Question

hello, in projectile motion, the particle in the x-motion has a constant velocity since acc.(x-motion)=0. why is the acc.(x)=0 in the first place?appreciate the help guys, thanks.

Answer 1

The x-component of acceleration is zero because there is no net force applied on the particle once it gets the initial push/firing/what have you. This is because we are assuming we are in a vacuum--so no friction or drag. In other words, when you throw a baseball in a vacuum, as soon as the ball leaves your hand, whatever speed it leaves your hand in the x-direction is the speed at which it will travel in the x-direction forever; it will never make progress in the x direction faster or slower (there will never be an x component of acceleration). The moment after it reaches your hand, without friction there's nothing slowing it down, and there's certainly nothing speeding it up because you are not longer touching it (and as far as I know there isn't a rocket attached to the ball!).

The reason why we must take into account acceleration in the y-direction is because there IS a net force on the particle...gravity.

Once we get into problems that DO involve friction, you will have to start talking about acceleration in the x-direction. Outside of a vacuum (in the real world), when you throw a ball, it actually has to move THROUGH air, so every air molecule it hits is like a tiny wall slowing it down. As such, a baseball thrown through air, with friction, will experience negative acceleration (also known as deceleration, or slowing down). A car at rest will accelerate until it reaches it's top speed if you step on the gas.

Answer 2

Because gravity pulls things in the (negative) y-direction, not the x-direction. Gravity is powerless to change the x-component of the projectile's velocity vector. So the x-acceleration is zero.

Answer 3

thanks for your help guys.