Which of the following statements correctly describes the role that friction plays in the motion of a train? *

A. Friction is necessary to make a train stop.
B. Friction is necessary for traction as the train starts moving.
C. Friction is necessary to keep the train on the track as it is moving.
D. All of the above.

An engineer designs a roller coaster so that a car travels horizontally for 162 ft, then climbs 127 ft at an angle of 31.0° above the horizontal. it then moves 127 ft at an angle of 46.0° below the horizontal. if we take the initial horizontal motion of the car to be along the +x-axis, what is the car's displacement? (give the magnitude of your answer, in ft, to at least four significant figures and give the direction of your answer in degrees counterclockwise from the +x-axis.)

Metals are good conductors of electricity because they

For two variables in a direct proportion, what is the result of doubling one variable?