Answer:
4 hertz
Explanation:
The defination of freqyency = the total no of cycle made by a wave in one second .
so,
cycle or vibrations=60
tame taken = 15
now,
frequency = no. of cycle / time taken
= 60/15
=4 hertz
hence, the its frequency = 4hertz
Answer:
Frequency of the light will be equal to 
Explanation:
We have given wavelength of the light 
Velocity of light is equal to 
We have to find the frequency of light
We know that velocity is equal to
, here
is wavelength and f is frequency of light
So frequency of light will be equal to 
So frequency of the light will be equal to 
(a) No, because the mechanical energy is not conserved
Explanation:
The work-energy theorem states that the work done by the engine on the airplane is equal to the gain in kinetic energy of the plane:
(1)
However, this theorem is only valid if there are no non-conservative forces acting on the plane. However, in this case there is air resistance acting on the plane: this means that the work-energy theorem is no longer valid, because the mechanical energy is not conserved.
Therefore, eq. (1) can be rewritten as

which means that the work done by the engine (W) is used partially to increase the kinetic energy of the airplane (
) and part is lost because of the air resistance (
).
(b) 77.8 m/s
First of all, we need to calculate the net force acting on the plane, which is equal to the difference between the thrust force and the air resistance:

Now we can calculate the acceleration of the plane, by using Newton's second law:

where m is the mass of the plane.
Finally, we can calculate the final speed of the plane by using the equation:

where
is the final velocity
is the initial velocity
is the acceleration
is the distance travelled
Solving for v, we find

We know,
V= f× wavelength
10.5= f×0.15
f=10.5/0.15
f= 70 Hz