# Homework Help: Velocity Ball In Air

1. Dec 9, 2011

### C42711

A ball is thrown with an initial upwards velocity of 40.0 m/s.
a)How long is it in the air?
b)How fast is it going when it hits the ground?

I decided my given was:
vi= 40.0 m/s
vf=0 m/s
a= -9.8 m/s2

I solved for time and distance using the big four and for
a)4s
b)81.6m

Is this correct?

2. Dec 9, 2011

### PeterO

Not really. You have calculated how long it takes to stop travelling up, and how high it gets.
The questions were how long is it in the air; it has to come down again, and how fast it is traveling when it gets back down.

3. Dec 9, 2011

### C42711

Okay, how is this?
a) 8 s

And then for b, I'm not sure what I'm looking for to be how fast. I would have thought velocity, but they gave me velocity, so that's not it. And it can't be acceleration because that's gravity. So I am confused on what I'm looking for.

4. Dec 9, 2011

### PeterO

OK, you said they gave you the velocity. what was it then? remember you want the velocity just before it hits the ground, not the velocity at which it was thown.

5. Dec 9, 2011

### PeterO

Just realised you may have said vf=0 m/s since when the ball hits the ground it will stop.
The question is really asking you at what speed the ball is travelling as it reaches the ground.

6. Dec 9, 2011

### C42711

So I am looking for final velocity and it's not zero?
If so, I got it to be .61 m/s, but that seems awfully low, so I don't know. I used this equation:
vf= vi2+2*a*d

7. Dec 9, 2011

### PeterO

I trust you actually used

vf2= vi2+2*a*d

What value for d did you use?

I hope it was zero. [notice what happens to that formula when d = 0]

Hint:

put something in your hand, a coin, a pen, a ball.

Now toss it up as if you were juggling a single object - ie repeatedly.

While you are doing that, watch how fast the object leaves your hand, then how fast it arrives back at your hand.