i hate this question.
Conservation of momentum - is when the total momentum before and after collision is equal.
Here is the formula darling,
p = p
mv = mv
See the pic for example. HmpH
The first: alright, first: you draw the person in the elevator, then draw a red arrow, pointing downwards, beginning from his center of mass. This arrow is representing the gravitational force, Fg.
You can always calculate this right away, if you know his mass, by multiplying his weight in kg by the gravitational constant

let's do it for this case:

the unit of your fg will be in Newton [N]
so, first step solved, Fg is 637.65N
Fg is a field force by the way, and at the same time, the elevator is pushing up on him with 637.65N, so you draw another arrow pointing upwards, ending at the tip of the downwards arrow.
now let's calculate the force of the elevator

so you draw another arrow which is pointing downwards on him, because the elevator is accelating him upwards, making him heavier
the elevator force in this case is a contact force, because it only comes to existence while the two are touching, while Fg is the same everywhere
Force = mass × acceleration
To find acceleration, we can divide the speed by the time it took:
acceleration = 2.40×10^7 / 1.8×10^-9
acceleration = 1.33×10^16
the mass is equal to the mass of an electron
force = (9.11×10^-31)(1.33×10^16)
force = 1.21×10^-14 N
Answer:
The voltage across a semiconductor bar is 0.068 V.
Explanation:
Given that,
Current = 0.17 A
Electron concentration 
Electron mobility 
Length = 0.1 mm
Area = 500 μm²
We need to calculate the resistivity
Using formula of resistivity


Put the value into the formula


We need to calculate the resistance
Using formula of resistance



We need to calculate the voltage
Using formula of voltage

Put the value into the formula


Hence, The voltage across a semiconductor bar is 0.068 V.