# Homework Help: Two hard physics problems (Projectile motion and gravitation), for me

1. Feb 4, 2012

### physicsisgrea

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

Please refer to the figures. (The first one is multiple-choice, the latter is structured).

2. Relevant equations

For the first problem:
u = √(2gH)
x = uxsinθt + $\frac{1}{2}$gsinθt2
uycosθt = $\frac{1}{2}$gcosθt2 and maybe others
u = initial velocity
x = x-displacement (along the slope)
y = y-displacement (perpendicular to slope)

For the second:
FG= Gm1m2 / r2
m are masses
FG = gravitational attractive force between two masses
$G\ =\ 6.673(10)\ \times\ 10^{-11}\ m^{3} kg^{-1} s^{-2}$

and maybe others

3. The attempt at a solution

Sorry, I've been doing these two problems for hours, still can't get the answers:

For the first, I only found that the time between the release and the first impact might be √(2H/g).

For the second problem I even dont any idea about how to solve it:(

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Last edited: Feb 4, 2012
2. Feb 4, 2012

### BruceW

the first question is a bit tricky. Those kinds of questions get easier with practice. You are right about the time between release and the first impact, but I don't think that is useful for answering the question. It says that each of the collisions are elastic, so this tells you something that is conserved before and after each collision. What is that quantity? And from the diagram, what can you say about the direction of velocity after any of the collisions?