictitious" force that is an artifact of viewing things from a rotating reference frame. (No need to use a noninertial frame here.)

If there were no force acting on the object, it would keep going in a straight line at constant speed. But gravity pulls it in a curved orbit. If the object doesn't have enough speed, it will crash into the earth.

12. Oct 14, 2007

### Staff: Mentor

Use Newton's 2nd law. Hint: Consider the centripetal acceleration of the spacecraft (assuming it's a simple circular orbit).

13. Oct 14, 2007

### Shooting Star

The spacecraft does not need any force from the rockets to keep it orbiting the earth, provided the orbit does not pass through the atmosphere. In circular orbit the speed remains constant, in elliptical orbits the speed will vary. But the craft is revolving around the earth due to gravity only,which you had rightly pointed out, cannot be ignored

Does the moon need rockets to be in orbit around the earth? Or the earth about the Sun ? Now you understand, I hope.

14. Oct 14, 2007

### Boogeyman

Yeah man, you guys' info is great.

15. Oct 14, 2007

### Shooting Star

Quote:
Originally Posted by Boogeyman
Does a spacecraft in orbit around the Earth need a force (i.e from the rockets) to maintain constant speed? Why?

I think the answer is yes but I can't give a good reason..my mind keeps telling me the spacecraft will keep constant velocity but the acc. due to gravity (3N/kg) obviously can't be ignored...

How does it follow from Newton's 1st law?

If the orbit is not a circular one, then the force due to gravity is not perpenicular to the direction of motion, except at the perigee and the apogee.

16. Oct 14, 2007

### Boogeyman

r=12000km
mass of spacecraft=1000kg
grav. field strength= 3N/kg
So the acceleration can be shown to be equal to v²/r where v is the speed around the orbit and r is the orbit radius..and F=ma. So using Newton's second law, a=3000N/1000kg=3m/s²

If a=3m/s², then 3m/s²=v²/12000 => 3x12000=v² so v=190m/s

I think that's it guys?

17. Oct 14, 2007

### rcgldr

If the orbit is not circular, than work is being done, and there are changes in speed. The total energy, which is the sum of gravitational potential energy and kinetic energy will be a constant.

18. Oct 14, 2007

### Boogeyman

Okay to make it clear, the orbit is circular. This is 15 year old physics guys...(cause I'm 15):D

19. Oct 14, 2007

### Staff: Mentor

Almost. The radius is given in km, not meters. Convert! (But you have the right idea.)

20. Oct 15, 2007

### Shooting Star

You are merely repeating what I had said. That's not what I had asked. I was curious when the mention of Newton's first law was made in a post. See my original posting.

Let's just forget it. It's not important.