Let
upthrust = T
weight = W = mg
Air resistance = F
When balloon is descending, air resistance acts upwards (positive)
By Newton's first law, the net force on the balloon is zero, or
T+F-W=0......................(1)
Let w=weight of material dumped so that balloon now travels upwards at constant speed.
Air resistance acts against motion, namely downwards.
The Newton's equation now reads
T-F-(W-w)=0................(2)
Subtract (2) from (1)
T+F-W - (T-F-(W-w)) = 0
Solve for w
w=2F, or
the WEIGHT of material to be released equals twice the resistance of air.
As the question is about changing in frequency of a wave for an observer who is moving relative to the wave source, the concept that should come to our minds is "
Doppler's effect."
Now the general formula of the Doppler's effect is:

-- (A)
Note: We do not need to worry about the signs, as everything is moving towards each other. If something/somebody were moving away, we would have the negative sign. However, in this problem it is not the issue.
Where,
g = Speed of sound = 340m/s.

= Velocity of the receiver/observer relative to the medium = ?.

= Velocity of the source with respect to medium = 0 m/s.

= Frequency emitted from source = 400 Hz.

= Observed frequency = 408Hz.
Plug-in the above values in the equation (A), you would get:


Solving above would give you,

= 6.8 m/s
The correct answer = 6.8m/s
I am assuming this is a true or false question, to which the answer would be True.
Answer:
1)

2)

Explanation:
<u>Projectile Motion</u>
When an object is launched near the Earth's surface forming an angle
with the horizontal plane, it describes a well-known path called a parabola. The only force acting (neglecting the effects of the wind) is the gravity, which acts on the vertical axis.
The heigh of an object can be computed as

Where
is the initial height above the ground level,
is the vertical component of the initial velocity and t is the time
The y-component of the speed is

1) We'll find the vertical component of the initial speed since we have not enough data to compute the magnitude of 
The object will reach the maximum height when
. It allows us to compute the time to reach that point

Solving for 

Thus, the maximum heigh is

We know this value is 8 meters

Solving for 

Replacing the known values


2) We know at t=1.505 sec the ball is above Julie's head, we can compute




<span>Here the force that is applied between the electron and proton is centripetal, so equate the two forces to determine the velocity.
We know charge of the electron which for both Q1 and Q2, e = 1.60 x 10^-19 C
The Coulombs Constant k = 9.0 x 10^9
Radius r = 0.053 x 10^-9m = 5.3 x 10^-11 m
Mass of the Electron = 9.11 x 10^-31
F = k x Q1 x Q2 / r^2 = m x v^2 / r(centripetal force)
ke^2 / r^2 = m x v^2 / r => v^2 = ke^2 / m x r
v^2 = ((1.60 x 10^-19)^2 x 9.0 x 10^9) / (9.11 x 10^-31 x 5.3 x 10^-11 )
v^2 = 4.77 x 10^12 = 2.18 x 10^6 m/s
Since one orbit is the distance,
one orbit = circumference = 2 x pi x r; distance s = v x t.
v x t = 2 x pi x r => t = (2 x 3.14 x 5.3 x 10^-11) / (2.18 x 10^6)
t = 33.3 x 10^-11 / 2.18 x 10^6 = 15.27 x 10^-17 s
Revolutions per sec = 1 / t = 1 / 15.27 x 10^-17 = 6.54 x 10^15 Hz</span>