# Homework Help: Initial vs final for a projectile

1. Jul 18, 2013

### judas_priest

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

A projectile is fired through the air. It is launched from the ground, and travels without interference from wind or air resistance, landing on a raised platform above the field.

Initial speed vs. final speed:

Initial vertical velocity vs. final vertical velocity

Initial horizontal velocity vs. final horizontal velocity:

Initial acceleration vs. final acceleration:

The attempt at a solution

For initial speed vs final speed, going by the basic definition, that is distance by time, they always stay in a ratio, so it doesn't change

For Initial vertical velocity vs. final vertical velocity, going by the basic definition again, that is displacement by time, till the highest point there's maximum displacement, therefore the ratio of displacement by time is the highest there. Even higher than just before landing, because it comes down, and hence the displacement decreases, also decreasing the ratio of d/t

Initial horizontal velocity vs. final horizontal velocity, there's no change because there's no air resistance or wind or any disturbance.

Initial horizontal velocity vs. final horizontal velocity. They're both the same in magnitude, just signs differ.

Please tell me where I'm wrong and why.

2. Jul 18, 2013

### PeterO

In order:
Distance over time won't help - you should be able to work it out by understanding projectile motion..

Displacement over time won't help - you should be able to work it out by understanding projectile motion

Correct - you understand that side of projectile motion

Puzzling: you have addressed Horizontal velocity a second time rather than considering acceleration.

3. Jul 18, 2013

### judas_priest

Oops!
I meant "They're both the same in magnitude, just signs differ." for comparision of accelerations.

Anyways, how do I determine using projectile motion. No idea has struck me yet. If you could give me a start to my thinking.

4. Jul 18, 2013

### PeterO

Not quite for acceleration: What is causing the acceleration?

The Vertical velocity in projectile motion is the same as for an object thrown straight up.

If it comes back to original height, it slows down on the way up, then speeds up on the way back down - regaining all the speed it lost - arriving back at the same speed that it began (but different velocity because of the direction.

If it gets "interupted" on the way down, and doesn't come back to the original level, then it does not completely regain its original speed.

How was that for a hint?