Can External Forces Change an Object's Mass Without Altering its Velocity?

In summary, Newton's law of gravity may be more accurately described as the law of motion of a body with a non-zero mass under the influence of an external force. There are examples of the effect of an external force in changing the rest mass of an object while leaving velocity constant, but they are rare.
  • #1
wotanub
230
8
I was thinking about Newton's law. The case of changing mass is often ignored outside the textbook rocket problems. Are there and examples of the effect of an external force is to change the rest mass of an object while leaving velocity constant?

I am familiar with something like in a particle accelerator in which particle are moving near the speed of light, but I do know that in relativistic mechanics, the measurement of "mass" is different from the rest mass. I'm specifically inquiring about phenomena in the classical regime, although I guest that classically, mass doesn't change.
 
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  • #2
but I do know that in relativistic mechanics, the measurement of "mass" is different from the rest mass.
In relativistic mechanics, "mass" is the same as "rest mass".

The only example class I can see:
If wind leads to erosion, a small fraction of the force is used to reduce the mass of the object in the wind.
 
  • #3
What I meant was, in general, [itex]m = \gamma m_{0}[/itex], since [itex]m ≈ m_{0}[/itex] at non-relativistic speeds.

Isn't wind erosion basically sand blasting though? Effectively breaking off pieces of rock (moving/accelerating them). Perhaps I'm thinking of it wrong.

The more I think about this, the more I think it just really isn't possible because what I had in mind is some kind of field that would reduce the mass of a body placed in it. But where would the mass (elementary particles) go if it doesn't simply chip off? I guess it would be converted to energy or use some fancy quantum mechanical tunneling trick, but this would confound Newton. Maybe the problem comes from thinking of a mass as composed of many quanta as opposed to just a blob of stuff.

Then it get's me imagining a bizzaro-world where the norm is for the mass to change and it's hard to think of a case where only velocity changes.
 
  • #4
wotanub said:
What I meant was, in general, [itex]m = \gamma m_{0}[/itex], since [itex]m ≈ m_{0}[/itex] at non-relativistic speeds.
This "relativistic mass" is not used in physics any more.

Isn't wind erosion basically sand blasting though? Effectively breaking off pieces of rock (moving/accelerating them).
Right
It is the same at rockets, you accelerate the exhaust away from the rocket.
 
  • #5
wotanub said:
I was thinking about Newton's law. The case of changing mass is often ignored outside the textbook rocket problems. Are there and examples of the effect of an external force is to change the rest mass of an object while leaving velocity constant?
Do you mean an external net force F on a body of total mass m which centre of mass has velocity v? How could the velocity not vary, since F = ma (non relativistic regime)?
 
  • #6
lightarrow said:
How could the velocity not vary, since F = ma (non relativistic regime)?

Velocity can remain constant since [itex]F = m \cdot \dot v + v \cdot \dot m[/itex] (even in non relativistic regime). It seems unusual but it corresponds to the original definition of force.
 
  • #7
DrStupid said:
Velocity can remain constant since [itex]F = m \cdot \dot v + v \cdot \dot m[/itex] (even in non relativistic regime). It seems unusual but it corresponds to the original definition of force.
If [itex]v[/itex] doesn't vary, [itex]F = v \cdot \dot m[/itex] which means that the system's centre of mass varies and this is a different case from the one I asked, but I have understood better the OP' question now.

If we are not necessarily talking of a linear trajectory but we can also consider an object spinning about a fixed axis, another example could do this: the object spins at constant angular speed while there is a constant non-zero axial torque on it, provided that the moment of inertia varies in the appropriate way.
 
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1. What is the force that changes mass?

The force that changes mass is known as the net force. It is the total force acting on an object and can cause a change in an object's motion or shape.

2. How does force affect an object's mass?

Force can cause an object's mass to change in two ways. Firstly, it can cause the object to accelerate or decelerate, resulting in a change in velocity. Secondly, it can cause an object to deform or change shape, resulting in a change in its density.

3. What are some examples of forces that can change an object's mass?

Some common examples of forces that can change an object's mass include gravity, friction, and applied forces like pushing or pulling an object. These forces can either increase or decrease an object's mass, depending on the direction and magnitude of the force.

4. How does the force that changes mass relate to Newton's Second Law of Motion?

According to Newton's Second Law of Motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This means that the greater the force acting on an object, the more its mass will change.

5. Can the force that changes mass be negative?

Yes, the force that changes mass can be negative. This means that the force is acting in the opposite direction of the object's motion, causing a decrease in its mass. For example, when a car is braking, the negative force of friction is acting against the direction of motion, causing the car's mass to decrease as it slows down.

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