Complete Question: Why do we use a spaceship in outer space, far from other objects, to illustrate the principle that an object that does not interact with anything travels at constant speed in a straight line (Newton's first law)? Why not a car or a train? (Select all that apply.)
(1) A car or train touches other objects, and interacts with them.
(2) A car or train can't travel fast enough.
(3) The spaceship has negligible interactions with other objects.
(4) A car or train interacts gravitationally with the Earth.
(5) A spaceship can never experience a gravitational force.
Answer:
(1), (3), (4), (5)
Explanation:
In order to be able to move in a straight line at constant speed forever, as stated by Newton's first law, the object can't be subject to any external net force that can change its momentum.
1) A car, or train, interacts with other objects (the air, the road surface, or the rails, for instance) which means that sooner or later, it will come to an stop, so, for this reason, is not a good fit for that purpose.
3) As it is assumed that the spaceship has negligible interactions with another objects, it will continue moving in a straight line at a constant speed, forever, so it's a good fit to explain Newton's first law.
4) As the train, or a car, or any earthling object, is subject to the gravitational attractive force from Earth, it is not possible for them to move along a straight line at a constant speed forever, as stated by Newton's first law, so a train or a car definitely aren't a good fit in order to explain it.
5) Even though a spaceship can actually experiment a gravitational force from any mass close enough to it, as stated by Newton's Universal Law of Gravitation, in order to simplify things, in this case, usually we neglect any of them.
