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

[kbncp]

Homework Statement

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)

Homework Equations

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.

 

[borgwal]

That's not an attempt at a solution.

 

[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.

 

[borgwal]

correct!

 

[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?

 

[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.

 

[HallsofIvy]

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

 

[03myersd]

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

 

[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?