Velocity and acceleration of a ball while it is rising in the air

1. Oct 11, 2008

kbncp

1. The problem statement, all variables and given/known data
A cannonball is shot vertically upward with a velocity of 10m/s. While it is rising in the air (choose one and explain your choice)

2. Relevant equations

3. The attempt at a solution
a. its velocity and acceleration are both upward.
b. its velocity is upward and its acceleration is downward.
c. its velocity and acceleration are both downward.
d. its velocity is downward and its acceleration is upward.
e. its velocity and acceleration are both decreasing.

2. Oct 11, 2008

borgwal

That's not an attempt at a solution.

3. Oct 11, 2008

kbncp

i chose b.. becasue the velocity is slowing down as it is going up, the the acceleration is downwards.. also, velocity is upward because velocity is a vector and up means positive and upward.

4. Oct 12, 2008

borgwal

correct!

5. Oct 13, 2008

03myersd

Would e not also be correct in this case (assuming it means magnitude)? As it is rising in the air the velocity and acceleration would be decreasing? Both would start to increase in magnitude again as it descended, but the question states that it is only when the ball is rising?

6. Oct 13, 2008

borgwal

IF the question was about the magnitudes of $\vec{v}$ and $\vec{a}$, then e would indeed also be correct [the frictional force would decrease in magnitude, acting in the same direction as gravity] but as stated it's not. One point of the question must be to distinguish speed from velocity.

Edit: on the way down the acceleration would keep decreasing in magnitude if there is friction, until it reaches a constant value.

Last edited: Oct 13, 2008
7. Oct 13, 2008

HallsofIvy

Staff Emeritus
Neglecting air resitance, of course, the acceleration would NOT be decreasing. It would remain constant at about -9.81 m/s2.

8. Oct 13, 2008

03myersd

Yeah I can see that now... Must have been having a slow moment earlier! Thanks.

9. Oct 13, 2008

borgwal

I assumed that both 03myersd and the question itself included the effects of air friction. Why else mention air explicitly and not mention "neglect air friction" explicitly?