Why Does a Thrown Stone Have Constant Acceleration?

[Stripe]

Homework Statement

ok so on a practice test it states that:

http://img4.imageshack.us/img4/797/98387316.png

During its flight, the graph of the acceleration of a stone thrown is best shown by:

Homework Equations

The Attempt at a Solution

Ok so basically there are 4 possible solutions, 4 graphs.

The first one is in the form of a negative parabola, that starts midway on the y intercept and goes up and down until it reaches 0 after 5 seconds.

The second one is a positive parabola that starts on the same y intercept, but goes down to 0 after 2 seconds then goes up.

The last one is one that has a constant line at the same y intercept.

And some other one which i am sure isn't the answer.


Anyway so i chose the first one, also suspecting it may be the second one. The answer says it's the last one.

Now what i don't understand is, how the acceleration can be constant? it firstly travels up, and then down, which means that the acceleration goes from positive to negative?

UNLESS the question is talking about the acceleration across not up and down.

I know this sounds confusing this question is very misleading...

Thanks anyway~!

 

[ideasrule]

The fact that it first travels up, then down means that SPEED isn't constant. That doesn't mean acceleration isn't constant.

The only force acting on the object is gravity, and acceleration due to gravity is a constant value.

 

[Stripe]

oh...i get it!

Thanks that was really helpful.

Let's hope that's what the jibbed question meant!

thanks again