Muscles function only by contracting. This makes it necessary for one end of the muscle to be fixed and the other mobile.
Take the bicep for example.
Its origin is at the shoulder and its two heads connect to the bones of the forearm, the radius and ulna.
Now, had the muscle not been fixed at one end, and contracted, it would pull both our shoulder and forearm together resulting in an ineffective movement. The desired motion is to lift the forearm (proximal and distal movement) which can only be achieved if the bicep is fixed at the shoulder and allowed to move at the forearm.
Answer: 
Explanation:
According to Newton's law of universal gravitation:
Where:
is the module of the force exerted between both bodies
is the universal gravitation constant.
and
are the masses of both bodies.
is the distance between both bodies
In this case we have two situations:
1) Two bags with masses
and
mutually exerting a gravitational attraction
on each other:
(1)
(2)
(3)
2) Two bags with masses
and
mutually exerting a gravitational attraction
on each other (assuming the distance between both bags is the same as situation 1):
(4)
(5)
(6)
Now, if we isolate
from (3):
(7)
Substituting
found in (7) in (6):
(8)
(9)
Simplifying, we finally get the expression for
in terms of
:
Answer:
4.6834625323 m/s
0 m/s
Explanation:
s = Displacement
t = Time
Velocity is given by

The bird's average velocity for the return flight is 4.6834625323 m/s
In the whole episode the bird went 5220 km away from its nest and came back. This means the displacement is zero.
Hence, the average velocity for the whole episode is 0 m/s
When a swimmer pushes through water to swim they are propelled forward because of the water resistance against the hand and feet. It's A. The water doesn't automatically push the swimmer forward. It releases a reaction after the swimmer pushes through the water.