# 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?
Original post by Dhshsnsksnskxn
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?

For vertical motion onoly, if a ball is projected upwards then the initial velocity, u, is non-zero.

If youre considering horizontal and vertical motion and a balll is projejcted vertically upwards, then the initial horizontal velocity would be zero.

However, it would help to see a u=0 question youre asking about.
(edited 1 month ago)
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.
Original post by Dhshsnsksnskxn
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.
Original post by mqb2766
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
Original post by Dhshsnsksnskxn
Okay thank you

The ball can't go upwards unless it is given a velocity so u never equals zero. When a ball is dropped then u = 0.
Original post by mqb2766
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?
Original post by Dhshsnsksnskxn
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 vertlical velocity is zero. If its projected vertically upwards or downwards, then the initial vertical velocity is non-zero.

Usually at rest refers to horizontal motion (with zero initial horizontal velocity).
(edited 1 month ago)
Original post by mqb2766
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
Original post by Dhshsnsksnskxn
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.
Original post by mqb2766
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?
Original post by Dhshsnsksnskxn
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.
Original post by mqb2766
Exactly.

Thank you so much I can finally answer these questions now
Original post by Dhshsnsksnskxn
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?

If the ball goes UP it cannot have u = 0 vertically - it wouldn;t go upwards without being given an initial velocity.