Scientific laws are statements of facts produced after observed events of nature. There are usually no exceptions to how the outcome of these events unravels. Laws explain HOW nature works.
Explanation:
ON THE OTHER HAND, Theories are usually blanket principles, usually overarching, on how nature has been observed to work. Theories can evolve over time as more information is discovered, but laws are more or less static. Theories explain WHY we see nature to behave in a certain way.
In the case of this question it is scientific law that if a fluid is heated, it becomes less dense as its temperature rises.
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Well it seems like this problem gives you what you need. You said the car was going 4m/s and then accelerated to 60m/s... so 4m/s would be your answer for the initial velocity