Answer:
Electromagnets are special types of magnets that are made by passing current through coils of wire. To make an electromagnet, the minimum requirements are:
1. A nail (usually made of iron, steel or zinc)
2. Dry cell batteries
3. Wire (Usually copper wire)
Other things could be:
1. Electric tape to hold both ends of the wire properly at the battery terminals.
2. Scissors to cut the wire into desired length.
3. Iron fillings for testing purposes.
The answer is D. As the ambulance gets closer, the sound waves are compressed relative to the person; so the frequency increases.
Answer:
The rate of change of the shadow length of a person is 2.692 ft/s
Solution:
As per the question:
Height of a person, H = 20 ft
Height of a person, h = 7 ft
Rate = 5 ft/s
Now,
From Fig.1:
b = person's distance from the lamp post
a = shadow length
Also, from the similarity of the triangles, we can write:

Differentiating the above eqn w.r.t t:
Now, we know that:
Rate = 
Thus