What does kg x m/s mean?


Im having a hard time understanding momentum with people defining it in terms that don’t really explain what it is. I think that getting a feel of what the unit for momentum is might help me

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8 Answers

Anonymous 0 Comments

It’s simply a given amount (mass in kg) of something moving at a given velocity (m/s).

The heavier it is, the more momentum it has.

The faster it is going, the more momentum it has.

Anonymous 0 Comments

The momentum is equal to the mass times the speed of an object.

If an object is heavier or faster it will have more momentum.

Half the mass but twice the speed means you have the same momentum.

Momentum is similar to kinetic energy but the kinetic energy of an object is half its mass times its velocity squared (1/2 x kg x m^(2)/s^2 )

Anonymous 0 Comments

Momentum is a measure of how hard it is to stop something.

(Technically, it is a vector, so there’s also the element of changing direction)

As you can imagine, the heavier something is, the harder it is to stop it. Like when you’re driving, when the car is empty, you can break it easily, but when the car is super full (think of moving houses), then the car might slide a bit more. The way we measure mass is kg, so that’s why it is there.

Another component of momentum is speed. The faster something goes, the harder it is to stop it. The unit for momentum is m/s, so that’s why it is there.

Anonymous 0 Comments

Would this work?
Take a 1L water bottle, have it roll for one meter in a second, and hit your hand.

Presumably, that impact is what 1kg*m/s feels like.

Anonymous 0 Comments

As many other have said, momentum is a vector given by mass times velocity and it gives you an indication on how hard it is to stop (or accelerate) something with a specific momentum.

What I still haven’t found in the comments is that there is a relationship between momentum and force.
If you want to stop an object with 100kgm/s momentum in 1 second you have to exert a force of 100N. Dividing momentum by time you get a force. And multiplying a force by time you get a momentum (in the same exact way you can switch from acceleration to velocity).

I don’t know if it would still be ELI5, but from a more rigorous standpoint, force is defined as the derivative of momentum.
While in most cases this information is pointless, it becomes very important when studying rocket motion and there’s variation in momentum due to the heavy fuel consumption (and subsequent mass reduction) during the flight.

Anonymous 0 Comments

Before looking at Momentum lets look at what Velocity and Acceleration mean since it’ll help us.

Velocity is how fast something is going in a given direction. We define Velocity thus as displacement, not distance, divided by time. The SI unit for this would be Meters per Second. m/s, Acceleration is the measure of how much the Velocity changes over time. If you’ve studied calculus you’d recognize this as the derivative of Velocity. Acceleration is measured in the SI unit Meter per second squared, m/s^2

Thus we can move onto Force and Momentum. Momentum is how much mass is moving in a given direction in a given time. From this we can see that Momentum is defined as Velocity times Mass, or Kg*m/s. Force is how quickly a mass is accelerating, and is thus defined as Mass times Acceleration. These are not the same thing! If an object is moving, it does not necessarily have a force acting on it. Equally, an object could have a force acting on it, but not be moving. From this we can see that force is the change in momentum over time. Note we don’t generally refer to change in momentum as force, but rather the impulse as Force does not have a time component.

These can be a bit unintuitive so a quick look at them. If we launch a rocket into space, it will accelerate as long as its engines are burning. It will get faster and faster and faster, until we run out of fuel. The engine is applying a force to the rocket to make it move faster in a direction. Once the rocket engine cuts out, there is no longer any force acting on the rocket (assume its in a perfect vacuum far enough way from any mass for gravity to act upon it). The rocket however is still moving, and will continue to move in whatever direction it was flying at whatever final speed it had reached, forever. For the case of force with no motion, an example would be friction. If we attempt to push a heavy object, if the amount of force were applying to the object is less then the opposing force of friction, it won’t move.

Anonymous 0 Comments

There is another way to write the same unit: `N*s` (Newton-seconds). It corresponds to alternative formula: `momentum = force * time`

That hints on the physical meaning of the momentum: momentum is a “push-time” of an object – bigger momentum means that the object can push for longer (in time) before it stopped.

Anonymous 0 Comments

It is much easier to understand if you look at it as lbs x ft/s…

The unit is telling you how to calculate the momentum if you know its mass and velocity.

As usual, the definition they give you for momentum in high school is not fully correct since massless particles (e.g. photons) can have and transfer momentum.