# Homework Help: Zero velocity and zero acceleraton

1. Nov 18, 2008

### Throwback24

If velocity is the change in position per unit time and acceleration measures the change in velocity of an object, can an object zero velocity or zero acceleration?

In terms of a baseball, the baseball stands still for a moment at the top. Would that mean it has zero velocity?

But does a baseball ever have zero acceleration? It doesn't speed up nor speed down at the at the arc, so would that be zero as well?

2. Nov 18, 2008

### G01

Is the baseball being thrown like projectile?

If it is thrown as a projectile it does always have a velocity, at least in the x direction.

In the y- direction it's y velocity is 0 at the highest point on the path, but the total velocity will still be non zero, unless it was thrown straight up.

Also, there is a constant nonzero force on the ball (gravity) at all times. What does this tell you about the acceleration of the ball at all times?

3. Nov 18, 2008

### curly_ebhc

You are fairly close to understanding what is going on. If you throw the projectile straight up, it will have zero velocity at the top but the acceleration will always = 9.8 m/s/s downwards.

First it is only stationary (v=0) for a fraction of a fraction of a second.

Second it has an upwards velocity immediatly before and a downwards velocity immediatly afterwards. So it has to have a downwards acceleration.