Why is vertical speed decreasing and increasing true but speed at top zero is false?

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Homework Statement


Which one of the following statements is true concerning the motion of an ideal projectile launched at an angle of 45 degrees to the horizontal?

ok so, it seems simple enough, this was a multiple choice questions and i narrowed it down to 2 solutions:
1) the speed at the top of the trajectory is zero.
2) the vertical speed decreases on the way up and increases on the way down.

I got stumped here; both of them seem to be true, so when i looked in the answers it said that 2 was right because "the acceleration of the projectile is always down..[the rest of the answers] are wrong. I don't get this, anyone explain this better?
 
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Speed is simply the absolute value of velocity, which is a VECTOR quantity. This is to say it has direction, and components, which is referred two in part 2. Consider that all components figure into the total speed of the projectile. If a projectile's speed was zero at the top, why does it continue going? :)
 
watsup91749 said:
"the acceleration of the projectile is always down.."

Hi watsup91749! :smile:

The horizontal speed is always the same.

In other words, v.costheta is constant.

It starts at vi.cos45º (vi is initial speed), and it stays at that value, however large v is.

"the acceleration of the projectile is always down.." means that the horizontal acceleration is zero!

So, using Newtons's second law, the change in horizontal momentum must be zero.

In other words, the horizontal speed is contant. :smile: