Explain why your body feels like it is being pushed back when the car starts back up again

This push is felt due to inertia

Inertia is the resistance, of any physical object, to any change in its velocity. This property is also called the first law of motion which says a body in rest will remain at rest and a body in motion will stay in motion until an external force acts upon them. The same thing happens when our body is at rest and the car suddenly starts backing up again. Because the rest state of our body experiences a sudden push and our body resist it due to inertia.

an electric circuit

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.