Consider energy. When you throw the ball in space, you give it kinetic energy (m*v^2/2) now unless an other force is applied on the ball at some point, this energy has no reason to disappear. It will not fade nor change. So it will keep moving forever.

On earth though, you throw the ball applying kinetic energy, but it also has potential energy (made by gravity as mgh) and air will apply friction energy opposite to the movement vector(proportional to f*v). As energy is conserved, it does not disappear, it will switch from kinetic to potential and inversely. Friction and hits will turn to thermal energy.

For every action there is an equal and opposite reaction. Basically gravity is the external force which can (and does) change the motion of an object; recall ‘gravitational constant’.

Yes you need to just accept it as fact.

The view that you have is a common misconception because in our daily lives, we’re surrounded by invisible forces that make it appear as though objects stop moving when we aren’t interacting with them. The common sense explanation is that the universe works as your describing. Fortunately, through rigorous science, we can discover the true (or closer to truth) laws of the universe by controlled experimentation and reproducible measurements that hedge against our inherent biases.

I like the energy response. A lot of things make more sense when you look at them from that perspective. Quite simply put: things move through the universe on a path of constant energy. A moving object has a certain amount of kinetic energy and will continue moving in a way that exactly preserves that energy, unless some force is applied to it.

You know how the path of light can bend in the presence of a black hole or other massive object. I’ve heard a lot of explanations for why this happens, and one is that the light follows the path of zero change in energy. Like contour lines on a map showing elevation.

Hope this helps!

Velocity is relative. That includes a velocity of zero. There's no such thing as being "at rest" in an absolute sense. Right now you consider yourself at rest, but to the billions of neutrinos passing through your body every second you're travelling at nearly the speed of light! So the idea that things at rest stay at rest is the same idea that things moving with a constant velocity maintain their velocity. If they don't feel like the same idea, then you haven't yet come to a full appreciation of the principle of (Galilean) relativity.

The reason that it's hard at first to wrap your mind around the principle of relativity is that the forces due to gravity and friction dominate our lives. In our everyday experience, there's very much a difference between being at rest and moving with respect to the air, nothing we push keeps moving in a straight line forever at a constant speed, and everything that goes up comes down. Coming to terms with Newton's 1st Law requires understanding that we're surrounded by complex "special cases" that hide the underlying simplicity of inertia. It's not intuitive, but you can build up an intuition here.

On force...

While force is related to phenomena you're familiar with (e.g., pushing and pulling), ultimately it's an abstract quantity, and it's simply a fact that (net) force is directly proportional to acceleration, not velocity. If it helps, you can regard Newton's 2nd law as a definition: the net force acting on a body is defined as the body's acceleration scaled by its mass.

Also, don't confuse the net force acting on an object with any particular force acting on it. When you push the box and it moves at a constant velocity, it has no net force, but you are still applying a force to it. Think of it like this: the table is trying to bring the box to rest, and you're applying the force that's preventing that from happening. As soon as you stop pushing, the only remaining force acting on the box is due to friction with the table, and since the net force is now not zero, the box's velocity changes as it slows to a stop.

For some reason objects remember their state of motion. This is what Newton's first law says. We do not know why this is the case, just that it is. It is an unsolved puzzle. So either accept it or solve it.

>If I throw a ball in space, I know my hand exerts force on the ball for it to accelerate, but when I let go, it will keep moving forever in a straight path, with no force acting on it. How is that?

Why did the ball start moving?

You exerted force on it.

What would cause the ball stop moving, or otherwise change it's motion?

An external force acting on it, like someone catching it.

Why does the ball move in a straight line with constant speed?

>No force acting on it.