# What does time=0 represent in kinematics

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Talha
So basically i am confused about time 0 in terms of kinamatics graphs
Lets say i throw a ball upwards so what does time t=0 represent, the time after which the ball is released or the time at which ball is about to be released?
Secondly in projectile motion if i throw a ball horizontally at a height then time zero has zero vertical velocity but a non zero horizontal velocity, why is that(considering the first option that it represents time after ball is released so it must have some vertical velocity even in points due to 9.8m/s^2 acceleration)?

mfb
Mentor
Typically t=0 is chosen to be the point where the ball is released. The velocity at t=0 is then the initial velocity you give the ball, unaffected by gravity.
The velocity for t>0 is affected by gravity.

Keep in mind that this choice is completely arbitrary. You can define that the ball is released at t=6436.83 s. Then gravity only affects the velocity for t>6436.83 s. It doesn’t change the motion of the ball, it just makes the calculation a bit more complicated.

ChocolateScrub
Normally, the complete statement would be "t represents the time in seconds after the ball is released".

So t=0 is when the ball is about to be released and anything >0 is when the ball will have vertical velocity. Since this is the last instant where the ball is still in contact with your hand, this is the initial velocity given to the ball. Afterwards, it will experience acceleration/decelleration due to net force from external factors (air resistance and gravity)

CWatters
Homework Helper
Gold Member
So basically i am confused about time 0 in terms of kinamatics graphs
Lets say i throw a ball upwards so what does time t=0 represent, the time after which the ball is released or the time at which ball is about to be released?
The first case is just before the ball is released.
The second case is just after the ball is released.

In the limit (eg when "just before" or "just after" means an infinitely small amount of time) these are the same thing.

Secondly in projectile motion if i throw a ball horizontally at a height then time zero has zero vertical velocity but a non zero horizontal velocity, why is that(considering the first option that it represents time after ball is released so it must have some vertical velocity even in points due to 9.8m/s^2 acceleration)?
It's so soon after release that the vertical velocity might as well still be zero.