Simple acceleration problems - requesting a check

[RyanJF]

This is a bit of reading, but the math is simple. I only need my work checked, except for problem number three, in which case I'm simply asking for clarification on which equation I should use.

Homework Statement

Problem number one:

"A flower pot falls from a windowsill 25m above the sidewalk. A) How fast is the flower pot moving when it strikes the ground? B) How much time does a passerby on the sidewalk below have to move out of the way of the way before the flower pot hits the ground?"

a = 9.8 (gravity)
x = 25
Vi = 0

Problem number two:

"A plane, starting from rest at one end of a runway, undergoes a constant acceleration of 1.6 m/s^2 for a distance of 1600 meters before takeoff. A) What is its speed upon takeoff? B) What is the time required for takeoff?"

a = 1.6
Vi = 0
x = 1600

Problem number three:

"A boy sliding down a hill accelerates at 1.4m/s^2. If he started from rest, in what distance would he reach a speed of 7/ms?"

a = 1.4
Vi = 0
Vf = 7

Homework Equations

Problem number one:

v^2 = Vi^2 + 2ax

Problem number two:

v^2 = Vi^2 + 2ax

Problem number three:

Not sure, that's why I'm asking about it. Possibilities are:

v = Vi + at
x = 1/2 (Vi + V)t
x = Vi x T + 1/2at^2
v^2 = Vi^2 + 2ax

The Attempt at a Solution

Problem number one:

v^2 = Vi^2 + 2ax

Plugging in numbers for the variables gives me

v^2 = 0^2 + 2 x 9.8 x 25

so

v^2 = 490

sq(490 = 22.135

V = 22.135 m/s

Part b, all I did was:

25/22.135 = 1.129 seconds for the passerby to move out of the way of the falling flower pot.

Problem number two:

v^2 = Vi^2 + 2ax

Plugging in the numbers for the variables gives me

v^2 = 0^2 + 2 x 1.6 x 1600

so

v^2 = 5120

sq(5120

V = 71.554 m/s

For part B, all I did was:

t = Vf - Vi / A

Plugging in the numbers for the variables gives me

t = 71.554 - 0 / 1.6

so

t = 44.72 seconds for the plane to take off

Problem number three:

Not sure of the equation, so I didn't start it.

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If I didn't already say, all I'd like is to have my answers for problems one and two checked, with an equation(s) supplied for problem number three.

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Hopefully somebody will help me out here. Thanks!

 

[RyanJF]

Eh, I think the answer to problem three is 17.5 meters. All I had to do was take another of the equations and solve for T, which is what I originally thought. Answer makes sense.

I need to stop being lazy.