Final answer:
Relative to the car system, the backpack is not in motion and has no kinetic energy, but from an external observer's perspective, it does have kinetic energy while the car is moving.
Newton's first law applies to a car moving at a constant velocity because the net force is zero, and the forces are balanced. In an ideal system, total kinetic energy is conserved, and graphs of total momentum and energy vs. time would show horizontal lines. No.
Step-by-step explanation:
The question of whether a backpack in a moving car is in motion relative to the car system can be addressed by considering reference frames. When the car is moving at a constant velocity, the backpack inside the car, relative to the car system, is not in motion. However, from an external observer's frame of reference, such as someone standing on the sidewalk, both the backpack and the car have kinetic energy because they are moving with the car's velocity.
Newton's first law of motion states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. When a car moves at a constant velocity, Newton's first law does apply because the net force on the car is zero, indicating that the forces are balanced. This is true unless an external force, like friction or a collision, changes the state of motion.
In the case where a car and a truck are moving with the same kinetic energy, but the truck has more mass, the car would have a greater speed. Conversely, if both are moving at the same speed, the truck will have more kinetic energy because kinetic energy is proportionate to the mass of the object when the velocity is constant.
Examples of Energy
The energy of rushing water turning a turbine (kinetic energy).
A man running (kinetic energy).
The energy in a chocolate bar (potential energy).
If we examine kinetic energy conservation, in an ideal system without external forces, the total kinetic energy should remain constant. For an idealized graph of total momentum vs. time, the line would be horizontal reflecting constant momentum if no external forces are acting. Similarly, a graph for total kinetic energy vs. time would also be a constant horizontal line, indicating the conservation of kinetic energy in the absence of external forces.