I dont understand how objects lose their velocity when for example they’re in air. Shouldnt the momentum of it be constant?
In: Physics
Conservation of momentum applies to an entire system not to the individual parts. So when a moving object travels through air, some of the momentum is transferred to the air molecules. The total momentum of all the air molecules that the object encounters increases, while the momentum of the object itself decreases.
Gravity is a force and all objects on earth are affected by gravity. The same force that pulls things down, also slows them down.
Newton’s Second Law is the answer to this. It states that an object will maintain its momentum… so long as there are no other forces acting on it. There are quite a few forces acting on an object that moves in our world, just to name a few:
1. Gravity
2. Air pressure
3. Wind speed
4. Wind direction
5. Sound waves
6. The angular momentum of Earth itself
All of these and more have some level of influence on how an object behaves when interacting with it.
Don’t confuse velocity and acceleration. Velocity is speed and direction. Acceleration is a change in speed or direction. A force applied to an object is acceleration, so by definition, it changes the object’s speed and direction by speeding it up, slowing it down, or diverting it.
A cartridge accelerates a bullet when it ignites in the chamber and the gasses expand. This accelerates the bullet in the direction of the least opposing force, which is (hopefully) the barrel. But once the bullet leaves the barrel, this force is no longer acting on the bullet and is not accelerating it.
Other forces are still present, though, and they change its velocity. Gravity exerts a force that accelerates the bullet toward the gravitational center of the earth (sideways acceleration, direction). Air resistance exerts a force opposite to the bullet’s path (negative acceleration, speed) through the resisting medium.
When you wave your hand very quickly through the air you can feel the air around your hand as a wind. That is friction between the atoms of your hand and the air bumping into each other and slowing each other down. Friction slows you down and acts against your momentum. Mathematically described by Newton’s Second Law of Motion.
When you jump up and land, that is the force of Gravity accelerating you downwards which acts in reverse to your upwards velocity. Mathematically described by Newton’s Law of Gravity
If there was no gravity and you were in a perfect vacuum, your momentum would be constant. But matter attracts each other due to gravity (accelerating you towards the object) and rubbing against other matter causes friction which slows you down in reality.
If you’re moving through something (like air, water, or oil), you have to push the little bits of it out of the way. It takes energy to do that, so that slows you down.
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