D&D 5E Levitate is a save-or-die spell

[Xetheral]

But can you cast levitate on a stone that is being thrown at you with a readied action? If you do, what happens?

If it's mass is nullified temporarily (Magic!) then it stops moving forward. It also has no weight.

If it still has mass but is not being acted upon by gravity it has no weight. But the planet is still rotating, revolving around the sun and moving through space. Does it move in a semi-random direction relative to the planet and everyone else on it while still maintaining forward momentum?

Something else? Does it merely rise up 10 ft and continue on it's previous arc but in a straight line?

Inquiring minds want to know!

A thrown rock is already in freefall, and is therefore weightless. I would rule that the caster gains control over the vertical component of the rock's trajectory, within the velocity limits of the spell, but gains no control over the horizontal component of the trajectory (although the horizontal component can be indirectly affected by changing the vertical component so as to cause a collision).

 

[Guest 6801328]

A thrown rock is already in freefall, and is therefore weightless. I would rule that the caster gains control over the vertical component of the rock's trajectory, within the velocity limits of the spell, but gains no control over the horizontal component of the trajectory (although the horizontal component can be indirectly affected by changing the vertical component so as to cause a collision).

Ooh, that's a great example. Taken literally, you could not cast Levitate on a falling object (for example, if you readied action to cast Levitate when somebody jumped off a cliff) because it would weigh nothing at the moment of casting.

Which, I think, is evidence that we should read "weight" to really mean "mass". Or, to really be correct with our terms, “what the weight would be if at rest on the ground.”

At sea level.

If there were a sea.

SEE WHAT A SLIPPERY SLOPE THIS IS!?!??!

 

[Guest 6801328]

@Maxperson I do apologize for going after you too hard. Given our history of sparring, I took unseemly delight in finally cornering you on something definitive.

/guilty

 

[Maxperson]

@Maxperson I do apologize for going after you too hard. Given our history of sparring, I took unseemly delight in finally cornering you on something definitive.

/guilty

Thanks! And I likewise apologize for being overly stubborn about things. I was indeed wrong about the weight/mass thing. The Mage Hand spell is most likely conflating the two like people commonly do.

Or maybe it isn't and the next time I get to play, I can threaten to knock the world into the sun if the DM doesn't do what I want.

 

[Xetheral]

Or Orybe it isn't and the next time I get to play, I can threaten to knock the world into the sun if the DM doesn't do what I want.

Hmm. We know a mage hand can produce at least 10 lbs of force. If we assume the Earth is a valid target for mage hand because it is weightless, the spell should be able to deliver at least 10 lbs of force to the Earth. Converting to metric, that's 44.5 N. Pushing the Earth into the sun requires imparting roughly 30 km/s to the Earth. The formula to calculate how long it would take to accelerate the Earth by 30 km/s is:

(time force applied)=(desired velocity change)*(Earth's mass)/(force applied)

Plugging in all the values we get: (time force applied)=(30 km/s)*(5.97*10^24 kg)/(44.5 N)=1.28*10^20 years.

That's about 128 quintillion years. The Earth will collide with the Sun much sooner than that when the Sun starts expanding into a red giant, which suggests that your efforts are futile.

But wait! Colliding with the outer edge of the Sun becomes easier as the Sun starts to expand in about 5.4 billion years. Maybe you can speed up the engulfing processing by pushing the Earth into an orbit that intersects the expanding Sun sooner? Let's find out.

In 5.4 billion years of applying 44.5 N of force to the Earth, you can change it's velocity by:

(44.5 N)/(5.97*10^24 kg)*(5.4 billion years)=1.27*10^-6 m/s

Rats! That's not enough to make an appreciable change in the Earth's orbit. So, tragically, your threat to shove the Earth into the Sun with Mage Hand is toothless. You can't meaningfully speed up the inevitable collision.

You might have better luck if you campaign world is much less massive than Earth (about 7 orders of magnitude), and orbits (at the same distance as Earth) a red dwarf that will last 10 trillion years or so, giving you a longer window to push with mage hand. Sadly, that campaign world would be utterly frozen since it would orbit far outside the red dwarf's habitable zone. If you're serious about pursuing this idea, I recommend the 3.5 sourcebook "Frostburn".

 

[Maxperson]

Hmm. We know a mage hand can produce at least 10 lbs of force. If we assume the Earth is a valid target for mage hand because it is weightless, the spell should be able to deliver at least 10 lbs of force to the Earth. Converting to metric, that's 44.5 N. Pushing the Earth into the sun requires imparting roughly 30 km/s to the Earth. The formula to calculate how long it would take to accelerate the Earth by 30 km/s is:

(time force applied)=(desired velocity change)*(Earth's mass)/(force applied)

Plugging in all the values we get: (time force applied)=(30 km/s)*(5.97*10^24 kg)/(44.5 N)=1.28*10^20 years.

That's about 128 quintillion years. The Earth will collide with the Sun much sooner than that when the Sun starts expanding into a red giant, which suggests that your efforts are futile.

But wait! Colliding with the outer edge of the Sun becomes easier as the Sun starts to expand in about 5.4 billion years. Maybe you can speed up the engulfing processing by pushing the Earth into an orbit that intersects the expanding Sun sooner? Let's find out.

In 5.4 billion years of applying 44.5 N of force to the Earth, you can change it's velocity by:

(44.5 N)/(5.97*10^24 kg)*(5.4 billion years)=1.27*10^-6 m/s

Rats! That's not enough to make an appreciable change in the Earth's orbit. So, tragically, your threat to shove the Earth into the Sun with Mage Hand is toothless. You can't meaningfully speed up the inevitable collision.

You might have better luck if you campaign world is much less massive than Earth (about 7 orders of magnitude), and orbits (at the same distance as Earth) a red dwarf that will last 10 trillion years or so, giving you a longer window to push with mage hand. Sadly, that campaign world would be utterly frozen since it would orbit far outside the red dwarf's habitable zone. If you're serious about pursuing this idea, I recommend the 3.5 sourcebook "Frostburn".

I think perhaps you have underestimated things a bit! Mage Hand is a cantrip, so pausing only to eat, sleep and use the bathroom...

 

[Umbran]

Mod Note:

@Mistwell and @Elfcrusher - Please cut it out. Use the Ignore feature, if you have to. But the squabbling ends now.

 

[Umbran]

Ooh, that's a great example. Taken literally, you could not cast Levitate on a falling object (for example, if you readied action to cast Levitate when somebody jumped off a cliff) because it would weigh nothing at the moment of casting.

So - there are inaccuracies creeping in, because folks are not being consistent. There are two definitions of "weight".

Operational - weight is the force an object exerts on a thing supporting it. If nothing is supporting it, it has zero weight.

Gravitational - weight is the force exerted on an object by gravity. A thing in free fall still has gravitational weight.

The problem comes in that when you are saying that when an object is in free fall, it is "weightless", you are using the operational definition. If you are realizing that you need to talk about mass, rather than weight, you are implicitly using the gravitational definition. These two definitions are not interchangeable, and things will go awry if you blithely swap between them.

 

[Guest 6801328]

So - there are inaccuracies creeping in, because folks are not being consistent. There are two definitions of "weight".

Operational - weight is the force an object exerts on a thing supporting it. If nothing is supporting it, it has zero weight.

Gravitational - weight is the force exerted on an object by gravity. A thing in free fall still has gravitational weight.

The problem comes in that when you are saying that when an object is in free fall, it is "weightless", you are using the operational definition. If you are realizing that you need to talk about mass, rather than weight, you are implicitly using the gravitational definition. These two definitions are not interchangeable, and things will go awry if you blithely swap between them.

Given your conclusion, I'm surprised you snipped my next paragraph, which reached (effectively) the same point:

Which, I think, is evidence that we should read "weight" to really mean "mass". Or, to really be correct with our terms, “what the weight would be if at rest on the ground.”

I think it's possible to have fun tossing around the silliness that ensues when you try to rationalize magic with science without letting that get in the way of reasonable in-game adjudication.

 

[Guest 6801328]

Hmm. We know a mage hand can produce at least 10 lbs of force. If we assume the Earth is a valid target for mage hand because it is weightless, the spell should be able to deliver at least 10 lbs of force to the Earth. Converting to metric, that's 44.5 N. Pushing the Earth into the sun requires imparting roughly 30 km/s to the Earth. The formula to calculate how long it would take to accelerate the Earth by 30 km/s is:

(time force applied)=(desired velocity change)*(Earth's mass)/(force applied)

Plugging in all the values we get: (time force applied)=(30 km/s)*(5.97*10^24 kg)/(44.5 N)=1.28*10^20 years.

That's about 128 quintillion years. The Earth will collide with the Sun much sooner than that when the Sun starts expanding into a red giant, which suggests that your efforts are futile.

But wait! Colliding with the outer edge of the Sun becomes easier as the Sun starts to expand in about 5.4 billion years. Maybe you can speed up the engulfing processing by pushing the Earth into an orbit that intersects the expanding Sun sooner? Let's find out.

In 5.4 billion years of applying 44.5 N of force to the Earth, you can change it's velocity by:

(44.5 N)/(5.97*10^24 kg)*(5.4 billion years)=1.27*10^-6 m/s

Rats! That's not enough to make an appreciable change in the Earth's orbit. So, tragically, your threat to shove the Earth into the Sun with Mage Hand is toothless. You can't meaningfully speed up the inevitable collision.

You might have better luck if you campaign world is much less massive than Earth (about 7 orders of magnitude), and orbits (at the same distance as Earth) a red dwarf that will last 10 trillion years or so, giving you a longer window to push with mage hand. Sadly, that campaign world would be utterly frozen since it would orbit far outside the red dwarf's habitable zone. If you're serious about pursuing this idea, I recommend the 3.5 sourcebook "Frostburn".

Why stop at 30 km/s? Just keep pushing.