# Homework Help: Acceleration of a ball thrown vertically upwards

1. Sep 17, 2015

### Janiceleong26

1. The problem statement, all variables and given/known data

I chose A because since the acceleration of the ball is positive vertically upwards, the acceleration downwards is negative, and at the turning point, acceleration is zero.. The correct answer is B. Why?

2. Sep 17, 2015

### Staff: Mentor

Perhaps you are confusing acceleration (which is the rate of change in velocity) with velocity? Why do you think the acceleration changes? Does the force change?

3. Sep 17, 2015

### Staff: Mentor

If the acceleration were 0 at the maximum height, the ball would just sit there suspended. Does that happen?

If the acceleration were positive as the ball rises, then it would get faster and faster as it does. Does that happen?

4. Sep 17, 2015

### Janiceleong26

Oh..I get what you mean.. I thought acceleration was zero at the maximum height, because v=0.. but I shouldn't see into velocity, it's the change, thanks for the help!

5. Sep 17, 2015

### Janiceleong26

But why, whenever we do Kinematics questions, where an object falls, we take acceleration as g= + 9.81ms^-2 ?

6. Sep 17, 2015

### Staff: Mentor

I'm not exactly sure what you are asking, so I'll mention two things:
(1) g is usually taken as a positive constant; g = 9.81ms^-2;
(2) for a falling body, the acceleration is downward with magnitude g; whether you express that downward acceleration as + or - depends on your sign convention. (Sometimes it's convenient to use "down = positive".)

7. Sep 18, 2015

Ok thanks !