Why do astronauts feel weightless in space?

[Boogeyman]

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

 

[Doc Al]

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.)

Read this for more: Weightlessness

 

[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...

 

[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.

 

[russ_watters]

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...

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.

 

[Boogeyman]

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.

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.

 

[Boogeyman]

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.

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

 

[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).

 

[Doc Al]

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

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!)

 

[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..

 

[Doc Al]

The net force on an object (satellite, astronaut, moon) in orbit around the Earth is zero.

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

The Earth's gravity is balanced by the centrifugal force resulting from being in 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 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).

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.

 

[Doc Al]

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..

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

 

[Shooting Star]

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...

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.

 

[Boogeyman]

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.

Yeah man, you guys' info is great.

 

[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...

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.

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.

 

[Boogeyman]

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

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?

 

[rcgldr]

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.

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.

 

[Boogeyman]

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

 

[Doc Al]

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?

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

 

[Shooting Star]

If the orbit is not circular, than work is being done, and there are changes in speed.

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.

 

[russ_watters]

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.

The post I responded to asked about a constant speed scenario, which must be a circular orbit.

 

[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?

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

Russ, I am still curious about the answer given. Were you replying to some other question?

 

[D H]

The answer is no. The Moon doesn't need thrusters to stay in orbit around the Earth. Why should a satellite?

 

[Shooting Star]

The answer is no. The Moon doesn't need thrusters to stay in orbit around the Earth. Why should a satellite?

Everybody by this time knows the answer. My curiosity is about Russ's using Newton's first law to get to the answer, viz., that the speed is constant in a circular orbit.

 

[D H]

Russ was a bit mistaken. The first law talks about what happens to objects that are not subject to any forces. The second law pertains here. A force perpendicular to the velocity vector does not change the magnitude of the velocity vector. Write the velocity vector as

$$\vec v = v(t)\hat v(t)$$

Assuming a constant mass,

$$\vec F = \dot{\vec p} = m\dot v(t)\hat v(t) + mv(t)\frac{d}{dt}\hat v(t)$$

The time derivative of the unit vector $\hat v$ has no component along $\hat v$. Thus if the force is always normal to the velocity vector the speed v(t) must be constant.

 

[russ_watters]

Yeah, you know what - I had the first and second law reversed.

What I meant was simply that along the direction of motion, f=ma=0 in a circular orbit.

Still, the first law is about just that special case. They are so related, I tend to forget which is which.

 

[Boogeyman]

Wait wait wait how does Newton's 2nd law have anything to do with it. I don't see it, cause this law deals with the rate of change of momentum being directly proportional to the force producing the change.

Hmm, or is it because momentum is just mass by velocity(speed with direction)? So it's like the line of action(the force) is affecting the speed, which is part of velocity which is part of the entire momentum thing? As you can tell, I'm not very good at this...

 

[D H]

Force and momentum are vectors. A force that is always normal to the momentum vector cannot change the magnitude of the momentum vector. It only changes the direction.

 

[Boogeyman]

Oh, but if it was another momentum that was parallel to the momentum, would the magnitude then be changed? (parallelogram law etc.)