# Homework Help: Kinematics:Interpreting graphs and deriving equations(Check my solutions please)

1. Jan 26, 2012

### supernova1203

1. The problem statement, all variables and given/known data
Marian who is standing on her balcony is surprised by a pigeon, and throws a flowerpot up, in the air at 2.1 m/s. It takes a total of 3 s for the flowerpot to smash to the ground. The flowerpot experiences acceleration due to gravity of 9.8m/s

a) How high is Marians balcony?
b)How fast was the flowerpot moving just before it smashed into the ground?
2. Relevant equations

Δd=V1Δt + 1/2 aΔt^2

V2=V1+aΔt

Δd=V1Δt - 1/2 aΔt^2
(By the way id like to know what this third equation is for, when is it appropriate to use it? I know when to use it when i see a v-t graph and am required to find displacement, and geometrically there are 2 objects in graph, i have to find area, of one and to do so would require me to subtract the area from other geometrical object, in such a scenario i would use the 3rd equation, but if the quest is in written word problem format, i dont know when to use the 3rd formula.)

3. The attempt at a solution

a) v=2.1m/s

Δt=3 s

a=-9.8m/s

Δd=V1Δt + 1/2 aΔt^2

=(2.1)(3)+1/2(-9.81)(3)^2

=6.3+1/2(-9.81)(9)

=6.3+1/2-88.79

=6.3-44.14

Δd=-37.82m

b) v1=2.1m/s

Δt=3s

a=-9.8m/s

v2=v1+aΔt

=2.1+(-9.8)(3)

V2=-27.3 m/s

2. Jan 26, 2012

### tiny-tim

hi supernova1203!

yes that all looks fine
my recommendation is that you should always use the first one, if necessary of course using a negative number for a

3. Jan 26, 2012

### CallMeMarvin

as far as i know the 3rd equation is used for upward motions to make it a bit easy cause others usually forget to use a negative sign for the magnitude or amount for "a" when using the 1st equation that you stated earlier :]

so i think its better to use the 3rd one when you know it the object is thrown upwards or something. to just substitute it directly though it may be confusing at times but you'll get used to it.

and yes they are all fine :)