Because although they cannot see it, they can see it's influence on objects that can be seen, and it's effects.
1) C. velocity
Acceleration is defined as the rate of change of velocity per unit time. In formulas:

where
is the change in velocity
is the time interval
Therefore, the correct answer is C. velocity.
2) A. 9.8m/s/s
Earth's gravity is a force, so it produces an acceleration on every object with mass located on the Earth's surface. This acceleration can be calculated, as it is given by the formula

where
is the gravitational constant
is the Earth's mass
is the Earth's radius
By substituting these numbers into the formula, one can find that the acceleration due to Earth's gravity is
.
We can solve the problem by using the first law of thermodynamics, which states that:

where

is the change in internal energy of the system
Q is the heat absorbed by the system
W is the work done by the system
In our problem, the heat absorbed by the system is Q=+194 kJ, while the work done is W=-120 kJ, where the negative sign means the work is done by the surroundings on the system. Therefore, the variation of internal energy is
Given Information:
Initial speed = u = 3.21 yards/s
Acceleration = α = 1.71 yards/s²
Final speed = v = 7.54 yards/s
Required Information:
Distance = s = ?
Answer:
Distance = s = 13.61
Explanation:
We are given the speeds and acceleration of the runner and we want to find out how much distance he covered before being tackled.
We know from the equations of motion,
v² = u² + 2αs
Where u is the initial speed of the runner, v is the final speed of the runner, α is the acceleration of the runner and s is the distance traveled by the runner.
Re-arranging the above equation for distance yields,
2αs = v² - u²
s = (v² - u²)/2α
s = (7.54² - 3.21²)/2×1.71
s = 46.55/3.42
s = 13.61 yards
Therefore, the runner traveled a distance of 13.61 yards before being tackled.
Answer: a) 
b) 
Explanation:
A nuclear fission reaction is defined as the reaction in which a heavy nucleus splits into small nuclei along with release of energy.
a) The given reaction is 
Now, as the mass on both reactant and product side must be equal:


Thus three neutrons are produced and nuclear equation will be: 
b) For the another fission reaction:

To calculate A:
Total mass on reactant side = total mass on product side
235 + 1 = A + 94 + 2
A = 140
To calculate Z:
Total atomic number on reactant side = total atomic number on product side
92 + 0 = 56 + Z + 0
Z = 36
As Krypton has atomic number of 36,Thus the nuclear equation will be :
