# Homework Help: Throwing a ball in the air

1. May 4, 2012

### influx

When you throw a ball up (vertically; with no horizontal component), when does the speed of the ball start decreasing? I mean it has to decrease for it to stop (momentarily) and then come back down.. Does the speed start decreasing as soon as the ball leaves your hand? or is it at another stage during the journey?

2. May 4, 2012

### tiny-tim

welcome to pf!

hi influx! welcome to pf!

as soon as it leaves your hand …

its constant acceleration is g downward the whole time it's in the air

if it starts with speed v upward, then after time t its speed is v - gt (ie downward after t = v/g)

3. May 4, 2012

### influx

Re: welcome to pf!

Thanks :)

So the fastest speed the ball can achieve is the initial speed (the speed you launch the ball with)?

Also, on the journey back down, the speed of the ball at the instant it hits your hand (assuming you have not moved your hand since the launch) has to be less than the launch speed right? Is this due to the work done against air resistance?

Thanks :P

4. May 4, 2012

### tiny-tim

only if you catch it!!
erm … in exam questions, there's never any air resistance!

(but yes you're correct … in real life, air resistance reduces the energy, and so the speed on the way down will be less)

5. May 5, 2012

### influx

Oh yeah true! Thanks :)

So in exam questions (where air resistance = 0), its safe to assume that the launch speed = the speed of the ball at the instant when it hits your hand?

6. May 5, 2012

### tiny-tim

yup!

(for that reason, if you're asked to find the speed on landing of something thrown out of a window, it doesn't matter whether it was thrown up or down!)

7. May 5, 2012

### influx

Thanks :). One non-physics related question, how do I stop getting emails every time someone posts in a thread I posted in?

8. May 5, 2012

### tiny-tim

at the top of each thread there's a secondary menu-bar …

click "Thread Tools" and then "Unsubscribe from this Thread"

9. May 9, 2012

Thank you