Projectile motion HELP (1 Viewer)

[PandA_]

When doing calculations with projectile motion, when it asks for e.g. maximum height at a certain time? what formula do i use
can some1 plz explain how certain times fall into the projectile motion formulas

 

[PandA_]

my teacher taught us without 's'
should i start using s?
what is the way to find wat time it wil reach this height without use 's'

 

[k02033]

To get the time needed to reach maximum height, you use this simple logic :

if your vertical velocity is higher than zero at a certain point, then you are still moving up, so that point is not max height.

if your vertical velocity is lower than zero at a certain point, then you have already fallen from the max height.

so you are only at the maximum height when your vertical velocity is zero.

Vy=Uy+at

set Vy=0

so at=-Uy

t=-Uy/a

 

[PandA_]

tyvm, just a quik question k02030 when u say lower then 0, is that becuz -9.8? not actually velocity is negative?

 

[k02033]

when i say that Vy is negative, its actually bias. when we do projectiles, we usually use the coordinates in the 1st diagram. since the vertical velocity is pointing in the -ve y axis direction in that second half of the flight, we say that vertical velocity is negative in direction. and since gravitational attraction is always pointing towards -ve y axis, we say that the acceleration has a magnitude of 9.8, but we say its -9.8, we added to minus to signify direction relative to our axis. This notation is arbitary, notice in my 2nd diagram(look at the y axis) the gravitational acceration relative to that axis is just 9.8 not -9.8