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.

For this question, we're told that two skydivers of different masses are jumping out of an airplane. At some point, both skydivers slow down to the point at which they reach constant velocity.

To answer this, we have to realize how constant velocity affects the net force. Since the velocity for both people is constant, that means that no acceleration is occurring. Moreover, because either person is not accelerating, we see that the net force on each person is zero. Consequently, the net force for both person A and B is the same.

According to Newton's first law, an object will remain at rest until a force is applied. If the floor of the boxcar is perfectly motionless, than the object will remain in the same spot in relation to the earth. This will make it appear to move to the back of the boxcar.

The correct answer is "Newton's First Law: every object continues in its state of rest or of uniform velocity in a straight line as long as no net force acts on it." On a slippery surface, we can assume there is no friction. If there is no friction working against the box's movement, the box will continue to move at its initial velocity forever.  If the box were on a rough surface, friction from the surface would act against the box, causing the box's velocity to slow and eventually stop (if the force acting on the box is not greater than that of the frictional force). 

Force is defined as

Since the velocity is constant, the acceleration is zero. Therefore the force required to keep this object in motion is zero.

This is also stated in Newton's First Law: "An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force."

According to Newton's First Law of Motion, an object that is in motion will stay in motion unless acted on by another force. When the van slows down, the ball will want to continue moving forward, and the friction between it and the floor of the van is not strong enough to keep the ball back.

According to Newton's First Law of Motion, something in motion will stay in motion unless acted on by another force. As the van slows down, we expect free object in the van to continue moving forward within the van. However, helium is lighter than air so the air in the van will continue to move forward, leaving the light balloons in the back of the van.