Wood isn't as tough as rock. Wood also breaks down in weather and cracks under pressure. Plus rocks were more accessible.

Where r is the radius of balloon.
Here mass of woman = 68 kg
Mass of air displaced by a balloon with volume V = 1.29*V
Mass of helium inside balloon = 0.178*V
Total mass to be lifted by balloon = 68 +0.178*V
Buoyant force = 1.29V-0.178V=1.112V
So we have 1.112 V = 68+ 0.178*V
0.934 V = 68
V = 72.81 
\frac{4}{3} \pi r^{3}[/tex]= 72.81
r = 2.59 m
So radius of helium balloon = 2.59 m
Answer:
a.) 1567.2 m/s
b.) 149.4 m/s
Explanation:
Given that a 26 kg body is moving through space in the positive direction of an x axis with a speed of 350 m/s when, due to an internal explosion, it breaks into three parts. One part, with a mass of 7.8 kg, moves away from the point of explosion with a speed of 180 m/s in the positive y direction. A second part, with a mass of 8.8 kg, moves in the negative x direction with a speed of 640 m/s.
The x-component of the third part can be calculated by assuming that it moves in a positive x axis.
The third mass = 26 - ( 7.8 + 8.8)
The third mass = 26 - 16.6
The third mass = 9.4kg
since momentum is conserved, the momentum before explosion will be equal to sum of the momentum after explosion
26 x 350 = -8.8 x 640 + 9.4V
9100 = -5632 + 9.4V
9.4V = 9100 + 5632
9.4V = 14732
V = 14732/9.4
V = 1567.2 m/s
(b) y-component of the velocity of the third part will be
7.8 x 180 = 9.4 V
1404 = 9.4V
V = 1404/9.4
V = 149.4 m/s
Answer:

Explanation:
n = Order = 1
c = Speed of light = 
f = Frequency = 
= Angle = 
Lattice spacing is given by

The lattice spacing of the crystal is 