In a vacuum, there is no friction due to air resistance. Newton's first law states that an object in motion stays in motion unless acted upon by a net force. Thus the spaceship will travel at the constant speed (zero acceleration) of  indefinitely and the net force on the spaceship must be zero. This can also be shown mathematically:

Since both you and your friend are traveling at a constant speed, the acceleration of you and your friend is zero. Thus, neither you nor your friend experiences any acceleration. This can be shown mathematically using the equation for acceleration:

Since there is no change in velocity over time acceleration is zero. Also note that when acceleration is zero, so is the net force.

Because an object at rest tends to stay at rest, when your car is hit your body/neck will 'want' to stay where it was. This will cause your body and neck to 'whip' as it will take time for it to speed up from being hit. 

The acceleration of the person in the elevator is determined by the net forces and his/her mass. The net force is calculated to be 20N upwards. To find the mass of the passenger, use the following formula:

Then, to find the net acceleration, use Newton's second law.

Recall Newton's first law of motion: an object will remain in its state of uniform motion unless acted upon by an external force. The car's motion is described as having a constant velocity which is a uniform state so there are no external forces.

If the box is moving at a constant velocity, we know that the sum of the forces acting up and down the inclined plane must add to zero. If the only forces acting are gravity and friction, we can show:

We can gain information about the normal force  by looking at the forces acting perpendicular to the plane, shown by: 

Putting all of this together lets us write the following expression for the forces in acting along the plane, and finally, our answer:

Use conservation of momentum:

Plug in values.

Solve for :

If the diver has reached terminal velocity, her acceleration is .

Thus, according to Newton's second law:

Her net force is equal to zero.

The only forces acting on her are gravity and wind resistance, which must add up to zero.

Where is pointing down and thus negative

Plug in values:

Solve for

From the definition of Newton's first law, for an object to not be accelerated, .  Therefore, for an object to be accelerated, .

Newtons first law states than an object in motion tends to stay in motion unless acted upon by an external force. Because we are disregarding air friction, there is no external force to slow the ball down in the horizontal direction after it falls off the train. The acceleration of gravity would only affect the ball in the vertical direction.