What are the main components of the central nervous system? A. brain and cranial nerves
B. brain and spinal cord
C. motor nerves and spinal cord
D. spinal cord and spinal nerves

it doesn't. it was imagined up in the middle ages.

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

                                              ⇒      

here   k is the proportionality constant whose value is 1.

                                                            [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it