Physics general question..

Hi, I'm confused when i have to use acceleration as 9.8m/s^2 or 0?

What does it mean by the acceleration is constant? i know that that means no air resistance and no effects and ect.. but when i get the question i had to find out what the acceleration then put into other equations to find x or ect.. and why??

and one of the question: A soccer ball is kicked with a velocity is 25ms^-1 at an angle of 30degree above the horizontal ground. how far does ball travel horizontally before striking the ground?

i found the t = 2.55s

now i had to use x= x0 + Vx0 * t
but where is that equation is from? in this case this is the full equation x= x0 + Vx0 * t + 0.5at^2
and why a = 0?

This is a projectile problem. in a projectile, there is no horizontal acceleration as the ball travels at the same horizontal velocity throughout its flight. the only acceleration acting upon the ball is gravity (9.8m/s) and this only affects the vertical component of the projectile. Therefore, when you only look at the horizontal component, a = 0.

I'm not an expert, just doing intro physics myself. However, I can tell you that you use the acceleration due to gravity (that is the constant you have listed) whenever you are dealing with items in free fall. So when the only force acting on an object is it's falling towards the earth, or being thrown up away from the earth.

Just be watchful of your signs for direction.

Hope this helps even a little bit, and someone who can explain better will give you a better answer :)

the acceleration is 0 because acceleration always act's vertically downward but as you want to find the horizontal distance , the component of acceleration is zero in horizontal direction as the angle b/w horizontal & vertical is 90degree