21) Acceleration from D to E: 
22) The acceleration of the bus from D to E is 
Explanation:
21)
The acceleration of an object is equal to the rate of change of velocity of the object. Mathematically:

where
u is the initial velocity
v is the final velocity
t is the time elapsed
In this problem, we want to measure the acceleration of the bus from point D to point E. We have:
- Initial velocity at point D: u = 0
- Final velocity at point E: v = 5 m/s
- Time elapsed from D to E: t = 21 - 16 = 5 s
Therefore, the acceleration between D and E is

22) This question is the same as 21), so the result is the same.
Learn more about acceleration:
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Answer:
the density of mobile electrons in the material is 3.4716 × 10²⁵ m⁻³
Explanation:
Given the data in the question;
we make use of the following expression;
hall Voltage VH = IB / ned
where I = 2.25 A
B = 0.685 T
d = 0.107 mm = 0.107 × 10⁻³ m
e = 1.602×10⁻¹⁹ C
VH = 2.59 mV = 2.59 × 10⁻³ volt
n is the electron density
so from the form; VH = IB / ned
VHned = IB
n = IB / VHed
so we substitute
n = (2.25 × 0.685) / ( 2.59 × 10⁻³ × 1.602×10⁻¹⁹ × 0.107 × 10⁻³ )
n = 1.54125 / 4.4396226 × 10⁻²⁶
n = 3.4716 × 10²⁵ m⁻³
Therefore, the density of mobile electrons in the material is 3.4716 × 10²⁵ m⁻³
Answer: 
Explanation:
In the image attached with this answer are shown the given options from which only one is correct.
The correct expression is:

Because, if we derive velocity
with respect to time
we will have acceleration
, hence:

Where
is the mass with units of kilograms (
) and
with units of meter per square seconds
, having as a result 
The other expressions are incorrect, let’s prove it:
This result has units of
This result has units of
This result has units of
and
is a constant
This result has units of
This result has units of
This result has units of
and
is a constant
This result has units of
and
is a constant
because
is a constant in this derivation respect to
This result has units of
and
is a constant