Answer: object B is negatively charged, object C is positively charged and object D is also positively charged
Explanation: since unlike charges attract and like charges repel, for object A which is positively charged and B to attract B must be negatively charged and then for B which is negatively charged and C to attract C must be positively charged and for C and D to repel they have to be of thesame charge which means D is positive as well.
 
        
             
        
        
        
Answer:
momentum=mass×velocity 
momentum =400kg×20m/s=8000kg.m/s
 
        
             
        
        
        
Answer:
we learned that an object that is vibrating is acted upon by a restoring force. The restoring force causes the vibrating object to slow down as it moves away from the equilibrium position and to speed up as it approaches the equilibrium position. It is this restoring force that is responsible for the vibration. So what forces act upon a pendulum bob? And what is the restoring force for a pendulum? There are two dominant forces acting upon a pendulum bob at all times during the course of its motion. There is the force of gravity that acts downward upon the bob. It results from the Earth's mass attracting the mass of the bob. And there is a tension force acting upward and towards the pivot point of the pendulum. The tension force results from the string pulling upon the bob of the pendulum. In our discussion, we will ignore the influence of air resistance - a third force that always opposes the motion of the bob as it swings to and fro. The air resistance force is relatively weak compared to the two dominant forces.
The gravity force is highly predictable; it is always in the same direction (down) and always of the same magnitude - mass*9.8 N/kg. The tension force is considerably less predictable. Both its direction and its magnitude change as the bob swings to and fro. The direction of the tension force is always towards the pivot point. So as the bob swings to the left of its equilibrium position, the tension force is at an angle - directed upwards and to the right. And as the bob swings to the right of its equilibrium position, the tension is directed upwards and to the left. The diagram below depicts the direction of these two forces at five different positions over the course of the pendulum's path. 
that's what I know so far
 
        
             
        
        
        
Answer:
we can say that wind energy is due to
D) Severe thunderstorms
Explanation:
As we know that wind energy is converted into kinetic energy of wind mills
This kinetic energy of wind mill is then converted into electrical energy using turbine
now we can consider here  energy conservation theory that energy is only converted from one form to other form 
it neither be destroyed nor be created but it can transfer from one form to other form
So here we can say that wind energy is due to
D) Severe thunderstorms
 
        
             
        
        
        
Answer:
20m/s
Explanation:
acceleration=final velocity-initial velocity/time
 4.0m/s²=v m/s-0m/s/5.0sec
 5.0sec×4.0m/s²=v m/s-0m/s×5.0m/s/5.0m/s
 20m/s=v