Answer:
618.03 J
Explanation:
Potential energy can be calculate using the following formula.
PE=m*g*h
where m is the mass, g is the acceleration due to gravity and h is the height.
In this situation, we have a 3 kilogram rock on a 21 meter high cliff. Therefore, the mass is 3 kg and the height is 21 m.
On Earth, the acceleration due to gravity is 9.81 m/s^2.
m=3 kg
h=21 m
g=9.81 m/s^2
Substitute these values into the formula.
PE=3*9.81*21
Multiply all the numbers together
PE=63*9.81
PE=618.03
The potential energy is 618.03 kg m^2/s^2, or 618.03 Joules
Assuming conservation of energy:
Energy before = 7000 + 4000 = 11 000 MJ
Energy after = 2000 + Kinetic = 11 000
Therefore Kinetic = 11 000 - 2 000
Kinetic = 9 000 MJ
Answer:
Destructive interference
Explanation:
Destructive interference occurs when two waves traveling along the same medium meet and cancel each other out. The resultant wave has a smaller amplitude than the individual waves. In this case, the maxima of two waves are 180 degrees out of phase.
Hence, the correct option is (A) "destructive interference"
Explanation:
A charge alters the space around it. This alteration of space is called the electric field. It is also defined as the electric force acting on a charged particle per unit test charge. It is given by :

Where
F is the electric force, 
The direction of electric field is in the direction of electric force. For a positive charge, the direction of electric field lines are outwards and for a negative charge, the direction of field lines are inwards.
Hence, the correct option is (c) "electric field".