Answer:
0.358g
Explanation:
Density of Helium = 0.179g/L
ρ=m/v
m=ρv
when the volume was 2L
m1= 0.179*2
m1=0.358g
when the volume increased to 4L
m2= 0.179*4
m2=0.716g
gram of helium added = 0.716g-0.358g
=0.358g
The frequency of the wave is 
Explanation:
The frequency, the wavelength and the speed of a wave are related by the following equation:

where
c is the speed of the wave
f is the frequency
is the wavelength
For the radio wave in this problem,


Therefore, the frequency is:

Learn more about waves here:
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Answer a would be correct since velocity is a vector and has a magnitude and a direction. In this case v₁ = - v₂.
Answer:
54 × 5/18 = 15m/s
Explanation:
to convert km/hr to m/s you multiply by 5/18
Taking the copper wire, he has to wind it around the nail made of iron. After which, he then connect both ends of the copper wire to the battery, so an electric charge travels through the wire. This is the basic electromagnet. Since a current is now flowing through the wire, a magnetic field is produced. Placing the electromagnet near the mixture of copper and iron, the magnet should attract the pieces of iron, as iron is more magnetic compared to copper. This is done over a period of time, so that only the copper pieces are left in the mixture.