Move the objects faster to get more friction.
In technical terms, every coil of wire increases the "magnetic flux density" (strength) of your magnet.
So it's A (magnetic field increase)
To solve the problem it is necessary to apply the concepts related to Force of Friction and Tension between the two bodies.
In this way,
The total mass of the cars would be,


Therefore the friction force at 29Km / h would be,




In this way the tension exerts between first car and locomotive is,



Therefore the tension in the coupling between the car and the locomotive is 
Answer:
Both physical and chemical changes
Explanation:
From the definition of the law of conservation of mass, total mass of all the substances taking part in a chemical reaction is conserved and the number of atoms of each element in the reaction does not change when a new product is formed. Both physical and chemical changes follow the law since when the system is closed to all transfers of matter and energy, the mass of the system must remain constant over time, irrespective of the state.
To solve the problem it is necessary to take into account the concepts related to beat frequency, i.e., The number of those wobbles per second.
The equation that describes the beat frequency is

For our given case we have that the frequency of the instrument is 440Hz and the Beat frequency is 5Hz therefore,
A) The frequency of the violin would be given by




B) <em>The violinist must loosen the string.</em> As the tightening increases the frequency, thereby increasing the number of beats from 5 to 6, i. e, on thightening the string, the frequency further increases as high frequency will be produced by short trings.