# Homework Help: When equation = 0

1. May 26, 2013

### j-fusion

Hi, im a student in grade 12 physics and im a little bit confused on when a system will be equal to 0

For example, Fnet=fy.

I know that a system that is not accelerating will = 0 following newtons second law of motion, however, iv found that using other equations such as centripetal acceleration:Ac= (4π^2 r)/T^2 seems to give me a hard time. Can anyone explain to me exactly when a system is = 0 or not?

Last edited: May 26, 2013
2. May 26, 2013

### CherryTrooper

A system experiences centripetal acceleration when it is changing direction (even if the speed is constant). So if the direction remains the same, the system's acceleration equals zero. All it takes is the change in velocity direction for the centripetal acceleration to be a non-zero number since it's pointed toward a center of a circular path.

3. May 28, 2013

### Andrew Mason

I assume you mean: when is the net force on a system = 0

Remember Newton's first law. A body continues in its state of rest or uniform motion unless a force acts on it. So if there is no force acting on the body, its velocity (which has both direction and magnitude) does not change with time. Conversely, if a body's velocity is changing (in direction or magnitude or both) with time, there must be a force acting on it ie. the sum of all forces acting on it is non-zero.

A central force is a force that is always directed toward the same point. So the acceleration (change in velocity vector / unit time) is always toward that point. That is centripetal acceleration. The body's speed may not change (if the body's path is circular) but its direction is always changing - so there is a force acting.

AM