AS Level Mechanics Question

When answering questions with SUVAT, say for example a man throws a ball up into the air, why is it that sometimes we can assume that u = 0, whilst other times we cant.
Continuing my example, a question may say a ball is vertically projected into the air from rest, and since it says from rest I can sometimes safely assume that u=0,
However sometimes a question wont say that, and will instead just say that a ball was vertically projected into the air (without stating its from rest) and i don't know if I'm allowed to assume that u=0.
Sometimes i assume u=0 and it works, sometimes it doesn't, can anyone help me to understand when I'm actually allowed to make this assumption?

Here is an example from a TLMaths video. Here it states that the body is initially at rest so I'm assuming thats why you can say u = 0, however what i dont get is that surely every body is initially at rest, whether the question states it or not.

If it says initially at rest, then u=0. However if it says it is projected upwards or has initial velocity or ... then u is non-zero.

Okay thank you

If it says initially at rest, then u=0. However if it says it is projected upwards or has initial velocity or ... then u is non-zero.

I've just found another question where it doesn't state that it starts from rest yet we still assume u=0 and now I'm lost again.
So now my understanding of vertical motion is that when an object is dropped, i can assume u = 0.
But when an object is projected vertically upwards, i can not assume u = 0 unless it is stated that it was initially at rest.
Is this correct or am I over complicating a simple concept?

Dropped means that the initial velocity is zero. If its projected vertically upwards or downwards, then the initial velocity is non-zero.

Okay right so initial velocity is 0 when an object is moved without any force being applied by someone to make it move (like dropping a ball).
And initial velocity is non-zero when an object is made to move by someone exerting a force onto it (like chucking a ball into the air).
Please say this is right lol

Unfortuately not. A force produces an acceleration (like gravity) and that is independent of the initial velocity (which could be zero). You can throw a ball up or down or drop it, and the effect of gravity (force) is to accelerate the ball downwards at 9.8m/s^2.
The intiial velocity is what happens at t=0, so the instant the experiment starts. The constant force or acceleration is applied to the object during the experiment.

So to find out whether the initial velocity is going to be 0 in a situation, I need to think about what is the objects velocity going to be as soon as the timer starts, and that's why you said that u does not = 0 when an objected is projected vertically upwards or downwards, because when the timer starts it will already be in motion due to the fact that it has been thrown by someone?

Exactly.

I've just found another question where it doesn't state that it starts from rest yet we still assume u=0 and now I'm lost again.
So now my understanding of vertical motion is that when an object is dropped, i can assume u = 0.
But when an object is projected vertically upwards, i can not assume u = 0 unless it is stated that it was initially at rest.
Is this correct or am I over complicating a simple concept?